Copyright by Francis Leo Lynch III 1994 THE EFFECTS OF DEPOSITIONAL ENVIRONMENT AND FORMATION WATER CHEMISTRY ON THE DIAGENESIS OF FRIO FORMATION (OLIGOCENE) SANDSTONES AND SHALES, ARANSAS, NUECES, AND SAN PATRICIO COUNTIES, TEXAS by FRANCIS LEO LYNCH III, B.S., M.S. DISSERTATION Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY THE UNIVERSITY OF TEXAS AT AUSTIN May, 1994 THE EFFECTS OF DEPOSITIONAL ENVIRONMENT AND FORMATION WATER CHEMISTRY ON THE DIAGENESIS OF FRIO FORMATION (OLIGOCENE) SANDSTONES AND SHALES, ARANSAS, NUECES, AND SAN PATRICIO COUNTIES, TEXAS This big, fat, book, for whatever its worth, is dedicated to my mom and aunties whose completely unwavering love and support over the past (gasp) 35 years, even through times characterized by sheer stupidity and lunkheadness, makes me understand the wordfamily. And also to FLL Jr., who had to leave before getting a chance to finish up his own this one's for you too. - Acknowledgements This project was supported by DOE grants to Land, by oil company grants to Land and Mcßride, by UTDOGS TAs, and by a GCAGS grant to me. Thanksgoto: BossLandandBossMcßride,whogavemeplentyofropeto either build something diagenetic, or make a really nice noose. Whichever way it That went, and I guess the jury's still out, it was my project, from beginning to end. kind of freedom is a rare thing in our own department, let alone other places. Kitty - Milliken (who was in a large way responsible for me coming to UT so blame her) dumb idea I was who I went to first with anv analytical question, or with any was thinking about. Luigi Folk harassed me the whole time, and Shirley Dutton was a good contact at the bureau. Other BEG people who went out of their way to help mewereRip Langford,Robert,Jon, andtherestofthe guys at the CRC (Iletyou jokers win at ping-pong). Mike Guest (Mobil), Kurt Geitzenauer (ARCO), and Tim Dave Pevear cores. Diggs (Shell) were particularly helpful with gaining access to (Exxon) proves that all mid-level oil pigs don't lose their sense of humor. My mother continuously pestered me to finish. The nine UTDOG professors who came to my tech talk were Carlson, Folk, Galloway, Gutierrez, Kirkland, Land, Long, Mcßride, and Walker. High binding costs and punitive libel decisions prevent me from listing all the people who've been "significant" to me over the past (holy shit!) 2,452 days, so let me just say thanks to the many of you who've been my friends. I hope you got from me as much as I got from you. ("I don't know half of you half as well as I should like; and I like half of half as well as you deserve." B. Baggins.) you ­ Special thanks go to Morlog, Otto, Bart the obsequious, the members of SVEM and PANCREAS, and especially Egeron and Kuron. Extra special thanks to Galador Greycloak, who, assisted only by Woof, the slavering, blood-thirsty rotweiller, was twice responsible for the Austin American Statesman not ending an article on Claynac with "an Ozzy record was found on the turntable," or for some network reporter not finding out that his neighbors thought "he was a regular guy but he kept to himself." Darmak on the ocean. V THE EFFECTS OF DEPOSITIONAL ENVIRONMENT AND FORMATION WATER CHEMISTRY ON THE DIAGENESIS OF FRIO FORMATION (OLIGOCENE) SANDSTONES AND SHALES, ARANSAS, NUECES, AND SAN PATRICIO COUNTIES, TEXAS Publication No. Francis Leo Lynch III, Ph.D. The University of Texas at Austin, 1994 Supervisor: Lynton S. Land Shorezone and shelf facies sandstones of the Frio Formation from five growth fault blocks in the Corpus Christi, Texas, area were studied by petrographic, isotopic, and XRD techniques in order to determine the factors influencing their diagenesis. The distribution of quartz overgrowths, calcite, kaolinite and secondary porosity in the sandstones suggests that permeability and fluid flow largely govern the diagenetic modification of the rocks. The most important control on the original permeability of a sandstone is clay content and is related to depositional environment. Mixed-layer I/S is the most common clay mineral in both Frio sandstones and shales. During burial diagenesis, the amounts of I/S and chlorite increase and the amounts of kaolinite and discrete illite decrease in both rock types. Mass balance calculations indicate the Frio shales act as open systems during diagenesis and require significant import of potassium from a yet unidentified source. The calculations further show that Frio shales can supply all the SiC>2 needed for quartz overgrowths in the associated sandstones. The quantitative relationships between the clay minerals in Frio shales do not support a smectite-cannibalization mechanism for the smectite to illite reaction. VI 8 180 values of calcite cement in sandstones become progressively depleted with depth, implying that the calcite recrystallizes throughout the burial history of the rock. The 87Sr/86Sr value and trace element chemistry of diagenetic calcite is largely a function of the chemistry of the formation water causing The recrystallization. presence of chemically distinct calcites and formation waters from adjacent growth fault blocks indicates that these blocks have been hydrologically separate from each other since at least the time of calcite precipitation (~26 m.y.b.p.). There is no evidence that sandstone diagenesis is different in fault blocks characterized by organic acid-rich formation waters. Most of the diagenetic modification of Frio sandstones occurs between 6,000 ft (1,829 m) and 9,000 ft (2,743 m). The 5 180 of quartz overgrowths The indicates that they precipitated from hot, rapidly ascending formation water. water was probably part of a flow (convection?) system that developed in the transition zone from hydrostatic to nearly lithostatic fluid pressure. The 5 180 values ofcalcite and kaolinite/dickite support this model for the diagenesis of Frio Formation sandstones. VII Table ofContents Pass List of Figures xi List of Tables xiv 1 Chapter 1. Purpose of Study and Dissertation Organization Chapter 2. Mineralogy and Diagenesis of Frio Formation Shales, Texas Gulf Coast 3 Introduction 3 4 Sample Preparation and Data Interpretation Results 7 %\in I/S 7 Clay Mineral Abundances 12 12 Mixed-layer I/S Discrete Illite 13 Kaolinite 16 Chlorite 16 Regional Differences in Clay Mineralogy 16 19 Basinward Variation in Clay Mineralogy 19 Factors Effecting Quantitative Analysis of Clay Minerals 22 Whole Rock Mineralogy The Smectite to Illite Reaction 27 Hower Revisited: 35 I/S Chemistry Mass Balance Calculations 44 Discussion 50 54 Summary Chapter 3. Introduction and Previous Work 59 59 Geologic Setting and Sedimentology General Geology 59 Geology of the Frio Formation in the Corpus Christi area 61 64 Depositional Facies Shorezone Sandstone Facies 64 64 Shelf Depositional Facies 65 Frio Shelf Depositional Processes Distal-shoreface/inner-shelf sandstone facies 66 Shelf Siltstone Facies 66 Shelf Sandstone Facies 70 Frio Sandstone Composition and Diagenesis 70 70 Detrital Mineralogy Diagenesis 74 75 Chapter 4. Sandstone Petrography Methods 75 Detrital Composition 75 VIII Diagenesis 84 Compaction 84 I/S (matrix) 88 Chlorite 97 Calcite 97 Kaolinite 104 Analcime 106 Quartz Overgrowths (QOGs) 108 Porosity 110 Core Descriptions 116 Shell TST 346 #1 116 Shell TST 392 #4 126 Cities TST 51 #1 129 Cities TST 10 #2 144 Cities TST 4 #2 143 Cities TST 49 #2 148 TST 49 #2 8,011 ft 148 TST 49 #2 8,521 ft 159 TST 49 #2 8,710 ft 150 TST 49 #2 8,975 ft 155 TST 49 #2 9,076 ft 155 Diagenesis 155 Arco TST 430 #5 157 Cecelia Kelley #2 163 Wainco Mutchler#! 163 Minnie S. Welder #67 166 Minnie S. Welder #7O 168 180 Copano TST 104 #7 Discussion 180 Calcite 185 Chapter 5. Petrography 185 186 Trace Element Chemistry Isotopes 192 813 C 192 6180 201 87Sr/*6Sr 203 Formation Water 206 Discussion 209 Chapter 6. Clay Minerals in Frio Sandstones Detrital Clay Matrix 212 Mixed-Layer I/S 216 Texture 219 Kaolinite 223 Dickite 223 Chlorite 228 234 Mineralogy of Shale Core 234 Clay Mineralogy Whole Rock Abundances 235 Discussion 238 Chapter 7. Diagenetic Model and Conclusions 246 Oxygen Isotope Geochronology 246 Evidence Against Deep Meteoric Water Invasion 248 249 Diagenetic Model Convection? 253 256 Relationship Between Diagenesis and Formation Water Chemistry Summary and Unanswered Questions 260 263 Appendix A. XRD analyses of shale cuttings 266 Appendix B. Sandstone framework petrography 270 Appendix C. Microprobe analyses of calcite cement Appendix D. XRD analyses of sandstone and shale core 277 References Cited 281 Vita 304 X List of Figures # Title Page 2.1 Sample locations of analyzed Frio Formation shales 5 2.2 %I in I/S vs depth in the o interstratification. Squares are Rio Grande embayment samples, triangles are San Marcos Arch area samples, circles are Houston embayment samples. Most of the conversion of smectite to illite occurs within the transitional pressure regime (see Chap. 7). 8 Figure 2.3. XRD patterns of <1 pm fraction clays from shale cuttings from Kenedy County well. Oriented samples are Mg saturated and glycol solvated. Labelled peaks are I/S = illite/smectite, I = illite, K = kaolinite, = C chlorite,Qtz=quartz,A=albite. Seetextforexplanationof4and.. A. 8,565 ft, B. 10,035 ft. 9 Figure 2.3 (cont). XRD patterns of <1 pm fraction clays from shale cuttings from Kenedy County well. Oriented samples are Mg saturated and glycol solvated. LabelledpeaksareI/S=illite/smectite,I=illite,K=kaolinite, C = chlorite, Qtz = quartz, A = albite. See text for explanation of * and .. A. 14,055 ft, B. 17,685 ft. 10 I/S ordering occurs between 60%1 and ~75%1 at depths between 10,000ft and12,000ft(Figure2.2). Mudweightinformationfromlogheadersandregional information from Bebout et al. (1975), and Bebout et al. (1978), indicates that "hard" geopressureisreachedbetween9,000ftand 12,000ftinmostofthewells, which corresponds to the approximate depth of I/S ordering. Only four samples deeper than 11,000 ft show random I/S interstratification. The three random I/S samples in the Rio Grande embayment come from the Cameron County well wheregeopressureisnotreacheduntil 13,000ft,soeventheseanomaliessupport the relationship between geopressure and ordering. (The deep random Houston embayment sample is also anomolously low in %I and has clay mineral abundances more characteristic of shallower samples and is probably due to hole caving, improper sample picking, or some other contamination.) This relationship between the smectite to illite reaction and geopressure has been documented by many others (Kerr and Barrington, 1961; Freed, 1981, 1982; Bruce, 1984; Pollastro, 1985). Some investigators have proposed that the occurrence of itself is a result of the conversion of smectite to illite (Hanshaw and geopressure Bredehoeft, 1968; Burst, 1969; Magara, 1975; Bruce, 1984) though others (Weaver, 1979; Colton-Bradley, 1987) make equally strong cases against this hypothesis. The relationship between overpressure and diagenetic change in sediments has been observed for other minerals as well. Quartz overgrowths in Frio sandstones are more abundant at and below geopressure (Land, 1984), diagenetic calcite in some Green River sandstones is chemically and isotopically different above and below overpressure (Dickinson, 1988), and the recrystallization of calcite in Frio sandstones is occurring in the overpressured regime (Lynch et al., 1993).While theeffectofincreased pressureonthesemineralogicchangescannot easily be determined, it is more likely that the hydrologic conditions encountered forced convection in the overpressured rocks, such as the possibility of free or(Wood and Hewitt, 1984; Blanchard and Sharp, 1985) are responsible for these relationships. Temperature is considered to be the primary factor controlling the smectite to illite reaction (Perry and Hower, 1970, 1972; Eberl and Hower, 1976; Eberl and Hower, 1977; Hoffman and Hower, 1979; Weaver, 1979; Hower, 1981; McCubbin and Patton, 1981; Roberson and Lahann, 1981; Burtner and Warner, 1986; Velde and lijima, 1988; Pytte and Reynolds, 1989; Glassman et al., 1989; Pollastro, 1989, 1990; Elliott et al., 1991; Huang et al., 1993; see review in Eslinger and Pevear, 1988; and "Geothermometry and Geochronology using Clay Minerals" (Cla>s and Clay Minerals, 1993, vol. 41, #2)), though secondary factors (such as water/rock ratio, fluid composition, mineral composition) and especially time (Huff and Turkmenogliu;l9Bl, Srodon and Eberl; 1984, Ramseyer and Boles; 1986; Whitney and Northrop, 1987; and others) are also important. Figure 2.4 shows the relationship between temperature and %I. Temperature data come from well log headers and has been corrected according to the method of Kehle, 1971. This figure shows that ordering takes place at between 115° and 130° which is hotter than the temperatures published by Freed (1981, 1982), but about the same temperature of ordering proposed by Hower et al. (1976). ClaY Mineral Abundances Mixed-Layer Illite/Smectite The most abundant clay mineral in the <1 pm fraction from all the shales investigated ;s mixed-layer I/S. Figure 2.5 shows that in all three regions the amount of I/S increases with depth from -60% to -75%, though this trend is less well defined in the samples from the two Houston embayment wells. Discrete Illite In the Rio Grande embayment samples, discrete illite is the second most abundant component of the ointerstratification. Squares are Rio Grande embayment samples, triangles are San Marcos Arch area samples, circles are Houston embayment samples. Well log temperatures corrected by method of Kiehle (1971). 13 14 clay —is S abundant that most implies the is depth Illite/smectite with abundance VS cuttings. in shale increase of fraction The shales. |irct in <1Formation reaction. I/S % Frio in 2.5. neoformation Figure mineral a 15 embay- Grande Rio the from shales in abundant most is Illite cuttings. regions. shale all in <4im depth of content with illite decreases Discrete content 2.6. Illite Figure ment. Kaolinite Kaolinite is the second most abundant clay in the fine fraction from the San Marcos arch area and from the samples from north Texas (Figure 2.7). Shallow samples from the San Marcos arch shales contain as much as 40% kaolinite. The abundance of kaolinite in the shales from all three regions decreases with depth, though again this trend is poorly defined in the two Houston embayment wells. Kaolinite is never fully removed from any of the shales. Chlorite Chlorite increases in abundance with depth in shales from north Texas and from the San Marcos arch area (Figure 2.8). The trend in chlorite abundance is less clear in the samples from the Rio Grande embayment. Chlorite is most abundant in Rio Grande embayment samples and least abundant in the Houston embayment. Regional Differences in Clay Mineralogy The data in Figures 2.5 through 2.8 show that significant differences do The exist in the clay mineralogy of shales from the three geographical regions. Norias delta in the Rio Grande embayment was the major depocenter for the Frio Formationthroughoutitstimeofformation(Gallowayetal., 1982). Thesourceof mostofthesedimentdepositedtherewasthevolcanicrocksofWestTexas and Mexico. The sediment deposited in Houston delta in the Houston embayment of North Texas was predominately of cratonic origin (Mcßride et al., 1962; Galloway, 1977). The Buna strandplain/shelf system on the San Marcos arch received sediment from both sources. Sandstones from the different depocenters also preserve these provenance differences (Lindquist, 1977; Loucks et al., 1981; Loucks et al., 1986). The high discrete illite content of the samples from the Rio Grande embayment correlates especially is Kaolinite cuttings. shale of fraction clfim of content Kaolinite 2.7. Figure depth. with decreases shales of content Kaolinite shales. Arch Marcos San in abundant Grande Rio of in abundance the chlorite The in cuttings. decrease effects. shale shallow provenance fraction The pm regions. to part <1 in allof due in be content depth may Chlorite with samples 2.8. increases Figure chlorite embayment well with a volcanogenic source. A volcanic source also probably explains the unusually high chlorite content of the shallow samples in this area. Weathering of cratonic rocks produces abundant kaolinite and is responsible for the high kaolinite content of the shales in the Houston embayment and on the San Marcos arch (Aoki and Oinuma, 1974). Basinward Variation in Clay Mineralogy Figure 2.9 shows the basinward variation in clay mineralogy from middle Frio samples from similar depths in the San Marcos arch wells. Kaolinite is preferentially concentrated in the nearshore MSW 27 well and I/S is more abundant in the offshore Mustang Island well. Illite also appears to be concentrated in the nearshore well. These are the same relationships seen in modem environments (Parham, 1966). These results are rather tenuous, however, because in order to preserve the same age and depth relationships, the number of samples used in this comparison is rather small. For chlorite and illite, the basinward variation is not statistically significant. The results of this investigation show that even though there probably were real differences in the clay mineralogy of these shales at deposition both along strike and down-dip, the diagenetic processes that modified the rocks during burial were the same in all three regions. The net result of these diagenetic processes was a decrease in the abundance of kaolinite and illite and an increase in I/S and chlorite with depth. Factors Effecting Quantitative Analysis of Clay Minerals Hower et al. (1976), and Freed (1981, 1982), report a decrease in the I/S and illite content of Frio shales with depth. Hower et al. (1976) reports that the amountofkaolinite decreasesand theamountofchlorite increases with depth. Freed (1981, 1982) only identified chlorite in one (of four) wells, and reported that Figure 2.9. Variation in Clay Mineral Abundance down depositional dip. Error bars are one standard deviation of the mean. The increase in "fine" I/S and decrease in "coarse" kaolinite basinward is the same seen in modem sediments. 20 the amount ofkaolinite actually increases with depth. How can these analyses be reconciled with the results of this study? In their section on quantitative analysis of clay minerals Moore and Reynolds (1989, p. 305) state, "quantitative analysis contains many pitfalls whose deleterious effects on the data are not obvious from an examination of the final results. Precision is easy but... 'accuracy comes from God.' The excellent repeatability that lull you can easily attain may you into thinking that the answers are correct, when in fact, internal consistency may be a manifestation only of your ability to obtain a wrong answer every time and do it with great precision." Within that statement, I believe, is the reason for the very different clay mineral variations seen in this study compared to the earlier work of Perry and Hower (1970), Hower et al. (1976), and Freed (1981, 1982). The quantitative techniques applied by Hower et al. (1976) and Freed (1981, 1982) are those of Johns et al. (1954), and Perry and Hower (1970). Since those methods were developed, the factors influencing X-ray diffraction of clay minerals andhow theyeffectquantitativeinterpretationofXRD patterns have been exhaustively investigated, principally by Reynolds (1983, 1985, 1986, 1989). Computer modeling ofXRD patterns using his conventions very closely match real clay XRD data. Quantitative analytical techniques summarized by Moore and Reynolds (1989) take into account variables in both sample preparation and interpretation that were overlooked in the older methods. Specifically, the older quantification techniques were based on diffraction peaks now know to suffer from mutli-phase interferences, and they used MIF values that have since been shown to be greatly in error. Additionally, the Hower et al. (1976) laboratory technique involved settling of the clays on glass slides, a procedure Reynolds (1985) has shown can result in size (and therefore mineralogic) fractionation that can produce quantitative errors greater than 50% relative! The procedures section of both papers by Freed does not state how the samples were prepared for analysis Considering the relative timing of those papers and the later work of Reynolds, one must assume that Freed's laboratory techniques were similar to Hower's. The most significant difference between this study and the previous work is in the relative change in abundance of I/S over the course of the smectite to illite reaction. Independent work of Lynch (1985) on a contact metamorphic I/S suite, and Awwiller (1992, 1993) on Wilcox shales in the Gulf of Mexico, both of whom used the techniques summarized in Moore and Reynolds (1989), show the same increase in I/S seen in this study. This is additional, albeit circumstantial, evidence that the mineralogic changes seen in this study are more nearly correct, and that previous results from Frio shales are less accurate because ofthe use of laboratory and interpretive techniques now known to be in error. Whole Rock Mineralogy The whole rock abundances of quartz, total clay, potassium feldspar, plagioclase feldspar and albite, and calcite for wells MSW 27 and Arco TST 36# 1 are shown in Figures 2.10 through 2.13. The amount of potassium feldspar decreases and the amount of albite increases in both wells. These relative changes in the feldspar content of shales are the same as reported by Milliken (1992), based on SEM and microprobe analyses of some of the same samples used in this study. These same changes in feldspar content are also seen in Frio sandstones (Loucks et al., 1981; Loucks et al., 1986; Milliken, 1989). Both wells show a decrease in the amount of total clay and increase in quartz content with depth, though these trends are best defined in the TST 36# 1 samples (Figure 2.12). These changes could be due to either original detrital differences or diagenetic modification of the original assemblage. The Frio Formation was deposited as a prograding clastic wedge which should produce a coarsening-upward sequence. Examination of well logs shows that each sampled interval does become more sand rich upwards. It is difficult to reconcile this relation with a purely detrital origin for the increased quartz content with depth. It is therefore assumed that the relative decrease in total clay and three of deviation standard shows bar Error cuttings shale 27 MSW of content quartz and clay Total 2.10. Figure data and data this of averages are calculations balance mass in used values clay and Quartz analyses. XRD independant depth. with decreases content clay total average and increases content quartz shale Frio Average 2.12. Figure in three of deviation standard one show bars Error cuttings. shale 27 MSW of content calcite and Feldspar 2.11. Figure to common albite in gain and feldspar potassium of loss diagenetic the shows data The analyses. XRD independant shales. Coast Gulf of deviation from decreases standard 2.10) show Figure bars and Error data (this cuttings shales %. to shale 24 #1 Frio 20% 36 of from TST content of clay increases content average content quartz The quartz clay and analyses. average Total the 2.12 69%; independant to Figure three 75% of deviation standard one show bars Error cuttings. shale #1 36 TST of content calcite and Feldspar 2.13. Figure by altered greatly been has shales Frio of composition and content feldspar The analyses. XRD independant diagenesis. three increase in quartz content of the shales is at least partially due to diagenetic changes in the mineralogy of the shales. The Smectite to Illite Reaction The clay mineral weight percent data given in Appendice A is calculated for 2-glycol smectite I/S. 2-glycol smectite I/S is not a natural mineral, it is what is produced during sample preparation, and what is analyzed for during data collection and interpretation. In nature, the interlayer space in smectite is occupied by hydrated cations, not ethylene glycol. Glycol interlayers account for 24% of the mass of 2-glycol smectite; a rock that is 50% 2-glycol smectite and 50% illite in the laboratory, is, in nature, really composed of 43% smectite (w/o glycol) and 57% illite (50 g smectite plus glycol per 100 g laboratory sample * .76 (weight percent smectite silicate-layers in 2-glycol smectite) = 38 g smectite (w/o glycol). 38 g smectite + 50 g illite = 88 g (88 g is the "natural," glycol-less, weight of 100 g of glycolated, laboratory sample.) 43 weight percent (38 g smectite / 88 g sample) of the "natural" sample is smectite.). This same kind of correction must be made for mixed-layer I/S, the correction is obviously much larger for low %I I/S. The real weight of any amount of I/S in nature is the weight in the laboratory multiplied by the %I, plus the laboratory weight multiplied by the %S and by 76%. This correction is completely necessary when calculating moles at depth from "laboratory" samples. Figure 2.14 shows the natural molar abundances of the clay minerals in the <1 jam fraction of the samples from the San Marcos arch and Rio Grande embayment. These values were derived by dividing the glycol-corrected weight percent of each clay mineral in a sample by its molecular weight (Table 2.1). Equations ofthe regression lines in Figure 2.14 and the calculated average molar clay abundances are given in Table 2.2. Figure 2.2 shows that at 7,000 ft the I/S is ~20%1andat15,000fttheI/Sis~80%1. Noticethatthemolecularweightsfor both Faux I/S 20 and Faux I/S 80 are calculated without ethylene-glycol smectite For this interlayers. range of depths and I/S compositions the natural molar changes in clay mineral abundances in the <1 pm fraction are: Figure 2.14. Molar changes in clay mineral abundance. Units are 10'2 moles <1 fim fraction. I/S and chlorite increase, illite and kaolinite decrease. per 100 g 28 O(10)OH(2 O(10)OH(2)O(10)OH(8) O(10)OH(2) calculations. Al(.15)Si(3.85) Al(.39)Si(3.61) balance l)Si(2.89) Al(.6)Si(3.4) Al(l.l in Al(1.33)Fe(.34)Mg(.34) Al(1.51)Fe(.22)Mg(.25) used Fe(.2)Mg(.2)Al(1.62) formulas mass Na(.7)Ca(.3)Al(1.3)Si(2.7)0(8) Al(2)Si(2)0(5)0H(4) Al(1.77)Fe(3.27)Mg(.96) SiO(2)KAlSi(3)0(8) NaAlSi(3)0(8) K(.75)K(.l)Na(.35)K(.6)Na(.l) Mineral 20 80 I/S I/S 2.1. Quartz Kspar Plagioclase Albite Kaolinite Chlorite Table Illite Faux Faux = I/S 11.941+3.9043*10(-4) * depth (ft) Illite = 7.0653-2.6519* 10(-4) * depth (ft) Kaolinite = 9.7128-2.9172*10(-4) * depth (ft) Chlorite = 0.5267+1.749*10(-5) * depth (ft) IZ& 111. K«h?1. Chlpr. 7,000 ft. 14.674 5.209 7.671 0.649 15,000 ft. 17.797 3.087 5.337 0.789 A +3.123 -2.121 -2.334 +0.140 Table 2.2. Molar changes in 0.1780 I/S(80%I) + 0.0014 Chlorite. Rewriting equation #1 in terms of the gain of 1 mole of illite in I/S yields 2) 0.7233 Smectite + 0.1874 Discrete Illite + 0.2060 Kaolinite > 1 Illite + 0.0124 Chlorite. Comparison ofthe clay mineralogy of the 0.1228 I/S(80%I) + 0.0010 Chlorite + 0.0666 Quartz + 0.0267 Albite. Table 2.3: Comparison of <1 um and <2 um clay fraction from shale cuttings. Sample Size %I %ys % Illite % Chlor. % Kaol. Kenedy 10,515 <1 57 74 14 4 8 <2 55 76 13 5 6 Kenedy 14,265 <1 81 68 24 5 3 <2 83 65 23 7 5 MSW 27 9,090 <1 46 59 10 2 29 <2 45 60 12 4 24 MSW 27 9,936 <1 75 58 7 1 35 <2 73 55 8 3 34 MSW 27 16,531 <1 72 70 12 7 11 <2 75 68 10 9 13 MSW 27 17,182 <1 71 56 17 7 20 <2 75 59 13 8 20 TST 36/1 8,810 <1 45 58 13 2 27 <2 49 62 12 4 22 Mustang 13,376 <1 79 77 7 4 12 <2 83 75 8 5 12 32 37 20 24 Average Ouartz Plaeioclase % % removal. Clav Kspar 75 69 carbonate Total 2 0 % % after to #1 1825 36 100% Ouartz Plagioclase % TST % normalized 36 Clav Arco Kspar 77 69 and 30 Total % analyses. 2.10-2.13 % 2123 37 XRD figures Ouartz Plagioclase in rock %% data Clav whole from #27 Kspar MSW 7470 20Total % Shale % calculated ft ft ft ft 2.4. Table Values 7,000 15,000 7,000 15,000 Rewriting equation #3 in terms of the gain of 1 mole of illite in I/S yields 4) 0.8320 Smectite + 0.2087 Discrete Illite + 0.2297 Kaolinite + 0.0945 Potassium Feldspar + 0.1483 Plagioclase > 1 Illite + 0.0131 Chlorite + 0.8740 Quartz + 0.3504 Albite. Hower et al. (1976) proposed that the smectite to illite reaction is a simple transformation which can be written as: 5) Smectite + A1+3 +K+ -—> Illite+ Si+4 Land et al. (1987), ignoring cations other than Al, Si, K, Mg, and H, rewrote this "closed system" equation as sa) Smectite + 0.15 Potassium Feldspar + 0.12 H2O + 0.01 H+ > 0.70 Illite + 0.11 Chlorite + 1.47 Sio2 + 0.005 Mg+2 Boles and Franks (1979) proposed that aluminum is conserved in the mixed-layer phase over the course of the reaction so that the relative changes are: 6) Smectite+0.25K++0.64H2O >Illite+0.1Na+ + 0.07 Ca+2 + 0.27 Mg+2 + 0.3 Fe+3 + 1.57 Sio2 + 3.63 O'2 + 7.26 OH' + 1.07 H+ as a means of Land et al. (1987) rewrote this equation, eliminating the use of O'­achieving charge balance, as 6a) Smectite + 0.19 K+ + 0.542 H+ + > 0.78 Illite + 1.07 SiCb + 0.366 Mg+2 + 0.5 H2O The mineralogic changes determined in this study do not support either of those mechanisms and in fact are similar to the illite neoformation reaction proposed by Lynch (1985) for contact metamorphic shales: 7) 0.5 Smectite + 0.5 Kspar + 0.33 mica + 0.3 Na+ > 1 Illite + 0.3 Albite + (cations + H2O) A neoformation-type smectite to illite reaction has also been proposed by Awwiller (1992, 1993)for Wilcox shale diagenesis: 8) 0.87 Smectite + 0.07 Kspar + 0.11 mica + 0.07 Na2o + 0.11 K2O > 1 Illite + 0.67 Quartz + 0.01 Chlorite + 0.14 Albite + (cations + water). Hower Revisited: I/S Chemistry The same way "all roads lead to Rome", all discussion of I/S chemistry leads to Hower et al. (1976). Almost 20 years after its publication, the mineralogic and chemical data contained therein is still the yardstick against It which all investigations on shale diagenesis (including this one) are compared. is the only source of wet chemical analyses of Frio I/S. The "Hower transformation mechanism" for the conversion of smectite to illite is still probably the most accepted form of the reaction. An alternative smectite cannibalization mechanism proposed by Boles and Franks (1979) is derived from recalculation of Hower's data. The concept that shales act as closed chemical systems comes from the Hower et al. (1976) paper as well. (Indeed, as a personal aside, my own entranceintothefieldofclaymineralogywaspartofReynolds'desireto seeifwe could "prove if John's right or wrong.") Table 2.5 is the chemistry data of the 2,K2O,A1203,andNa2obesuppliedtothedeepershale. Thesesame mineralogic changes result in an excess of CaO, MgO, Fe2C>3, and H2O. Milliken et al. (1991) report that there is a significantK2O increase in many Frio Formation shales with depth. The calculated shale mineralogy (Table 2.10) supports those chemical findings. In the deeper shales the major K2O reservoir is high %I I/S. In the shallow shales the K2O reservoirs are potassium feldspar, detrital illite, and, because of its large abundance, low %I I/S. The mass balance calculation (Table 2.11) shows that the calculated loss of 2% potassium feldspar and 7% detrital illite from the shallow samples can only provide -40% of the K2O needed to form the 80%1 I/S in the deeper shale. Table 2.12 shows mass In none balance calculations assuming different (and feasible) shale mineralogies. of these cases is a K2O mass balance achieved. Where can this extra K2O come from? During burial Frio sandstones lose between 2 and 3% K2O, due primarily to dissolution of detrital potassium feldspar (Milliken et al., 1994). Considering the low sandstone/shale ratio of the formation, the loss of 3% K2O from Frio sandstones can only provide about -25% oftheK2Odeficitintheshales. Awwilier(1993)hasshownthatasimilarK2O deficit is present in Wilcox shales so it is unlikely that the Tertiary section is the source ofthe extra K2O in Frio shales. Mesozoic evaporites are common in the Gulf, but K-bittem salts are noticeably rare. Weaver and Beck (1971) proposed that granitic basement is the source of the additional K required for smectite illitization in shales. Hower et al. (1976), Boles and Franks (1979), Mcßride (1989), and many others have proposed that reactions in shales are the source of quartz cement in associated sandstones, yet this mass balance calculation indicates that SiC>2 must be imported to the shales. Whatif the relative increase in the quartz is 2% not 4%? That difference would change the shale system from a SiC>2 importer to a ft 0079480 24 47 wt. 15,000 % ft % 20230 153 15 42 0 7,000 wt. 20 80 I/S I/S Quartz Kspar Plagioclase Albite Kaolinite Chlorite Illite Faux Faux 49 _­ sum 97.91 97.74 H20 calculated 2.95 2.93 % 9. Fe203 -Mineralogy 4.68 3.93 6.65 5.89 table % and MeO data 2.11 1.71 2.96 2.90 % this calculation CaO - _ 1.26 0.34 from % balance 0.83 1.01 1.37 1.65 Na20 mass calculated % comparison. K20 for 2.19 4.10 3.52 5.60 for chemistry % mineralogy chemistry Shale A1203 20.16 19.64 19.74 18.53 % 2.4. silicate and #2) 2.1 Si02 2.2 63.72 64.08 64.64 62.95 % Complete tables (Figure ft ft shale ft ft 2.10. in 7,000 15,000 8,146 14,715 data Table from Kenedy Kenedy Kenedy H20 -3.500 -0.400 -1.590 -11.000 12.280 -4.210 moles 10(-2) Small, Fe203 -0.164 -0.159 -1.870 1.351 -0.842 are deficit. MeO -0.096 -0.318 -3.740 3.070 -1.084 Units a CaO -0.336 -1.925 0.614 -1.647 indicate moles Na20 -0.392 1.335 -0.963 0.307 0.288 values exporter. 10(-2) net Sio2 a K20 -0.360 -0.596 -0.550 3.684 2.178 mineralogy. to positive shale A1203 -0.360 -0.728 1.335 -1.750 -0.144 -1.765 -8.140 11.666 0.114 shale in the excess, an Si02 6.660 -2.160 -3.024 8.010 -3.500 -0.289 -5.406 2.272 -42.350 44.331 changes changes indicate for content moles A moles values I/S 6.660 -0.720 -1.120 2.670 -1.750 -0.100 -1.590 -11.000 12.280 calculation net net or quartz balance Negative % in A 4 -2 -37 -61 -7 wt -42 47 Mass shale. changes 20 80 2.11. of I/S I/S Mineral Quartz Kspar Plagioclase Albite Kaol Chlorite Illite 100greasonable, Faux Faux Table per 50 mass 14%; a H20 2.95 2.93 2.90 2.93 3.08 2.93 2.63 2.80 2.94 2.93 2.97 2.96 from to achieve doubled illite increased Fe203 4.68 3.93 4.61 3.93 4.66 3.93 4.25 3.80 4.67 3.92 2.96 2.02 values discrete * of dissolution shales adjustments MgO 2.11 1.71 2.07 1.71 2.09 1.71 1.87 1.63 2.10 1.71 2.13 1.73 both these illite content in do results. I/S CaO 1.26 0.34 1.23 0.34 1.13 0.34 1.32 0.31 1.26 0.34 1.27 0.34 K2O of Discrete Never test D. compositions sensitivity B. composition Na20 0.83 1.01 0.82 1.01 0.76 1.01 1.01 1.36 0.83 1.01 0.84 1.02 5%; shales; mole. .1 both by Shale to K20 2.19 4.10 2.69 4.10 2.84 4.10 2.29 3.88 2.43 4.22 2.44 4.28 for A 1203 doubled E. calculation decreased 65% A1203 20.16 19.64 20.12 19.64 20.69 19.64 18.54 19.49 20.09 19.60 21.33 21.01 set moles; dissolution chemistry to compostion to Si02 63.72 64.08 63.60 64.08 62.93 64.08 66.42 65.49 63.59 64.01 63.97 64.40 Kspar proportions .45 Shale A. .375 Fe2o3 K2O. clay from ft. ft. ft. ft. ft. ft. ft. ft. ft. ft. ft. ft. for 2.12. 2.10. mole, Total .1 7,000 15,000 7,000 15,000 7,000 15,000 7,000 15,000 7,000 15,000 7,000 15,000 Table Table changed balance C. by * AB CDE Test 51 Sio2 exporter, and, assuming a rock density of 2.5 g/cc and a sandstone/shale ratio of 1:8, approximately 2 volume % quartz cement could be provided to each 100 g of sandstone (0.01059 m SiC>2 / 100 g shale (m Sio2 from Table 11, modified for only 2% SiC>2 increase in shales) X 60.066 g/m = 0.636 g SiO2/100 g shale exported to sandstones. 0.636 g SiC>2 / 100 g shale X 8 (shale/sandstone ratio) = = 5.09 g SiC>2 / 100 g sandstone. 5.09 g SiC>2 / 2.5 g/cm3 2 volume % SiC>2 / 100 g sandstone). The average quartz cement content of sandstones in the Frio The Formation is approximately 3% (Land and Fisher, 1987; this study). analytical techniques used in this study are not sufficiently accurate or precise enough to differentiate between 20% and 24% quartz and therefore no definite statement can be made about shales as Si(>2 suppliers to sandstones. These calculations show, however, that shales can be SiC>2 exporters without a smectite cannibalization I/S reaction taking place. In fact, I/S can be neoformed and the shales can still provide SiC>2 to the sandstones. These mass balance calculations based totally on XRD analyses, and admittedly an approximation due to unknown clay mineral chemistries and imprecise quantitative mineralogies, strongly suggest that shales in the Frio Formation behave as open systems in regards to K2O, SiC>2, and possibly other elements, as has been proposed by Weaver and Beck (1971), Boles and Franks (1979), Howard (1979), and Moncure et al. (1984). Discussion X-ray diffraction analysis is the best way to investigate the nature and diagenesis of clay minerals, but unless great care is taken in sample preparation and data interpretation the results can be misleading. Earlier studies of shale diageneis suffered from a lack of understanding of the potential errors. The results of this study are in direct contradiction with the often referenced smectite to illite conversion mechanisms proposed by Hower et al. (1976) and Boles and Franks (1979). An aluminum conservative reaction requires great changes in the abundances of I/S and quartz in the shale which are not seen in this study. If SiC>2 is being generated in the manner suggested by Boles and Franks (1979) and exported from the shales, the volume ofSiC>2 would be sufficient to fill almost all the porosity in associated Frio sandstones. Eberl (1993) uses the mineralogy and chemistry from Hower et al. (1976) three along with the K-Ar analyses of Aronson et al. (1976) to propose mechanismsofilliteformation,includinganOstwaldripening step,inFrioshales. a correction for K-Ar dates from Aronson et al. (1976) are partially a function of kaolinite content. The correction is derived from the mineralogic data presented in Hower et al. (1976) which we know is not completely correct. The data are even self-contradictory in that the amount of discrete illite contamination in the 2 to sandstones without an aluminum conservative, smectite cannibalization illitization reaction mechanism operating. 8. The mineralogy and chemistry of Miocene shales is different from the mineralogy and chemistry of Frio shales. Chapter 3 INTRODUCTION AND PREVIOUS WORK Geologic Setting andSedimentology General Geology The Oligocene Frio Formation was deposited along the northwest margin of the Gulf of Mexico sedimentary basin. It is underlain by the Oligocene Vicksburg Formation and overlain by the Oligocene/Miocene Anahuac Formation Deposition of the Frio Formation began 33.5-31.5 m.y.b.p. and continued until approximately 25 m.y.b.p (Galloway, 1986 b). It is one of the major Cenozoic to increased sediment clastic wedges that prograded out into the Gulf in response influx derived from the Cretaceous and Tertiary volcanic terrain in West Texas which later increased the flux of and Mexico and from the Laramide orogeny, sediment into the Mississippi embayment. This influx of sediment prograded the continental shelf edge more than 350 km basinward from the Cretaceous margin (Martin, 1978; Galloway, 1989). The Cenozoic section is composed ofrepetitive sequences of sandy coastal-plain (>30% sand) and marginal-marine (5-30% sand) units which thicken and grade gulfward into fine-grained marine shelf and slope deposits (<5% sand) (Galloway, 1989; Sharp et al., 1988). Figure 3.1 is a dip oriented cross section through the Cenozoic section of the Gulf which shows how successively younger units lie progressively gulfward of the Cretaceous shelf margin. The sand-rich progradational units are stratigraphically separated by shales that represent marine transgressions. The thickness of the Cenozoic section is greater than 5 km (Sharp et al., 1988). Maximum progradation of each Cenozoic depositional unit took place at one or more depocenters where major deltas focused sediment input and where, in many cases, detritus was deposited directly at the shelf edge. Depocenters are located in subtle broad basement sags called "embayments." The Frio Formation is thickest in the Rio Grande embayment in the south Texas gulfand the Houston basin, Coast Gulf deposition, Cenozoic (1982). of et Styleal. 3.1. Bebout Figure From embayment in the north Texas gulf. The San Marcos arch, a broad basement high and area of lessened sedimentation and subsidence, but with no evidence of uplift, lies between the depocenters (Martin, 1978; Sharp et al., 1988; Galloway, 1989). The Frio Formation has been divided into several related deposition systems. The Houston delta and Buna strand-plain, located in the Houston embayment, and the Norias delta located in the Rio Grande embayment, are separated by the Greta/Carancahua barrier/strandplain system located on the San Marcos arch (Galloway, 1982). Geology of the Frio Formation in the Corpus Christi area Stacked, aggradational, barrier and strandplain shorezone deposits are the most common sandstone facies in the Frio Formation on the San Marcos arch. Updip the shoreface sandstones grade into back-barrier lagoon and fluvial units. Basin ward the shorezone sandstones grade into shelf sandstones and shales (Galloway et al., 1982; Galloway, 1986). In San Patricio, Nueces, and Aransas counties the depth to the top of the Frio increases basinward from 3,000 ft to The thickness of the Frio Formation also increases approximately 8,000 ft. basinward from 2,000 ft to >B,OOO ft (Bebout et al., 1978). Figure 3.2 and Table 3.1 identify the sandstone cores used in this project. * Figure 3.2 also shows the distribution of growth faults in the Frio Formation in the study area. Displacement on the fault between block 1 and block 2 is over 2,000 ft. Major fault blocks are further divided by secondary growth faults (with displacements of a few hundred ft) into sections as small as 0.5 sq. mi. (Morton et al., 1981). There is a five-fold expansion of the section at the block 1/block 2 growth fault. The expanded section consists largely of shelf sandstones and shales (fault block 2). Barrier/strandplain shorezone facies occur at the NW limit of the study area (fault block 1). Diapiric shale ridges exist below fault block 2 and fault block 3 (Galloway and Morton, 1988). 61 here. shown cores identifies Table locations. sample core Sandstone 3.2. Figure Table 3.1 Sandstone core sample locations. Field ID Field Name Countv Cores A Portilla San Patricio Minnie S. Welder #67 Minnie S. Welder #70 B Wildcat Nueces Wainco Mutchler #1 C Wildcat Nueces Cecelia Kelley #2 D Fulton Beach SW Aransas Copano TST 104 #7 E • Corpus Channel Nueces Cities TST 4 #2 Cities TST 10 #2 F Encinal Channel Nueces Citites TST 49-2 Cities TST 51-1 G Wildcat Nueces Arco TST 470 #4 H Red Fish Bay Nueces Shell TST 346 #1 Shell TST 392 #4 I Wildcat Nueces Arco TST 430 #5 63 Depositional Facies Four facies assemblages can be identified in the sandstone cores from the Corpus Christi area. The shorezone sandstone facies is composed of shoreface and back-barrier environments; the shelf depositional facies consists of the shelf siltstone facies, the shelf sandstone facies, and the distal-shoreface/inner-shelf sandstone facies. Shorezone Sandstone Facies The genetic shorezone facies includes the shoreface environments (the barrier core and beach ridge) and the "back-barrier" environments, such as crosscutting channels, flood and ebb tidal deltas, and associated fluvial, lagoonal, and marsh deposits. Middle and upper shoreface sandstones represent high energy environments in the shorezone facies assemblage. Lower back-barrier and energy lower shoreface environments are characterized by less mature sandstones. Heterogeneity in porosity and permeability in shorezone sandstone reservoirs is due to the complex facies architecture produced by the vertical juxtaposition of sandstones from different depositional environments (Galloway and Hobday, 1983; Galloway, 1986 a; Tyler and Ambrose, 1986). Frio shorezone sandstones in the Corpus Christi area are prolific oil reservoirs (Galloway, 1983, 1986 a). Figure 3.3 shows examples of electric logs from shorezone sandstones. ShelfDepositional Facies The shale:sandstone ratio (which, in a very approximate way, can be thought ofas the shelf:shorezone ratio) in the Frio Formation is greater than 8:1 (Galloway et al., 1982). Thick shorezone and high-energy shelf sandstone packages are surrounded by lower-energy, clay-rich siltstones, very-fine sandstones, and shales deposited in the prodelta and shelf environments. The three shelf facies, the shelf siltstone facies, the shelf sandstone facies, and the distal-shoreface/inner-shelf sandstone facies are largely gradational (in terms of sandstone content and energy ofenvironment) with one another. Residence in the hydrocarbon generation window, high fluid-pressures, and close association with source mudstones (?) make Frio shelf sandstones (so- called "distal Frio plays") potential hydrocarbon reservoirs (Galloway et al., 1983; Galloway, 1986; Hamlin, 1989). By 1986, 1.8 billion boe had been produced fromthedistalFrioFormation,anamountthatisapproximately 11%ofthe production from the entire Frio (Hamlin, 1989). In fact, well completion techniques developed in the early 1990's are allowing significant production from deep distal sandstones, including some from the Red Fish Bay field used in this study (L. R. Billingham pers. comm., 1993). - The sandstone and siltstone faciesFrio Shelf Depositional Processes developed on the Frio shelf were produced by the mixture of long periods of little or no deposition and extensive biological reworking with short periods of high-energy redistribution of sand from the shorezone prism to the shelf. The depositional processes operating on the Frio shelf are analogous to those of the Holocene Texas gulfand Atlantic shelves (Morton, 1981; Swift et al., 1986a; Gaynor and Swift; 1988). During storms, sand is eroded from the shoreface and transported to the shelf by storm-induced, bottom return-flow currents. Each storm also reworks a certain thickness of preexisting shelf sediment as well. Storm-induced geostrophic The currents redistribute much of this material into strike-elongate belts of sand. thickness of an individual storm bed is a function of the size of the storm and how much shoreface and shelf erosion it can produce. In more distal environments individual event beds are less than 1 cm to ~10 cm thick. In the proximal inner­ shoreface environment giant long return-frequency storms can produce event beds up to 1 m thick (Thome et al., 1991). The number and thickness of discrete sandy storm beds decreases basinward (Morton, 1981). Because the shelf is, by definition, below fair-weather wave base, extensive biological reworking of shelf storm deposits ("tempestites") can thoroughly homogenize the section and produce a heavily bioturbated, massive, clay rich sandstone or siltstone. The trace fossil assemblage characterizing shelf facies sandstones includes Skolithos, Teichichnus, Thalassinoides, Planolites, Ophiomorpha, Rhizocorallium, and Diplocriterion, and indicates neritic water depths (Seilacher, 1978; Galloway and Morton, 1988). During times of decreased water depth or increased accumulation rate, deposition of shelf tempestites can keep pace with bioturbation, and thicker, stacked, relatively clean sandstone packages can be developed (Thome et al., 1991). Some of the more proximal Frio shelf tempestite sand packages are on the order of 100 ft thick. Distal-shoreface/inner-shelf sandstone facies Most of the sandstone in the - study area belongs to the distal-shoreface/inner-shelf sandstone facies and is composed of stacked and amalgamated storm beds. This facies commonly forms upwards-coarsening/upwards-fining sandstone packages surrounded and interbedded with shelf siltstone facies rocks. Figure 3.4 is an electric log showing a number of these upwards-coarsening/upwards-fining distal-shoreface/inner-shelf sandstone packages. Figure 3.5 is the core log of a distal-shoreface/inner-shelf sandstone. Sandstones from this facies are best developed as shore-parallel belts on the downthrown sides of growth faults. Distal-shoreface/inner-shelf sandstones typically have sharp, sometimes erosional, basal contacts. Siltstone and mudstone interclasts are common. Many of the sandstones are texturally upward-fining, both through a decrease in sand grain size and by an increase in bioturbation and clay content. Low angle and planar laminations are the most common sedimentary structure, though hummocky cross-stratification, ripples, and soft-sediment structures are locally present. Preserved laminations are much more common in the basal parts of these sandstones; completely massive sandstones are not uncommon. Bed thickness from a few cm to 5 ft. The thickness of individual sandstones and ranges ~ sandstone packages decreases basinward. At the time of deposition, individual tempestites are physically attached to their shoreface source. However, this connection is quickly severed by biological and physical reworking by later storms, and by penecontemporaneous movement of growth faults. Shelf Siltstone Facies This facies is composed of siltstones, very fine­ - grained sandstones, and silty shales. Thin (a few cm) laminated sandstones, Figure 3.3. SP logs from TST 49-2 and MSW 70. Thick, stacked, shoreface and back-barrier environments dominate both logs. Thin, isolated, distal-shoreface/inner-shelf sandstones are present below 8350 ft in TST 49 #2. Figure 3.4. SP logs from TST 49-2 and TST 346-1. TST 49-2 shows series of distal-shoreface/inner-shelf sandstones. TST 346-1 (normal and expanded scale) shows shelf siltstone facies. 68 IV Figure 3.5. "Weathered outcrop" profile and sedimentary structures for TST 392-4. Symbols: "V =bioturbation, =lamination, =fming-upwards, =coarsening-upwards. Numbers are thin section identifiers. core 69 representing distal storm beds, are contained within the heavily bioturbated clay-rich component of this facies. Discontinuous, partially bioturbated sandstone remnants and clay-poor burrows are common. Galloway and Morton's (1988) description of this facies in core as "monotonous" and "nondescript" fails to do justice to the dismal nature of these important rocks. These sandstones, siltstones, and shales are the "shale baseline" of electric logs (at least in the more proximal parts of the basin). This facies constitutes the great preponderance ofFrio clastic rocks on the shelf. - Shelf Sandstone Facies The shelf sandstone facies is intermediate between the common shelf siltstone facies and the distal-shoreface/inner-shelf sandstone facies. It is composed of very-fine grained, laminated sandstones surrounded by heavily bioturbated siltstones and clay-rich sandstones, and was deposited basinward and marginal to the distal-shoreface/inner-shelf sandstone facies. The sedimentary structures in these sandstones are the identical to those in the distal-shelf/inner-shelf sandstones. Differentiation of the three shelf facies is subjective. Classifying thick, stacked, sandstone-rich core such as TST 49-2 8975 (Figure 3.5) as distal­ shoreface/inner-shelf facies, or sandstone-poor, clay-rich core such as TST 10-2 Classification (Figure 3.6) as shelf siltstone facies is simple and straightforward. becomes more problematic for core such as TST 49-2 8521 and 8710 (Figure 3.7) or TST 346 #1 (Figure 3.8), in which there are similar amounts of preserved tempestite sandstones and heavily bioturbated siltstones. Utilization of e-logs and consideration of a core's spatial position within the study area assisted in the facies classification. Frio Sandstone Composition and Diagenesis Detrital Mineralogy Loucks and others (1986) showed how the detrital mineralogy of Frio Formationsandstonesisregionallyvariable astheresultofprovenance differences. The source area for the Norias delta was the volcanic terrain of West Figure 3.6. "Weathered outcrop" profile and sedimentary structures for core TST 10-2. Symbols: *** =lamination, =fining-upwards, =coarsening-upwards. Numbers are thin section identifiers. Figure 3.7. "Weathered outcrop" profile and sedimentary structures for core TST 49-2 8521 ft and 8710 ft. Symbols: "v~ =bioturbation, y~35%) are from sandstones that were cemented with early syndepositional calcite. Figure 4.4. QFL composition of fluvial Frio sandstones. From Grigsby and Kerr (1991). These nearby sandstones are noticably quartz poor compared to marine sandstones from this study (Figure 4.1), from both detrital and diagenetic processes. 82 Figure 4.5. Detrital and authigenic clay content ofsandstones. • indicates shorezone sandstone, o indicates shelf sandstone. Shelf sandstones and some low shorezone sandstones energy contain very large, and variable, amounts of detrital and Kaolinite and recrystallized (predominately I/S) matrix clay. chlorite are almost completely diagenetic in origin. Chlorite is common in the finer grained sandstones in each facies. more Absence of kaolinite in deeper sandstones due to diagenesis. 83 It is important to remember that only matrix-poor (<5%) sandstones were used by Loucks et al. (1986) in their study of Cenozoic sandstone reservoir quality. Figure 4.5 shows that applying that cutoff to this data would eliminate almost all of the shelf sandstones and a significant number ofthe shorezone sandstones. QFL values for sandstones from this study differ from those reported for the Middle Texas Gulf coast in Loucks et al. (1986). The restrictions put on the Loucks et al. (1986) data base by the 5% matrix cutoff probably eliminated sandstones from many low-energy environments from their study and may be partially responsible for this difference. The similarity in QFL and rock fragment compositions between this study and South Texas gulf sandstones supports the conclusions of Duncan (1986) and Loucks et al. (1986) that northward longshore transport from the Nonas delta was the source of most of the sediment in the Corpus Christi area. Diagenesis determined from this Figure 4.6 shows the general paragenetic sequence study. It is similar to the diagenetic sequence of events derived by Lindquist (1977), Loucks et al. (1981), Loucks et al. (1986), and Sullivan (1988) for Frio sandstones. The average diagenetic mineralogy by facies is shown in Table 4.3. A general discussion of the volumetrically important diagenetic minerals and events is presented in this section. More detailed discussions of the occurrence and significance of the diagenetic modifications of the sandstones are presented in the core descriptions section and in Chapters 5 and 6. Compaction Shorezone sandstones similar to the Frio have >40% depositional porosity (Atkins and Mcßride, 1991). Frio shelf sandstone depositional porosity was probably similar. Figure 4.7 shows the PCP distribution with depth for shelf sandstones with >5% calcite cement. There is no evidence of significant proximal compaction, and of shoreface in exception the only With occurred events sandstones. diagenetic Formation Frio for "syndepositional" events of reactions, sequence mineral clay paragenetic the General and dissolution, sandstones. 4.6. Figure minor shelf OCX} 1.5 1.6 4.7 5.8 3.2 4.6 9.2 catagory, (%). Total Calcite 11.0 17.3 7.7 10.3 14.1 each deviation 1.3 2.3 0.9 2.0 1.1 2.2 For Kaolinite standard 3.2 4.0 4.4 4.6 3.8 4.4 Chlorite one 14.0 12.7 11.3 facies. is Detrital Clav 4.4 6.2 9.5 number by Size cD 2.3 0.3 3.3 0.2 2.8 0.6 porosity size Grain and PCP 21.7 7.4 21.3 9.1 21.5 8.3 lower/smaller 13.6 8.4 9.1 8.0 11.2 8.5 mineralogy (%), Total Porositv average 2.7 1.7 1.7 1.7 2.2 2.4 is diagenetic Secondary Porositv number 10.9 7.5 6.8 7.3 8.7 7.6 Average Primary Porositv size 4.3: Facies Shelf Both Table upper/larger Shorezone Figure 4.7: PCP vs. depth for calcite cemented shelf sandstones. The large range in PCP at any depth is due to variable martix content. The data imply that significant compaction ceased at < 8,000 ft. 87 compaction after 8,500 ft. The large range of PCP's is due to variable amounts of matrix. I/S (matrix) Clay matrix is either an original constituent of a sandstone (concentrated as laminations or as clay rip-up clasts) or is introduced soon after deposition by bioturbation or infiltration (Figures 4.9 4.11). Compaction and recrystallization - of this material are two of the earliest, and most important, diagenetic processes operating in sandstones. Pseudomatrix is formed when ductile clay clasts are deformed between more rigid grains during compaction (Figures 4.12, 4.13). I/S is the most abundant component of depositional matrix. The mineralogy of matrix clay is discussed in Chapter 6. Figure 4.5 shows the matrix distribution with depth; the abundance of matrix clay is very facies dependent. Figure 4.14 shows recrystallizing matrix I/S in the intergranular space between quartz grains. Reactions involving matrix clays (which include compaction, smectite conversion to illite, precipitation of chlorite, organic matter diagenesis) begin in the earliest diagenetic environments and The continue throughout burial. presence ofreacting matrix clays greatly reduces the permeability of a sandstone, in fact, the distribution of recrystallizing detrital matrix clays has been identified as a/the major control on reservoir quality in sandstones from diverse environments and ages (Taylor, 1978; Colter and Ebbem, 1978; Winn and Bishop, 1983; Mcßride et al., 1987; Mcßride et al., 1988; Gardiner et al., 1990; Scotchman and Jones, 1990; Houseknecht and Ross, 1992; Howard, 1992). Figure 4.15 shows that matrix clay content affects the distribution of diagenetic QOG's, calcite, and kaolinite. None of these phases are present in abundance in sandstones with > ~15% matrix. Figure 4.15 also shows that significant amounts of diagenetic chlorite can be associated with abundant I/S matrix. This supports the shale mineralogy and mass balance conclusions (Chapter 2) and the clay mineral textural evidence (Chapter 6) that chlorite is being produced at the expense of smectite. Figure 4.8. Diagenetic mottles. Bizarre mottled texture is due to multiple epidsodes of calcite precipitation and dissolution in upper-shoreface shorezone sandstones. Figure 4.9. Shelf Sandstones. A. Distal-shoreface/inner-shelf sandstone. Vicksburg-like green stripe is due to the presence of authigenic chlorite and albite. See Chapter 6. B. Laminated and bioturbated shelf sandstone. The average detrital clay content of shelf sandstones is 14%. 90 Figure 4.10. More Shelf Sandstones and Siltstones. A. Fining-upwards, laminated and bioturbated, shelf sandstone. B. Bioturbatedremnant of a distal, laminated, tempestite sandstone surrounded by bioturbated shelf siltstone. Burrows and laminated sandstone remnants are usually completely cemented, due to preferential fluid flow. See text. 91 Figure 4.11. Lamination-exclusive diagenesis. A. Chlorite rosettes formed in matrix-poor lamination. B. All porosity filled by matrix and chlorite, QOGs or calcite. Heterogeneous diagenesis is due to variable permeability differences at the lamination scale. See Figures 4.34, 4.40, and 4.41. Figure 4.12. Secondary Porosity. Secondary porosity in Frio sandstones averages only 2.4%. A.Skeletal,feldsparerosionalremnantsandleachedrockfragments(surroundedby kaolinite) are evidence of dissolution. B. A yellow-stained potassium feldspar overgrowth surroundsapartially dissolvedfeldspar grain. Figure 4.13. Quartz Overgrowths. The average QOG content of Frio sandstones is 3.2%. Shelf sandstones average ~5% QOGs and can contain >20%. All the IGV in these two shelf sandstones is filled with QOGs and brown/black recrystallizing I/S matrix (and authigenic chlorite which has formed at the expense of the detrital clay). Figure 4.14. Recrystallizing detrital I/S matrix clay. MSW #7O, 7325 ft. Recrystallizing, originally detrital I/S clay, exhibiting both sheet and cornflake textures, fills the intergranular space between framework grains. Chlorite rosettes and diagenetic quartz are intimately associated with the recrystallizing I/S. Figure 4.15. The effects of detrital matrix on the abundance of diagenetic phases in sandstones. • indicates shorezone sandstone, o indicates shelf sandstone. Detrital clay content >~15% adversly effects fluid flow and the precipitation of QOGs, calcite, and kaolinite. Significant chlorite can be associated with abundant matrix. 96 Chlorite Figure 4.5 and Table 4.3 show that chlorite is a common diagenetic mineral in both the shorezone and shelf facies sandstones. Chlorite is present as discrete rosettes (Figures 4.8, 4.11, 4.16, 4.17), as an intimate mixture with recrystallizing matrix I/S (Figure 4.14), and as radial rims (Figure 4.18). The presence of chlorite grain rims inhibits the later precipitation of QOGs in the Frio (Sullivan, 1988; this study) and many other sandstones (Heald and Larese, 1974;Hastings, 1990;Winnetal., 1990;Warrenetal., 1990;Pittmanet al., 1992;Winn et al., 1993; Ehrenberg, 1993). In Frio sandstones the presence of chlorite also can inhibit calcite formation, though Figure 4.16 clearly shows that this is not always the case. In some sandstones the distribution of chlorite is related to depositional environment (Langford and Lynch, 1990; Thomson and Stancliffe, 1990; Longstaffe and Ayalon, 1991; Crossey and Larsen, 1992). In the Frio sandstones from the Corpus Christi area chlorite is common in all facies; shoreface and back-barrier (where Figure 4.5 shows it is more common in finer- grained rocks), and shelf. Diagenetic chlorite and its relationship to detrital matrix is further discussed in Chapter 6. Calcite Calcite is the most abundant diagenetic mineral in these Frio sandstones, averaging 9% by volume (Table 4.3, Figure 4.19). There are two episodes of calcite precipitation (Figure 4.6). Calcite is one of the first diagenetic minerals to form; high preserved PCP's (Figure 4.20), floating grain textures, micritic crystal size, association with ash-shard textures (Figure 4.21), isopachous rimming textures, association with soil textures (caliche clasts, root casts, desiccation cracks), nodular texture in core (Figure 4.8), and its location in back-barrier and upper-shoreface/foreshore facies sandstones indicates that this calcite precipitated soon after deposition (c.f. Matthews, 1971; Bathurst, 1975; Longman, 1980; Machette, 1985; Blodgett, 1988). Syndepositional calcite in sandstones from Figure 4.16. Burial Diagenetic Calcite. Burial diagenetic calcite sometimes stains one color in thin section (A, with inclusions of pre-existing chlorite), and sometimes stains in various shades of pink and purple (B). Microprobe analyses indicate that in most of these sandstones there is no regular association of stain color and calcite chemistry. (Chapter 5) 98 Figure 4.17. Calcite, Quartz, and Chlorite. A. Purple-stained, erosional remnants of burial diagenetic calcite. Late dissolution of earlier IGV-filling calcite cement is never more abundant than 1 or 2 %. B. The times of euhedral QOG and chlorite rosette precipitation commonly overlap. Figure 4.18. I/S. A. Recrystallizing I/S (-75% I) fills most of the IGV in this grey, distal-shoreface/inner-shelf sandstone. This is the host-rock for the unusual, authigenic, Vicksburg-like green stripes (see Figures 4.23 and 6.8 and Chapter 6). B. Recrystallizing matrix is composed of thin sheets and bulbous protrusions (see Figure 6.5 and Chapter 6). Figure 4.19. Diagenetic QOG and Calcite content of Frio sandstones. • indicates shorezone sandstone, ° indicates shelf sandstone. TheQOG contentofFrio sandstonesisveryfaciesdependant, theincrease inQOG content with depth is due to the presence of deeply buried, QOG-prone distal-shelf sandstones in the study area. Abundant syndepositional calcite is present is shore-zone sandstones. There is little relationship between grain-size and calcite content; QOGs are more abundant in coarser-grained sandstones in both facies. 101 102 sandstone shelf indicates o sandstone, shorezone indicates • PCP. vs. content Calcite and QOG 4.20. Figure some in abundant is calcite Early sandstones. shelf in space intergranular the of proportion large sandstones. a fill QOGs shorezone Figure 4.21. Syndepositional Diagenesis. Some shoreface sandstones show evidence of early, syndepositional, episodes of calcite and kaolinite precipitation. A. Dissolution of volcanic glass shards at the center of an early micritic calcite concretion. B. Kaolinite surrounds erosional remnants of pink-stained, syndepositioanl calcite. Minnie S. Welder #67 and Minnie S. Welder #7O has undergone extensive dissolution (Figure 4.21). The relationship between depositional facies and early calcite cement has been observed in many different sandstones (Blanche and Whitaker, 1978; Glassman et al., 1989; Howard and Whitaker, 1990; Barnes et al. 1992; Mancini et al., 1990; Cowan and Shaw, 1991). Burial diagenetic calcite precipitates relatively late in the paragenetic sequence of the sandstones (Figure 4.6). In shorezone sandstones burial diagenetic calcite is most common in back-barrier facies rocks, where it often precipitates on or around erosional remnants of syndepositional calcite. In both shorezone and shelf sandstones, burial diagenetic calcite distribution can range from patchy to completely filling all available intergranular space. Most samples of burial diagenetic calcite show some evidence of dissolution, though never abundant than 1 demonstrably more or 2% (Figures 4.16 and 4.17). There is no relationship between grain size and calcite content within separate facies (Figure 4.19). The calcite content of shorezone sandstones is higher than shelf sandstones, but this comparison is complicated by the presence of syndepositional calcite in the proximal rocks. In the Frio and many other sandstones,theoccurrenceand distributionofcalcitecementis aprimarycontrol on reservoir quality (Kantorowicz et al., 1987; Emery, 1987; Boles and Ramseyer. 1987; Saigal and Bjorlykke, 1987; Bryant et al., 1988; Glassman et al., 1989). Calcite chemistry and diagenesis is discussed further in Chapter 5. Kaolinite Authigenic kaolinite (Figure 4.22) is the least abundant clay mineral present in Frio sandstones, averaging only 1 %, however it is locally important, especially in Minnie S. Welder #67 and TST 51-1 (Table 4.3). Figure 4.6 shows that there are two episodes of kaolinite precipitation, a syndepositional event restricted to the shorezone facies sandstones (Figure 4.21) and a late diagenetic event seen in both facies (Figure 4.12). Figure 4.15 shows that kaolinite is not present in sandstones with abundant detrital matrix, and the occurrence of kaolinite is not related to grain size (Figure 4.5). Figure 4.22. Kaolinite (& Dickite) and Analcime. A. Kaolinite and dickite are locally important cements in TST 51 #1 and MSW #67. Note poorly hexagonal crystal shape. B. Analcime commonly occurs as small poikilotopic concretions. There are two episodes of analcime and kaolinite precipitation (Figure 4.6). The distribution of kaolinite in a sample appears entirely random. In some samples kaolinite is concentrated in matrix-rich laminations, in others it is concentrated in matrix-poor areas. In these sandstones, as well as the other Frio sandstones studied by Lindquist (1976) and Sullivan (1988), insufficient kaolinite is present in the sandstones to account for all the elements made available by feldspar dissolution. Kaolinite mineralogy is discussed'further in Chapter 6. Analcime Analcime, NaAlSi2o6-H2O, is a zeolite-like diagenetic mineral commonly associated with volcanogenic sandstones (Coombs, 1961; Hay, 1966; Coombs and Whetten, 1967; Surdam and Boles, 1979; Barrer, 1982). In thick, tuffaceous sandstones in New Zealand and Japan, analcime exists as an intermediate phase in the diagenetic series clinoptilolite-analcime-albite (Coombs, 1954; Coombs et al., 1959; Inoue et al., 1983; Aoyagi and Asakawa, 1984). Analcime is also commonly associated with saline lake deposits (Surdam and Eugster, 1976; Surdam and Sheppard, 1978; Remy and Ferrell, 1989; Turner and Fishman, 1991). Figure 4.6 shows that analcime precipitation occurs at two times in Frio sandstones. In fluvial Frio sandstones syndepositional analcime can be as abundant as 18% of the rock volume (Galloway and Kaiser, 1980; Grigsby and Kerr, 1991). In this study, syndepositional analcime is found in back-barrier sandstones from Minnie S. Welder #7O and is almost always associated with syndepositional calcite that shows ash-shard replacement textures. In the shorezone and shelf sandstones of this study, analcime usually occurs as small poikilotopic concretions (Figure 4.22) or is concentrated in clay-free laminations (Figure 4.23). Burial diagenetic analcime is relatively common in TST 49-2 sandstones. It often contains inclusions of, and is included in, kaolinite - and burial diagenetic calcite indicating that the three minerals precipitated at approximately the same time. Dehydration reactions convert analcime to albite. The temperature of this reaction is a function of P(H2O), P(CC>2), pH, fluid chemistry and concentration, and mineral chemistry (Hay, 1966; Coombs and Figure 4.23. Shoreface sandstones. Fine-grained, laminated, middle-shoreface, shorezone sandstones. A. Diagenetic lamination due to analcime cementation. B. Lamination predom inately due to concentrations of clay clasts and matrix. Whetten, 1967; lijima, 1978; Surdam and Boles 1979; Ghent, 1979; Gottardi and Galli, 1985). The entropy and Gibbs free energry changes of the analcime + quartz --> albite + water reaction are small relative to most dehydration reactions because water molecules lose only a small degree of freedom while a component of the zeolite system (Campbell and Fyfe, 1965). In individual thin sections, analcime can show both euhedral crystal faces or evidence of extensive dissolution. Analcime in Frio sandstones is commonly slightly birefringent and zoned. The refractive index of analcime is a function of its Si/Al ratio (Gottardi and Galli, 1985). Figure 4.24 shows that there is a significant range in composition of analcime in these Frio sandstones. Figure 4.25 shows the relationship of analcime peak position and depth and is discussed in Chapter 7. Paragenetic relations indicate that burial diagenetic analcime is much more abundant in Frio sandstones than syndepositional analcime, even in the back-barrier sandstones where the syndepositional phase occurs. This is also the case with burial diagenetic kaolinite abundance vs. syndepositionalkaolinite abundance. Apparently the early formed minerals serve as a metastably precursor for precipitation of their burial diagenetic equivalents. This relationship is also seen in both generations of calcite in shorezone sandstones; burial diagenetic calcite is more likely to occur associated with syndepositional calcite than as the only calcite in the sandstone. Quartz Overgrowths (QOGs) Diagenetic quartz is the most abundant cement in Frio sandstones from the Houston embayment (Loucks et al., 1981; Loucks et al., 1986; Sullivan, 1988); but in sandstones from the Rio Grande embayment and from the Corpus Christi area, ~3% of the rock volume. QOGs average only In the study area there is a strong facies control on the abundance of QOGs (Table 4.3), as much more diagenetic quartz is found in shelf sandstones. Figure 4.19 shows an increase in the QOG content of sandstones with depth. The implication that significant QOG precipitation occurred over a large depth range is incorrect. In Frio and other o\ . WOO D is-s a» w Esic 3> 8^ 0> T3*3 £§<“ § si E TD T3 G Oc« > e3 «s w - c *2r-.2 Z so -3 .3 ON g «w E ¦o c£ u3 o ¦wU'JH W •-££2 E c2 ¦gc 2.2 E "!r! ft) .3 C Co U c *2 -= (range from 3.20 to 3.60 O) and decreases with depth (Figure 4.29). The average QFL for the core is 48:24:28 (Table 4.2). Plagioclase feldspar and albite is more common in the coarser upper section of the core; rock fragments (predominately VRFs) are more common in the finer, deeper sandstones (Figure 4.30). Detrital clay content ranges from 3% to 37% and averages 12%. Pseudomatrix and burrowed matrix clays are both Figure 4.29 shows that detrital clay is more common in the finer size common. sandstones, and the PCP % is inversely proportional to the amount of detrital clay in the sandstone. Figure 4.29 also shows that there is no strong relationship between PCP (average 22%, range 5% to 31%) and depth. Intergranular porosity (range 0 to 10%, average 4%) shows only a slight decrease with depth, and shows no relationship with grain size or PCP. Secondary porosity decreases with depth and is proportional to total porosity (Figure 4.31). Sandstones with >~25% total clay have very little porosity, though significant porosity can exist in sandstones with 20% or 25% total clay (Figure 4.31). The diagenetic sequence of events in these sandstones is typical compaction chlorite QOGs calcite secondary porosity generation pathway Figure4.29. DetritalcharacteristicsofTST346#1sandstones. Grainsize decreases and clay content increases with depth. There is a strong correlation between PCP and detrital clay content. 117 118 the in common more is Feldspar sandstones. 346/1 TST of content fragment rock and albite, and Plagioclase 4.30 Figure sandstones. finer-grained the in common are VRFs core, the of part upper the in sandstones coarser Figure 4.31. Porosity in TST 346 #1 sandstones. Intergranular and secondary porosity is maximized in the coarser-grained, upper sandstones. Secondary porosity is proportional to total porosity. Sandstones with >25% total clay content have negligible porosity. 119 seen in most Frio sandstones (Figure 4.6). No kaolinite was identified, and Chlorite chlorite formation continued at least into the time of calcite precipitation. content is not statistically related to the amount of detrital clay in a sandstone, but on the thin section scale chlorite is much more common near clay-filled burrows and in clay-rich laminations (Figure 4.11). The chlorite content is 7% and average ranges from 15% detrital clay (Figure 4.32). Cathodoluminescence and total porosity but no relationship exists between chlorite and grain size or microscopy shows that the QOGs in this core, and in the other cores studied, are only slightly luminescent and show only minor zoning. The average QOG content in these sandstones is 5%, though -25% of the samples have 10% or more QOGs. The QOG content of TST 346/1 sandstones is not related to detrital quartz content (Figure 4.32). Diagenetic calcite averages 19% (range 2% to 39%) and occurs as both grain replacement and IGV fill. Calcite dissolution textures are common. Calcite is more abundant in the deeper, finer grained section of the core. Sandstones with >20% total clay and low PCPs contain very little calcite (Figure 4.33). What controls the distribution of the diagenetic minerals in these shelf sandstones? The only diagenetic phases that can be produced within the sandstones themselves are I/S and chlorite, which are formed during diagenesis of the depositional clay matrix (see above and Chapter 6). SiC>2 and calcite must be imported into the sandstones. Hazeldine et al. (1984) correctly point out that diffusion of elements into sandstones would produce a homogeneous distribution of cements. This relationship is not observed in Frio or most other sandstones. At any time during the diagenetic evolution of a sandstone package the preferred conduit for fluid flow is the sandstone with the highest permeability. That sandstone is therefore also the preferred unit for precipitation of authigenic phases forming at that particular time. Figure 4.34 shows that the abundance of Figure 432. QOGs in TST 346 #1 sandstones. QOGs are more abundant in the upper, coarser-grained sandstones. The abundance of QOGs is not related to detrital quartz content. Sandstones with abundant detrital clay contain few QOGs. 121 Figure 4.33. Diagenetic calcite in TST 346 #1 sandstones. Calcite is more common in finer-grained sandstones. Sandstones with >20% total clay and low PCPs contain very little calcite. 122 QOGs and diagenetic calcite in TST 346 #1 sandstones is related to permeability, represented by "Available PCP For QOGs, the available PCP is the original PCP of the sandstone minus the amount of diagenetic chlorite that formed before the QOGs precipitated. For calcite the value is the original PCP minus the amount of diagenetic chlorite and QOGs that formed before calcite began precipitating. Quartz oversaturated fluids flowed through the whole sandstone package at the time of quartz precipitation (Figure 4.34, Time B), however, the relationship between available PCP and QOG abundance indicates that many more pore volumes offluid passed through some of the more permeable sandstones. Calcite precipitation occurred after further burial of the sandstone package (Figure 4.34, Time C). The strong relationship between available PCP and calcite abundance indicates that fluid flow at the time of calcite precipitation was strongly focused in the more permeable sandstones. The relationship between dissolution and fluid flow is also indicated by the correlation between secondary porosity and its available PCP (Figure 4.34, Time D). The random distribution of chlorite and PCP (Figure 4.34, Time A) indicates that chlorite formation may not be as strongly related to fluid flow as are the other diagenetic phases and supports the (at least) partially autocthonous nature of diagenetic chlorite. Available PCP is not the only factor effecting the permeability of these sandstones. Coarser grained sandstones are more permeable than finer grained sandstones (Blatt et al., 1978; Gardner et al., 1990; Hartkamp et al., 1993). This relationship is one reason why some sandstones with high PCP have very few QOGs (Figure 4.34, Time B). The relationship between diagenetic heterogeneity and ease of fluid flow can be seen on the whole core, in individual sandstones, and even at the thin section scale. Samples 2 through 5 (Table 4.4 and Figure 3.8) are from one 2ft laminated sandstone. Sample 3 had the most original PCP and was least modified by pre-calcite cements, especially chlorite. Later fluid flow within the sandstone was easiest through the sample 3 part of the unit and calcite precipitation greatly reduced the porosity. More porosity was preserved in the parts of the sandstone with less available PCPs. The same relationship can be seen in samples 16-18 Figure 4.34. Fluid flow and diagenesis of TST 346 #1 sandstones. The "Available PCP" at Time B (when quartz precipitated) is the original PCP minus the amount of chlorite that formed at Time A, and is a measure of sandstone permeability. The association of abundant QOGs and calcite with highly permeable sandstones at the time of their precipitation indicates that fluid flow is an import­ ant control on the diagenesis of the rocks. Secondary porosity is also related to fluid flow, chlorite content is not. 124 Table 4.4 Diagenesis of TST 346 #1 sandstones. In all "diagenesis" tables, the values are volume % of thin section (except grain size (phi)) Framework grain composition for all samples is listed in Appendice A. Depth (ft) # PCP Grain Det. Tot QOG Tot Prim 2nd Tot Size Clay Chlor Cal Poros Poros Poros 14501.0 27 23.5 3.21 9.5 6.5 6.0 2.0 10.0 4.5 14.5 14502.0 26 32 3.36 2.5 3.5 18.5 8.5 4.5 6.5 11.0 14506.0 28 23.5 3.39 14.5 10.5 6.5 2.0 6.0 3.5 9.5 14507.0 25 31.5 3.23 13.0 6.0 15.5 6.0 5.5 4.0 9.5 14508.0 24 10.2 3.42 26.0 12.5 0.1 0.6 0.1 0.5 0.6 14509.0 23 31.5 3.21 3.0 9.0 14.0 11.0 5.0 5.5 10.5 14513.5 22 26 3.29 7.5 7.0 2.0 23.0 1.0 4.0 5.0 14515.0 21 4.5 3.55 29.0 4.5 0.5 1.0 0.0 0.5 0.5 14518.5 20 23.5 3.35 16.0 8.5 10.0 4.5 4.0 4.0 8.0 14519.7 19 25.5 3.37 8.5 10.0 9.0 5.5 5.0 4.0 9.0 14521.0 18 28.5 3.29 3.0 2.5 2.5 39.0 0.0 3.1 3.1 14522.5 17 26.5 3.38 6.5 7.5 7.5 7.0 7.5 6.5 14.0 14523.0 16 28.5 3.28 4.5 11.0 14.5 2.5 4.5 6.0 10.5 14529.0 15 26 3.40 5.5 9.0 4.5 7.0 10.0 5.0 15.0 14530.0 14 18.5 3.33 12.5 9.0 6.5 4.0 3.0 4.5 7.5 14533.7 13 23 3.20 12.5 7.0 2.0 9.0 9.5 4.5 14.0 14534.2 12 21 3.35 14.0 9.5 2.0 2.0 9.5 4.0 13.5 14534.5 11 21.5 3.36 6.0 9.5 4.0 4.5 8.0 4.5 12.5 14535.5 10 22 3.35 10.0 8.0 2.5 3.5 9.0 3.5 12.5 14538.0 9 28 3.40 8.0 1.0 1.5 34.0 0.0 3.7 3.7 14542.0 8 9 3.50 35.0 4.5 1.0 3.5 0.5 0.5 1.0 14543.0 7 14.5 3.58 16.0 7.5 1.0 4.5 6.0 2.6 8.6 14550.5 6 20 3.41 9.5 6.5 1.5 23.5 0.0 1.0 1.0 14552.0 5 20.5 3.44 9.5 8.0 2.0 20.0 1.5 2.0 3.5 14553.0 4 21.5 3.38 12.5 6.5 1.5 14.5 3.0 2.0 5.0 14554.0 3 25.2 3.29 3.0 0.1 0.5 37.0 0.1 0.1 0.2 14555.0 2 25 3.41 7.5 4.5 1.5 23.5 1.5 1.5 3.0 14564.0 1 10.1 3.60 37.0 9.5 0.5 2.5 0.1 1.0 1.1 125 from another sandstone. On the thin section scale, calcite and QOGs often are restricted to coarser, clay poor laminations (Figure 4.11). The net result of this of selective fluid flow and mineral process precipitation is that sandstones (or portions of sandstones) that are not the most the best reservoir porous or permeable at the time of deposition often preserve quality at depth because they are modified less by diagenesis. Shell TST 392/4 Two cores from Shell TST 392/4 from Redfish Bay Field, Nueces County,weresampled. Theuppercoredintervalis12,240ftto12,289ft(3,731m to 3,746 m) and is an example of the shelf siltstone facies (Figure 4.35). The core is predominately a heavily bioturbated silty mudstone. Horizontally bioturbated remnants of thin (1 cm) sand layers are common. The sandstones have sharp basal contacts, laminated bases and bioturbated tops. Seven thin sandstones ranging in size from -5 cm to 0.5 ft constitute a 5 ft thick coarsening upward/fining upward sequence in the middle of the core. The grain size of the sandstones is very fine to fine grained. (Grain size measurements were not performed on any TST 346/4 samples because of the large abundance of QOGs in the sandstones.) Several small 1-2 cm bioturbated sandstones and one massive, relatively thick (~0.5 ft) sandstone arepresent in the sandstonepoorparts ofthecore. The average QFL composition of sandstones from the upper core is 68:10:22 (Table 4.2). Detrital clay matrix content ranges from 2 to 27% and averages 12%. PCP averages 30% and ranges from 19% to 39%. Primary porosity averages 3% (~lO% with sandstones in abundant matrix. not is Calcite abundant 132 core upper indicates o core, lower indicates • sandstones. #4 392 TST in QOGs JB. 4 Figure content QOG between exists correlation posative slight A matrix. clay detrital >~lO% with sandstones in scarce are QOGs content. quartz detrital and Tot Poros 1.0 0.5 1.0 2.0 10.0 6.5 14.5 8.0 6.5 4.0 6.0 10.5 11.0 6.5 8.5 21.5 1.6 13.0 8.5 7.5 4.0 2nd Poros 1.0 0.0 0.0 0.5 1.0 1.5 2.0 3.0 1.5 0.5 2.5 4.5 2.0 3.0 4.0 3.0 1.6 7.0 6.0 0.5 1.5 Prim Poros 0.0 0.5 1.0 1.5 9.0 5.0 12.5 5.0 5.0 3.5 3.5 6.0 9.0 3.5 4.5 18.5 0.0 6.0 2.5 7.0 2.5 Tot Cal 20.5 16.0 9.0 1.0 9.0 18.5 13.5 14.0 6.5 22.5 3.5 2.5 4.0 28.5 7.0 1.0 0.2 1.5 1.0 1.5 2.5 QOG 6.5 21.0 25.5 0.5 3.5 2.5 6.0 11.0 8.0 10.5 31.0 20.0 20.5 3.5 16.5 4.0 1.5 8.5 25.5 2.0 11.5 Tot Chlor 3.0 0.5 0.5 32.0 6.5 10.5 4.5 6.0 5.5 3.5 2.0 3.0 2.0 2.0 2.0 2.5 5.5 8.0 1.0 7.0 17.0 sandstones. Det. Clay 15.0 4.0 4.5 16.0 20.5 6.0 9.5 11.0 26.5 15.5 2.0 4.0 6.0 6.0 1.5 11.0 45.0 16.5 6.5 37.5 16.5 392/4 Grain Size ndndndndndndndndndndndndndnd ndndndndndndnd TST of PCP 22 33 31.5 32 25 28.5 33 32.5 18.5 35 39 31 33.5 31 29 25 7.1 22 30 17 29 # 23 22 21 20 19 18 17 16 15 14 11 10987654321 Diagenesis 4.5 (ft) Table Depth 12259.0 12263.0 12668.5 12270.0 12270.2 12270.5 12271.0 12271.5 12272.0 12272.5 12288.0 12589.0 12591.5 12593.5 12595.0 12598.0 12599.0 12599.5 12601.0 12602.0 12607.0 Figure 4.39. Fluid flow and diagenesis of TST 392 #4 sandstones. . sandstones. indicates lower sandstones, o indicates upper Abundant QOGs and calcite correlate with available PCP % at the time of their formation, which is a measure of sandstone permeability and fluid flow. See Figure 4.34 and text. 134 - IGV content of TST 392 #4 Figure 4.40 Heterogeneous diagenesis upper sandstones. The heterogeneous distribution of IGV filling phases is due to perm­eability differences within and between sandstones, and position in sandstone package. B I/S matrix Chlorite f§§ QOG ¦ Calcite B Porositv 135 - IGV content of lower TST 392 #4 sandstones. Figure 4.41. Heterogeneous diagenesis The heterogeneous distribution of IGV filling phases due to perm­eability differences within and between sandstones, and position in sandstone package |I/S matrix |gf Chlorite fH QOG I Calcite H Porosity 136 laminated sandstones (0.5 to 20 cm thick) are present in the lower core. The upper part of the core consists of 1 ft to 5 ft thick laminated and bioturbated sandstones. Bioturbation and clay content increase towards the tops ofmost of these sandstones. Basal contacts are usually sharp. Vertical burrows are more common in the half of the core than in the lower half. 12 thin sections were studied upper from this core. The average grain size of TST 51 #1 sandstones is 3.42 d> (range 3.22 d> to 3.69 d>). The average QFL composition is 54:18:29. Detrital clay matrix 13% and from 6% to 26%. The averages ranges average PCP is 18% (range 0­ 29%). Figure 4.43 shows the positive correlation of PCP and grain size, and the negative correlation of PCP and detrital clay content. The average intergranular porosity of the sandstones is 7% (range 0-15%); total porosity ranges from 0% to 19% and 10%. averages Primary porosity is positively correlated to PCP and grain size (Figure 4.44). Diagenesis of TST 51 #1 sandstones follows the standard sequence of chlorite calcite kaolinite compaction QOG secondary porosity generation (Table4.6). Chloritecontentaverages4%anditsabundanceisnotrelatedtoPCP or grain size. QOG (3% average) and kaolinite (5% average, 10% maximum) content are both positively correlated to available PCP at the time of precipitation. Both diagenetic phases show somewhat poorer positive correlations with grain size, and negative correlations with detrital clay content (Figures 4.45 and 4.46). Secondary porosity is slightly correlated with available PCP. The only sample with abundant calcite (23%) is from the thickest sandstone in the heavily bioturbated lower part of the core. The relationships between the QOG and kaolinite contents and available PCP again show that permeability and fluid flow are important controls on diagenesis of Frio sandstones. The occurrence of the heavily calcite cemented "stand-alone" sandstone within the highly bioturbated impermeable sandstones in the lower part of this core is similar to the presence of QOG filled boundary sandstones in the cores from Red Fish bay (TST 346 #1 and TST 392 #4) and in other sandstones (Hawkins, 1978; Shew and Gamer, 1990). At some point during burial, thesepermeable sandstones were thefocusoflargeamountsoffluidflow 137 Figure 4.42. "Weathered outcrop" profile and sedimentary structures for TST 51-1. Symbols: = bioturbation, - lamination. Numbers are thin section identifiers. 138 139 sandstones. #1 51 TST in % Clay Detrital and Size Grain vs. PCP 4.43. Figure content. clay detrital and size grain framework by largly controlled is PCP Sandstone Tot Poros 0.5 19.1 11.0 19.0 8.5 5.0 11.0 5.0 0.0 16.5 15.5 6.0 1.0 3.5 0.1 0.0 0.1 0.0 0.0 2nd Poros 0.5 4.1 3.0 3.0 3.5 3.5 4.0 2.5 0.0 4.0 4.0 3.0 1.0 1.0 0.1 0.0 0.1 0.0 0.0 Prim Poros 0.0 15.0 8.0 16.0 5.0 1.5 7.0 2.5 0.0 12.5 11.5 3.0 0.0 2.5 0.0 0.0 0.0 0.0 0.0 Tot Kaol 1.5 5.0 6.0 5.0 4.0 4.0 6.0 7.5 0.1 10.0 6.5 6.0 1.0 7.0 0.0 1.5 0.0 0.0 0.0 Tot Cal 2.0 2.5 0.5 0.1 0.5 0.1 0.0 23.0 0.0 0.5 7.0 10.0 23.5 16.5 0.5 14.5 11.0 3.0 1.0 3.0 6.5 5.5 10.0 2.0 1.0 1.5 0.1 1.0 2.5 0.5 0.0 3.5 0.0 13.0 1.0 0.5 2.0 sandstones. QOG 2.0 10-2 Tot Chlor 6.0 3.5 8.5 2.5 12.0 1.5 6.0 1.5 1.0 4.0 3.0 4.0 1.5 2.0 1.0 0.0 0.0 0.0 0.0 TST Det. Clay 24.0 8.0 8.0 13.5 10.5 22.5 7.6 6.0 25.5 12.0 6.6 11.0 6.6 10.5 33.0 11.5 11.0 44.0 42.5 Lnd 51-1 Grain Size 3.69 3.33 3.26 3.26 3.22 3.67 3.51 3.27 3.62 3.29 3.33 3.59 3.46 3.32 3.66 3.61 3.23 3.71 4.54 TST 7 of PCP 8.5 29 22.5 31 20 16.5 22 0.1 21.5 26 14.5 14.5 24.5 1.0 18.0 6.5 2.0 3.5 1 # 12 11 1098765432 7654321 Diagenesis 51-1 10-2 4.6. TST TST (ft) Table Cities Depth 10454.0 10463.0 10466.0 10471.0 10476.0 10484.5 10489.0 10496.0 10499.5 10501.0 10502.0 10507.0 Cities 10399.0 10411.0 10418.0 10432.0 10438.0 10448.0 10449.0 141 PCP, and size grain coarse to correlated is porosity Primary sandstones. #1 51 TST in porosity Primary 4.44. Figure permeability. sandstone to related are which of both and % QOG betwen relationship posative The sandstones. #1 51 TST in abundance QOG on Controls 4.45. Figure diagenesis. on controls important are flow fluid and permeability that indicates PCP available 142 kaolinite between correlation posative The sandstones. #1 51 TST in abundance kaolinite on Controls 4.46. Figure content clay Detrital diagenesis. on control important the is flow fluid that indicates PCP available and abundance diagenesis. subsequent and permeability, sandstone effects strongly 143 while at the resulting in the great modification of their own intergranular space same time effectively shielding their less permeable or isolated (by position within the sandstone package) neighbors from extensive diagenetic modification by the same fluids. Cities TST 10-2 The core from Cities TST 10-2 (Corpus Channel Field, Nueces County) rangesfrom10,399to10,459ft(3,169to3,188m). Thecoreisacoarsening­ofburrowed upwards / fining-upwards sequence very fine grained sandstones and siltstones and is an example of the shelf siltstone facies (Figure 3.6). Thin (<5 cm thick), laminated, very well indurated sandstones are often the basal component of small (1 to 3 ft) fining-upward sandstones that comprise most of the core. These sandstones generally have sharp contacts with the underlying units, are heavily bioturbated and are often very clay rich towards the top (Figure 4.10). Sandstone grain size increases and clay content decreases upward in the core. Vertical burrows are more common in the upper part of the core. The average QFL composition for the seven thin sections studied is 52:16:31. for the heavily bioturbated The average PCP is 10% though the range sandstones that make up most of this facies is 1% to 7%. The sampled laminated sandstones have PCP's from 15% to 25%. Clay content of the bioturbated sandstones 33%, laminated sandstone clay content from 6% to averages ranges 12%. Almost no porosity is preserved in either type of sandstone, and laminated samples #7 and #6 have total porosity's of 1% and 3.5%, respectively. The grain size of the sandstones ranges from 3.23 d> to 4.54 O and averages 3.65 (Table 4.6). The laminated sandstones with significant PCP display the same sequence of diagenetic events seen in sandstones from other cores. Early chlorite is followed by QOGs, calcite, kaolinite, and the development of secondary porosity. Small amounts of poikilotopic barite (forming after QOGs) are also found in of the sandstones. many The intergranular volume of the laminated sandstones is completely filled with diagenetic minerals. In the heavily bioturbated, clay-rich sandstones, the little PCP that exists is always located in clay-poor burrows or in small remnant laminations. This intergranular volume is also completely filled with the same diagenetic phases as in the laminated sandstones. QOGs are especially common because they are the first cements to form and often completely fill the PCP. The reason the intergranular space in all these sandstones is almost completely filled with diagenetic minerals is because fluid flow is focused through clay-poor burrows and small, low PCP sandstones simply because the permeability of the homogeneously bioturbated clay-rich sandstones and siltstones surrounding them is so low. The heterogeneous distribution of QOGs, calcite, and kaolinite in the three laminated sandstones, #7 (tr. QOG, 14% IGV calcite, 1% kaolinite), #6 (4% QOG, 11% IGV calcite, 7% kaolinite), and #4 (13% QOG, 4% IGV calcite, 2% kaolinite) (Figure 3.6, Table 4.6), shows that different flow pathways were operating at different efficiencies during burial. The net result, however, almost complete loss of porosity and permeability, is the same in each sandstone. Cities TST 4-2 Another example of the shelf siltstone facies is Cities TST 4-2, also from Corpus Channel field, Nueces County (Figure 4.47, Table 4.7). The cored intervalis10,385ftto10,439ft(3,165mto3,182m). Thecoreiscomposedof bioturbated siltstones and very fine-grained sandstones. Very small (<3 cm) laminated sandstone remnants form the bases of upward fining sandstones and siltstones. Sandstone contacts are usually sharp and sometimes clearly erosional. The "sand-rich" bases of the fining-upward sequences are generally better indurated than the clay-rich tops. The sand content in TST 4-2 is less than in TST 10-2. PCP averages 5%, detrital clay matrix averages 52%, and no primary intergranular porosity is preserved in any sample. As in TST 10-2 sandstones, the only PCP in these samples occurs in clay-poor burrows and small preserved laminations. The PCP is filled with chlorite, QOGs, barite, calcite, and kaolinite. Figure 4.47. "Weathered outcrop" profile and sedimentary structures for cores TST 4-2 and TST 430-5. Symbols: =bioturbation, =lamination, A =fining-upwards, \7 =coarsening-upwards. Numbers are thin section identifiers. 146 Tot Poros 2.0 2.1 5.5 0.2 0.0 9.0 8.0 0.1 0.3 Tot 0.0 0.0 0.5 0.4 0.0 Poros 2nd Poros 1.5 0.6 1.0 0.1 0.0 1.5 2.0 0.1 0.2 2nd Poros 0.0 0.0 0.5 0.4 0.0 Prim Poros 0.5 1.5 4.5 0.1 0.0 7.5 6.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 Prim Poros Anal 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.5 1.0 Kaol. 0.0 0.0 1.0 0.0 0.0 Tot Cal 15.0 9.0 2.5 0.1 1.0 1.5 1.5 0.1 1.0 Tot Cal 0.0 3.5 3.0 0.0 4.5 1 000 000 Oil 1.5 Barite 0.0 0.0 1.5 1.5 0.0 12.0 2.0 1.5 0.5 1.0 1.5 3.0 1.0 0.0 QOG 0.0 2.0 1.5 tr 4/2 sandstones. QOG 1.5 0.5 Tot Chlor 0.6 3.0 13.0 4.5 1.0 7.0 14.0 1.5 3.0 Tot Chlor 0.0 0.5 2.5 1.5 1.0 TST and Det. Clay 5.0 28.5 20.5 45.5 42.0 33.0 12.0 54.0 51.0 Det. Clay 58.5 41.0 43.7 61.3 54.0 430/5 Grain Size 3.32 3.49 3.48 3.25 3.67 3.49 3.37 3.89 3.97 Grain Size nd nd nd nd nd TST 34 of PCP 23.1 14.5 19.5 6.6 16.6 23 5.6 PCP 2.0 4.5 7.1 3.8 2.5 # 123456789 # 12345 Diagenesis 430-5 4-2 4.7. TST (ft) TST (ft) Table ARCO Depth 10071.0 10077 10095.2 10095.8 10096 10107 10113 10119 10121 Cities Depth 10390 10398 10416 10427 10429 The total loss of PCP in the few originally permeable sandstones again attests to theimportanceoffluidflow in the diagenesisofFrio sandstones. Cities TST 49-2 Core from Cities TST 49-2 (Encinal Channel field, Nueces County), samples one shorezone sandstone sequence and four distal-shoreface/inner-shelf sandstone sequences. The sedimentology of the distal-shoreface/inner-shelf sandstones is discussed in Chapter 3. TST49-2 8,011ft - - The shallowest core (8,011 ft 8,061 ft; 2,442 m 2,457 m) is an example of the middle/upper shoreface facies. The sandstones from the lowest ~12 ft of the core coarsen upwards from fine-grained to almost medium-grained. Oyster shells are abundant at the base, and wood, biotite, and glauconite are present throughout. The sandstones are very well laminated at the top and only subtly laminated at the base (Figure 4.48, Figure 4.23). A decrease in grain size and a reasonably sharp contact separate the lower sandstones from the overlying unit. The next ~30 ft of core is grouped together because of the lack of obvious reactivation, erosional, or hiatial surfaces. The grain size of this section slowly decreases upwards from upper fine-grained to lower fine-grained. The base of this section is very well laminated, the upper half contains more clay, is only subtly laminated, and is moderately well bioturbated. Oyster shells are half of this section. present in the upper Approximately ten ft of laminated shales and bioturbated siltstones, capped by 1 ft of clay-rich, wavy laminated very-fine to fine grained sandstone, overlay the thick middle section of the core. The SP log from this core section is reproduced in Figure 3.3. The average grain size of this package is 2.19 d> (range 1.84 to 2.58 O), total porosity averages 13% (range 7% to 23%), PCP averages 20% (range 11% to 28%), detrital clay averages only 1% (range 0% to 8%) (Table 4.8). Figure 4.49 shows that in TST 49-2 sandstones Figure 4.48. "Weathered outcrop” profile and sedimentary structures for cores TST 49-2 8061 ft and MSW 67. Symbols: V =lamination, A =fining-upwards, =coarsening-upwards. Numbers are thin section identifiers. 149 there is a grain size effect on the detrital mineralogy, with VRF's being concentrated in the coarser, shorezone sandstones. TST 49-2 8,521 ft The core from 8,521-8,584 ft (2,597-2,616 m) samples a fining- upwards/coarsening-upwards distal-shoreface/inner-shelf sandstone package (Figure 3.7). The sandstones are generally between 3 and 10 ft thick and the contacts are sharp to erosional. This section of the core produces a strong deflection on the SP log (Figures 3.3 and 3.4). The upper half of the core, which produces a more subdued SP deflection, is a bioturbated very fine grained sandstone with variable amounts of clay. One 10 cm stand thick alone well-laminated sandstone occurs in the bioturbated upper core. Approximately 3ft of interlayered, thin (2-5 cm) well laminated sandstones and bioturbated siltstones separate the upper from lower half. The average grain size of this package is 3.20 (range 3.01 d> to 3.51 O), total porosity averages 17% (range 0% to 30%), PCP averages 21% (range 11% to 33%), detrital clay averages 8% (range 2% to 27%). TST 49-2 8,710 ft This TST 49-2 core (8,710-8,741 ft, 2,655-2,664 m) samples a small inner-shoreface/distal-shelf sandstone (Figure 3.7). The bulk of the core is a very well indurated bioturbated sandstone with minor preserved laminations. The top 4 ft is a well laminated, clay-poor sandstone, and two laminated sandstones form the base of the core. It is interesting that even a relatively thin and bioturbated sandstone produces so sharp a deflection on an SP log (Figure 3.4). The average grain size of this package is 3.23 d> (range 2.77 to 3.47 O), total porosity averages 16% (range 0% to 26%), PCP averages 29% (range 24% to 34%), detrital clay averages 7% (range 7% to 10%). Tot Poros 7.0 17.0 12.5 18.5 7.1 13.0 22.5 8.0 14.5 17.0 8.5 24.0 15.5 2.0 0.0 26.0 29.5 20.5 19.5 0.0 0.5 2nd Poros 3.5 5.5 4.5 5.5 1.6 5.5 5.5 5.0 3.5 4.0 4.0 1.5 2.0 1.0 0.0 2.0 1.5 2.5 3.0 0.0 0.5 ’ Prim Poros 3.5 11.5 8.0 13.0 5.5 7.5 17.0 3.0 11.0 13.0 4.5 22.5 13.5 1.0 0.0 24.0 28.0 18.0 16.5 0.0 0.0 Anal. 0.5 0 0 0 0 3 0.5 9 6.5 4.5 0 0 0 0 0 0 0 0 0 0 0 Kaol 3.5 6.5 4 1.5 0.5 2.5 2 0.5 0.5 0.5 2.5 0 0 0 0 0 0 0 0 0 0 Tot Cal 8.5 10.5 27.5 12.5 34.0 6.5 4.0 7.0 12.5 6.5 6.0 5.5 6.5 3.5 13.5 3.0 1.5 2.0 0.5 34.5 10.5 QOG 1.5 2.5 0.5 0.5 0.1 0.1 1.5 0.1 0.5 0.1 0.5 1.0 2.0 0.5 0.5 1.5 2.5 2.0 2.0 0.5 7.0 FOG 0.5 1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.0 0.0 0.5 0.0 1.0 0.5 0.5 0.5 0.0 0.0 sandstones. Det. Tot Clay Chlor 8.0 5.0 1.1 0.5 1.0 0.0 0.5 1.0 2.0 0.5 3.0 0.5 1.5 0.5 4.5 0.6 1.0 0.0 1.5 0.0 1.5 4.5 4.5 1.5 9.0 1.0 14.0 15.0 27.0 4.5 1.5 1.0 2.0 0.0 3.0 1.0 2.5 2.5 7.5 1.0 16.5 0.5 49-2 Grain Size 2.58 2.18 2.34 2.57 2.03 2.19 1.97 2.08 1.84 1.99 2.29 3.35 3.29 3.20 3.51 3.13 3.04 3.09 3.01 3.17 3.26 TST of PCP 17 24.5 28 19.5 23.6 15.1 22 15.2 23 22.1 11 25.5 20 16.5 11 29.5 32.5 23 22.5 20.5 14 Diagenesis # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 23 24 Figure 4.8 Depth (ft) 8029.0 8035.0 8036.0 8040.0 8040.5 8043.0 8047.0 8048.0 8049.0 8051.0 8056.0 8535.5 8539.0 8546.0 8553.0 8560.0 8561.0 8562.0 8562.5 8577.0 8580.0 151 Tot Poros 21.0 0.0 10.5 25.5 13.5 22.5 17.5 2.0 17.6 16.0 24.5 29.5 24.5 6.5 16.0 11.1 2nd Poros 3.5 0.0 1.0 1.0 0.0 3.5 4.0 0.5 2.1 2.5 4.5 5.0 2.5 3.5 3.0 3.6 Prim Poros 17.5 0.0 9.5 24.5 13.5 19.0 13.5 1.5 15.5 13.5 20.0 24.5 22.0 3.0 13.0 7.5 5 0 0 5 0000000 Anal. 0.1 0.5 0.1 0.1 0.5 00 0000 1 0 4 Kaol 5.5 4.5 0.5 0.5 0.5 0.5 0.5 Tot Cal 6.0 35.5 0.0 2.5 13.0 1.0 0.1 0.5 0.5 14.0 0.0 0.0 0.0 0.6 0.0 17.5 QOG 1.5 1.0 1.5 2.0 3.0 3.0 9.0 3.0 1.5 1.0 9.5 8.5 6.0 1.0 15.0 2.0 FOG 0.1 0.0 0.0 0.0 0.0 0.1 0.5 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.5 0.0 Tot Chlor 3.0 0.0 11.0 0.5 3.5 1.0 3.0 17.5 8.0 1.0 3.0 1.0 4.0 14.5 4.5 2.0 sandstones. 49-2 Det. Clay 6.5 9.5 9.0 7.0 8.0 8.0 5.0 18.0 11.5 8.0 3.5 5.0 3.0 12.5 3.0 10.5 TST of Grain Size 3.31 3.28 3.25 3.13 3.13 3.26 2.77 3.39 3.03 3.13 3.08 3.23 3.38 3.47 3.17 3.43 PCP 31.2 24 26 33.5 30.5 27.6 24.6 19.1 23.5 25.5 33 35.1 32.5 14.1 33.1 29.5 Diagenesis # 2526 27282930 31323334353638373940 (cont.). 4.8 (ft) Table Depth 8712.0 8722.0 8730.0 8732.0 8733.0 8738.0 8977.0 8981.0 8997.0 8999.0 9002.0 9006.0 9015.0 9017.0 9023.0 9026.0 Tot Poro! 16.0 22.5 22.0 20.0 0.0 22.0 0.1 18.5 25.0 18.0 6.6 15.1 6.1 2.1 8.0 20.5 12.0 25.0 25.0 2.5 19.0 14.0 15.0 2nd Poros 1.5 4.0 3.0 3.5 0.0 3.0 0.1 4.0 4.5 3.5 2.1 4.1 1.6 0.1 2.0 1.5 2.0 1.5 3.0 1.5 1.0 2.5 0.5 Prim Poros 14.5 18.5 19.0 16.5 0.0 19.0 0.0 14.5 20.5 14.5 4.5 11.0 4.5 2.0 6.0 19.0 10.0 23.5 22.0 1.0 18.0 11.5 14.5 21 0000000 0 10 0000 1.5 Anal. 0.1 0.5 0.5 3.5 0.1 1.5 0.1 1 01 00000 080900 00 02 Kaol 2.5 0.5 0.5 0.5 4.5 Tot Cal 0.2 0.6 0.5 0.0 41.0 0.1 33.5 0.1 0.2 0.0 10.0 0.5 12.0 31.5 1.5 0.0 0.0 0.0 0.0 0.2 0.0 0.0 0.0 QOG 3.0 2.0 1.5 10.5 1.0 9.5 1.0 1.5 4.0 11.0 1.5 9.5 2.0 1.0 1.0 5.5 11.0 3.5 4.5 1.0 2.0 12.0 2.5 FOG 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 0.0 0.0 0.0 0.5 0.0 0.5 0.0 0.1 0.0 0.1 0.5 0.0 0.0 0.5 0.0 Tot Chlor 4.0 1.5 0.0 0.5 0.0 3.5 9.0 6.5 0.5 0.0 0.0 0.0 0.0 0.0 0.0 2.0 2.5 1.0 1.5 9.5 0.0 5.0 0.0 sandstones. 49-2 Det. Clay 16.0 10.5 6.5 6.5 5.0 7.5 11.5 14.0 7.0 5.0 16.0 3.5 5.5 3.0 11.0 4.5 4.0 1.0 1.5 27.0 2.5 3.5 7.0 TST of Grain Size 3.08 3.19 3.19 3.20 3.16 3.09 3.20 3.02 3.17 2.99 3.28 3.07 3.02 3.05 3.09 3.04 3.24 3.07 2.98 3.37 3.02 3.12 3.05 PCP 23.6 24.1 23.6 27 25 31.1 35 23.6 25.6 26 22.5 21.1 25 32 18 27.7 23.5 28.1 29.1 11.6 23.5 30.5 21.6 Diagenesis # 4243444748495051525354555657586061626364656667 (cont.). 4.8 (ft) Table Depth 9079.0 9081.0 9086.0 9106.5 9107.0 9107.2 9107.5 9112.0 9114.0 9117.0 9119.0 9120.5 9124.0 9126.0 9127.0 9132.0 9137.0 9138.0 9138.5 9142.0 9143.0 9144.0 9145.0 finer- fragments the in Rock than #2 sandstones. core 49 #2 TST TST 49 shorezone in composition coarse-grained the detrital in on abundant control more sandstones. size Grain are facies CRFs) 4.49. and shelf Figure (VRFs grained 154 TST 49-2 8.975 ft This cored interval (8,975-9,038 ft, 2,735-2,755 m) is a thick fining-upwards/coarsening-upwards distal-shoreface/inner-shelf sandstone package (Figure 3.5). The lower part of the core is composed of bioturbated siltstones with interlayered 1 ft sandstones that become more abundant upward. The middle of this package consists of laminated and bioturbated 3-8 ft sandstones that are some ofthecoarsestofthisfacies. Thegrainsizeofthesandstonedecreasesandthe clay content increases upwards so that the top of the core is very similar to the base. The average grain size of this package is 3.21 d> (range 2.77 d> to 3.47 d>), total porosity averages 17% (range 2% to 30%), PCP averages 27% (range 14% to 35%), detrital clay averages 7% (range 3% to 18%). TST 49-2 9,076ft - Another thick fming-upwards/coarsening-upwards distal shoreface/inner-shelf sandstone package is sampled in this core interval (9,076­ 9,147 ft, 2,766-2,788 m) (Figure 3.5). Most of the sandstones in this package are between 3and5ftthick,thoughafewthinnerandthickersandstonesarepresent. Most of the sandstones fine-upwards from well-laminated, clay poor bases to bioturbated,clay-richtops. Approximately 10ftofwell-laminatedsandstones overlay 40 ft of siltstones and shales and cap the core. The average grain size of 14% this package is 3.16 d> (range 2.98 d> to 3.37 d>), total porosity averages (range 0% to 25%), PCP averages 25% (range 12% to 31%), detrital clay averages 7% (range 3% to 27%). Diagenesis - Figure 4.50 shows that there is a small increase in grain size in distal shoreface/inner-shelf sandstones with depth. The wide range of PCP's in any sandstone package (Figure 4.50) is due to the variability in detrital clay content (Figure 4.51). Figure 4.51 also shows that the porosity range is the same in all the 155 sandstone packages and is independent of depth. The positive relationship between total porosity and PCP, coupled with the absence of a relationship between grain size and porosity, implies that it is the material between the framework grains, not the size of the grains, that is the primary control on the diagenesis and resultant preserved porosity in these sandstones (Figure 4.50). Feldspar overgrowths (FOGs) form during early diagenesis of many sandstones and are probably related to the presence of meteoric water (Galloway, 1984; Loucks et al., 1986; Bjorlykke et al., 1986; Glassman et al., 1989 a,b; Milliken, 1990). The occurrence of (subsequently compacted) FOGs in these distal-shoreface/inner-shelf Frio sandstones indicates that there was some influx of meteoric water early in the diagenetic history of the rocks. The absence of FOGs in of the more distal shelf sandstones implies that meteoric water only effected any the shorezone and the most proximal inner-shelf sandstones. I/S, chlorite, QOGs, analcime, calcite, and kaolinite are all formed in the burial diagenetic environment. Ashasbeen seeninthepreviouscores,diagenesisofindividualTST49­2 sandstones can be incredibly heterogeneous. Samples #47 through #5O (Figure 3.5, Table 4.8) come from a 1 foot section of sandstone, are approximately the same grain size and contain similar amounts of detrital clay. Fluid flow at the time of quartz precipitation was clearly focused through the structureless samples #47 and #49 (11% and 10% QOG respectively) as opposed to the laminated samples #4B and #5O (1% QOG each). Later fluids traveled exclusively through the then more porous and permeable laminated samples and precipitated calcite which completely filled the intergranular space. Samples #l6, #l7, and #lB are another laminated, massive, laminated sample series from a single sandstone (Figure 3.7, Table 4.8), however, in this unit there has not been any segregation of diagenetic changes. Figure 4.23 shows how analcime precipitation has been concentrated on certain laminae. Kaolinite also often shows a strong laminar control. Figure 4.52 shows that the lowermost distal-shoreface/inner-shelf sandstones contain the most QOGs and the range in calcite content in all the sandstone packages is approximately the same. Analcime and kaolinite are presentinallthesandstonesexceptthoseinthe8535ftcore(Figure4.53). These relationships indicate that the same factors that operate to concentrate certain diagenetic products in specific sandstones (or even parts of sandstones) also work on the scale of the whole sandstone package. In separate sandstone packages secondary porosity is related to available PCP and permeability (Figure 4.54). Arco TST 430-5 This core (from a wildcat well in Nueces county) samples a coarsening upwards distal-shoreface/inner-shelf sandstone package (depth rangelo,o7l­-ft; 3,069-3,087 m). Very well laminated fine-grained sandstones are between 3 and 10 ft thick and commonly have erosional contacts (Figure 4.47). Reactivation surfaces are common within the sandstones. The upward-coarsening nature of the core presents itself as an actual increase in framework grain size which is accompanied by an increase in the sandstone unit thickness, and a decrease in the amount of bioturbation. Abundant planar and trough cross-beds, delineated by clay clasts as large as 10 mm, are slightly disrupted by minor bioturbation. The average grain size of this package is 3.55 (range 3.25 d> to 3.97 d>), total porosity averages 2% (range 0% to 9%), PCP averages 13% (range 3% to 23%), detrital clay averages 32% (range 5% to 54%). The diagenesis of these sandstones is dominated by I/S recrystallization and chlorite precipitation (trace amounts of kaolinite are also present in most 430­ 5 sandstones). Chlorite rosettes in these sandstones can be larger than 20 mm and commonly fill the little intergranular space not occupied with I/S matrix. Table 4.7 shows that PCP, and consequently the presence of significant amounts of QOGs, calcite, or preserved porosity, is directly related to the abundance of matrix and pseudomatrix in the sandstones. The relatively high porosity and PCP in clay-rich sample #6 is due to the fact that most of the detrital clay in this sandstone occurs as uncompacted clay clasts which are less detrimental to permeability than sandstones true matrix or pseudomatrix. Preferential fluid flow in the clay poor was maximized in the coarsest-grained units (sample #1). 157 Figure 4.50. Porosity, grain size, and PCP relationships in TST 49 #2 sandstones. The wide range of PCPs in any sandstone package is due to the variability in detrital clay content. The primary control on diagenesis of these sandstones is PCP, not framework grain size. 158 159 range porosity The depth. of sandstones. #2 49 independent TST is in and content packages clay detrital sandstone and #2 49 Porosity TST all in 4.51. same the Figure is QOGs most the contains package sandstone lowermost The sandstones. 49-2 TST in Calcite and QOGs 4.52. Figure 160 the is sandstones the of content calcite the in range The text). (see sandstones other the to relative position its of because packages. sandstone the all in same 161 water formation to due be may analcime Abundant sandstones. #2 49 TST in kaolinite and Analcime 4.53. Figure apparant not is sandstones 8535.5 core in kaolinite or analcime of lack the for reason The 7). Chapter (see chemistry themselves. rocks the from 162 between relationship The sandstones. #2 49 TST in flow fluid and Porosity Secondary 4.54. Figure mineral on control important an is flow fluid that indicates PCP available and porosity secondary dissolution. Cecelia Kelley #2 This wildcat core from Nueces county samples two small distal­shoreface/inner-shelf sandstone intervals (11,815-11,862 ft; 3,601-3,615 m and The 12,176-12,206;3,711-3,720m). uppercoreisaslightlyupward-coarsening, heavily bioturbated clay rich sandstone. The lower core (Figure 4.55) is composed of fine-grained sandstones and siltstones that thicken upwards from ~ ft to ~10 ft. Sandstones from both cores are interlayered with shales. The average grain size of these sandstones is 3.23 d> (range 2.95 d> to 3.53 d>), total porosity averages 4% (range 0% to 10%),PCP averages 22% (range 15% to 31%), detrital clay averages 7% (range 1% to 14%) (Table 4.9). The normal sequence of burial diagenetic changes is seen in these cement is calcite. The sandstones though by far the most significant average calcite content of these samples is 23%. The only sample with significant porosity is #2 (Table 4.9), which shows evidence of approximately 2% calcite dissolution. This sample also contains the most kaolinite in the core, indicating that this sandstone was the preferential flow path for late diagenetic fluids. The high of abundant CRT's in the calcite content ofthese samples is due to the presence sandstones themselves (Table 4.2) and to the high calcite content of the interlayered shales (see Chapter 6). Wainco Mutchler#1 This wildcat core samples another coarsening-upwards/fining-upwards distal-shoreface/inner-shelf sandstone package from Nueces county (Figure 4.55). The cored interval is from 8,775-8,817 ft (2,674-2,687 m). Most of the sandstones are heavily bioturbated with little preserved laminations. Contacts between very the sandstones are usually sharp. The average grain size of these sandstones is 3.21 0 (range 3.04 to 3.62 d>), total porosity averages 8% (range (range 0.86 d> to 2.58 d>), total porosity averages 8% (range 0% to 22%), PCP averages 31% (range 3% to 46%), detrital clay averages 4% (range 0% to 35%) (Table 4.10). Tot. Poros 4.0 2.0 1.0 2.0 1.5 0.0 22.0 9.5 7.5 18.0 5.0 14.5 7.5 0.0 8.0 14.0 4.5 20.0 1.0 25.0 2.2 18.0 2.0 11.0 2.0 32.0 2nd Poros 2.0 0.0 0.0 0.5 0.5 0.0 6.0 4.5 1.5 4.0 1.0 1.5 2.5 0.0 3.0 6.5 3.0 6.0 0.5 4.0 2.1 1.5 1.5 2.0 3.5 0.5 Prim Poros 2.0 2.0 1.0 1.5 1.0 0.0 16.0 5.0 6.0 14.0 4.0 13.0 5.0 0.0 5.0 7.5 1.5 14.0 0.5 21.0 0.1 16.5 0.5 10.5 0.0 28.5 01 1 Anal. 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 0.1 u 1.5 Kaol. 0.2 2.0 0.2 2.0 2.5 0.5 5.0 7.5 9.5 8.5 1.0 9.0 11.0 0.0 5.0 4.0 0.2 8.0 0.5 0.0 0.0 3.0 0.0 0.1 0.0 0.0 Tot Cal 1.5 49.5 50.0 34.5 55.0 68.5 18.0 12.0 0.0 1.5 57.0 1.0 19.5 53.0 32.0 20.0 39.0 1.5 51.0 0.0 22.5 13.5 2.0 22.0 30.5 1.5 sandstones. QOG 0.5 0.0 0.0 0.5 0.2 0.0 3.0 3.5 1.0 2.0 0.0 3.5 1.0 0.0 0.5 1.0 0.2 1.5 0.2 1.0 0.0 0.0 0.5 0.5 0.1 0.5 104/7 Tot Chlor 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.0 0.0 3.0 0.5 2.0 1.5 Copano Det. Clay 34.50 7.00 3.00 4.50 0.00 0.00 0.00 1.00 19.00 0.00 0.00 0.50 0.50 0.00 0.00 2.00 0.00 0.00 0.50 11.0 13.0 0.0 25.5 2.0 1.0 1.0 and 67 Grain Size 2.58 2.52 2.44 1.92 0.84 2.04 2.02 1.92 2.61 1.83 2.05 1.69 2.00 2.21 2.08 1.79 1.96 1.70 2.21 2.96 2.73 2.11 2.70 2.19 1.87 2.47 MSW of PCP 2.7 38 39.2 24.5 42.7 45.5 36 25 16.5 25 45.5 26 32 38.5 34.5 29.5 24.9 24 42.7 22 16.1 28.5 1.6 26.7 20.6 30.5 Diagenesis whole caliche "grey" "beige" "white" black/blue concretion outside only 7654321 total ss 104-7 4.10. 67 (ft) ST Table MSW Depth 8048 8053 8070 8072 8073 8075 8075 8080 8086 8088 8090 8094 8095 8102 8102 8103 8103 8103 8104 Copano 6908.0 6917.0 7154.0 7158.0 7163.0 7163.5 7171.0 Figure 4.8 shows the extensive diagenetic mottling common to the lower Minnie S. Welder #67 sandstones. Most of these calcite concretions are white or gray in color and show the textures previously discussed as characteristic of early, syndepositional cements. These early cemented sandstones have very high PCP's (#l-43%, #6-39%, #9-46%, #l4-45%). This calcite is extensively dissolved (Figure 4.21), leading to the bizarre textures seen in Figure 4.8. Kaolinite precipitation overlapped and continued after calcite dissolution. In some of the sandstones, FOGs were formed at about this time as well. Later, after further burial and the formation of minor QOGs, burial diagenetic calcite precipitated. This calcite precipitated in the porosity formed by the dissolution of the syndepositional cement, preserving PCP's of 35%, 31%, and 36% (samples #5, #7, and #l3), and in primary pores never filled by early calcite, preserving PCP's of -25% (samples #2-#4, #l2). Burial diagenetic calcite also shows evidence of later dissolution. Where burial diagenetic calcite does not fill When it all the intergranular space the core appears beige colored (Figure 4.8). completely fills the IGV the core appears black (Figure 4.8). Another episode of kaolinite precipitation occurred soon after the calcite was dissolved and calcite remnants can be found within kaolinite patches. Dissolution of feldspars continuedpastthetimeofkaoliniteprecipitation. Thediagenesisofcalcite cement is discussed further in Chapter 5. Minnie S. Welder #70 The core from Minnie S. Welder #7O (Portilla Field, San Patricio county) is a 238 ft (7,445 ft 7,211 ft, 2,269 m 2,198 m) shorezone sandstone package composed of stacked shoreface and back-barrier facies rocks. The core log for Minnie S. Welder # 70 is shown in Figure 4.56; shoreface facies units are labelled "S" and back-barrier facies units are labeled "B" in this figure and in Tables 4.1 and 4.11. The SP log (Figure 3.3) shows that the sandstone package is part of the sand-rich, stacked, barrier and strandplain depositional system. The grain size of Minnie S. Welder #7O sandstones is highly variable. In general, back-barrier sandstones are finer-grained than shoreface sandstones, 168 Figure 4.56. "Weathered Outcrop" profile of Minnie S.Welder#7O. Letters correspond to different depositions facies, S = shoreface, B = bac-barrier (Table 4.1). Back-barrier facies sandstones are more likely to be modified by syn­depositional diagnesis than high-energy shoreface sandstones. The average porosity of sandstones from this core is 15%, but the porosity in shoreface sandstones is much higher. 169 however, lower-shoreface sandstones from unit SI have the smallest average grain-size in the core (Figure 4.56, Table 4.1, Table 4.11). The is no relationship between a unit's average grain size and QFL composition, in fact, the two coarsest units, S 2 and S4, have considerably different framework compositions (75:6:19 vs. 57:13:29). Figure 4.57 shows the distribution of porosity in Minnie S. Welder # 70 sandstones. All units show a large range in total porosity, however, middle­shoreface units S2, S3, and S 4 have the highest average and smallest range. Relatively high-energy back-barrier facies sandstones (B 2 and B4) also have a significant amount of preserved porosity. Diagenesis of Minnie S. Welder # 70 sandstones proceeds along the pathway shown in Figure 4.6. These shorezone sandstones have been extensively modified by syndepositional diagenetic events. The lowermost unit, SI, is a very fine-grained, clay-rich, lower-shoreface sandstone. Ophiomorpha burrow density is high at the base of this unit and decreases upwards. Syndepositional, isopachous, calcite cement is common towards the top of the unit. Preserved porosity and sandstone reservoir quality of this unit is strongly controlled by detrital matrix content. An erosive contact separates back-barrier facies B 1 rocks from the underlying, lower-shoreface S 1 facies sandstones. B 1 is composed of several stacked 2-4 ft thick sandstones. The upwards fining nature of the log and the core, the of presence numerous reactivation surfaces and basal-lag intraclasts, and the absence of extensive bioturbation indicates that these back-barrier sandstones were deposited in a relatively high-energy environment, perhaps a distributary or tidal-inlet channel. Basal-lag intraclasts in the B 1 channel fill are eroded pieces of isopachous-calcite cemented S 1 sandstones. This occurrence attests to the very early and shallow nature of syndepositional calcite precipitation. Syndepositional calcite cement also occurs in B 1 sandstones and is common towards the top of the unit, though as a whole, reservoir quality is a function of detrital clay content. The thickest unit in this core package is the middle-shoreface facies S2. Coarse grain size and lack of detrital clay combine to yield -20% preserved back- indicates marker open sandstone, shorezone indicates marker Closed porosity. #7O Welder S. Minnie 4.57. Figure Secondary average. lower and range larger show sandstones back-barrier - depth vs. porosity Total sandstone. barrier porosity. secondary more have and grained coarser are sandstones shorezone - size grain vs. porosity 171 Kao 0.2 0.2 0.3 0.2 0.2 0.0 0.0 0.0 1.0 0.0 0.0 0.6 1.1 1.6 0.0 0.3 1.0 Anal 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Tot Cal 0.0 0.0 0.0 0.0 0.0 28.5 44.0 1.0 5.0 3.7 2.0 0.0 1.7 0.0 0.0 0.3 0.0 FOG 0.0 0.0 0.0 0.0 1.0 0.5 0.0 1.0 0.5 0.0 0.2 0.0 0.0 0.5 0.0 0.3 2.0 QOG 3.0 3.7 0.7 1.0 2.5 3.0 0.0 5.9 1.0 1.3 1.5 0.6 2.8 1.0 1.2 5.0 4.0 Tot I/S 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Tot Chlor 14.0 9.7 1.3 0.5 1.0 0.5 0.0 0.5 1.0 11.3 3.5 3.3 3.4 2.1 0.6 1.0 3.5 Dep Clay 9.0 6.7 17.3 5.5 0.0 0.0 0.0 0.0 0.5 2.7 4.5 13.3 18.4 10.5 20.8 5.0 0.5 sandstones. 2nd Poros 2.3 3.3 4.0 5.0 3.5 3.0 0.0 5.9 2.2 5.7 5.0 2.8 3.4 4.2 2.3 4.0 5.5 #7O Prim Poros 8.0 9.0 1.7 16.5 15.0 5.5 0.0 12.8 20.9 21.7 20.9 6.1 7.3 12.6 2.9 18.7 13.4 Welder S. lot Poros 10.3 12.3 5.7 21.4 18.5 8.5 0.0 18.7 23.2 27.3 25.9 8.9 10.6 16.8 5.2 22.7 18.9 Minnie PCP 25.1 22.5 4.0 18.2 19.7 28.5 30.0 19.7 25.4 35.0 26.2 7.2 11.2 16.8 4.6 25.3 23.9 of Grain Size 2.44 2.48 2.23 2.23 2.15 1.93 1.96 1.90 2.41 2.73 2.68 2.81 2.47 2.43 2.71 1.74 1.87 Diagenesis Unit B6 S5S5S5S5S5S5S5S5S5 B5B5B5B5B5 S4S4 4.11. Table Sample 7214.0 7218.0 7223.0 7228.0 7231.0 7235.0 7235.0 7235.0 7238.0 7245.0 7255.0 7261.0 7265.0 7266.0 7267.0 7268.0 7270.0 172 Kao 0.5 2.7 2.0 0.7 3.5 2.5 0.0 0.5 0.0 0.0 0.0 0.0 0.5 2.5 0.5 8.4 0.5 4.0 Anal 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 Tot Cal 0.0 0.0 0.0 0.0 0.0 0.5 0.0 0.2 0.0 0.0 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 FOG 0.8 0.0 0.0 0.0 0.0 0.0 0.2 0.2 1.6 0.7 1.2 1.5 0.0 0.0 1.7 0.2 0.0 0.0 OOG 4.0 1.7 1.0 1.3 2.0 2.5 2.0 6.0 5.8 5.5 2.0 4.5 1.5 1.0 4.0 0.2 2.0 2.0 Tot Chlor Tot l/s 5.0 0.0 2.0 0.0 4.5 0.0 1.0 0.7 1.0 0.0 4.5 0.0 8.5 0.0 6.5 0.0 4.2 0.0 3.0 0.0 0.5 0.0 4.5 0.0 1.0 0.0 1.0 0.0 3.0 0.0 3.5 0.0 1.5 0.0 1.5 0.0 Dep Clay 0.5 9.6 8.5 4.0 7.5 2.0 2.0 5.0 0.5 0.0 0.0 0.0 14.0 11.1 8.0 12.4 5.0 5.0 of MSW 70 sandstones. Tot Poros Prim Poros 2nd Poros 17.0 13.0 4.0 16.6 14.6 2.0 20.5 17.0 3.5 24.3 18.7 5.7 24.5 21.0 3.5 20.0 17.5 2.5 26.9 23.9 3.0 10.0 9.0 1.0 15.8 15.8 0.0 26.4 23.9 2.5 30.4 25.9 4.5 20.5 15.5 5.0 14.5 12.0 2.5 12.1 8.5 3.5 20.9 16.0 5.0 5.4 4.5 1.0 24.0 21.0 3.0 20.2 17.1 3.0 Table 4.11 (cont.). Diagenesis Sample Unit Grain Size PCP 7272.0 S4 2.34 19.8 7276.0 S4 2.15 20.9 7278.0 S4 1.99 24.5 7281.0 S4 1.98 22.3 7282.0 S4 2.23 27.5 7285.0 S4 2.33 27.0 7286.0 B4 2.53 30.2 7287.0 B4 2.54 18.7 7290.0 B4 2.53 25.3 7292.0 S3 2.60 33.2 7295.0 S3 2.57 29.7 7297.0 S3 2.30 24.0 7300.0 B3 2.27 15.0 7301.0 B3 2.02 13.1 7302.0 B3 2.17 25.2 7304.0 B3 2.69 15.8 7305.5 B3 2.36 25.0 7306.0 B3 2.36 25.2 0.5 0.5 0.0 0.0 0.0 0.2 0.0 0.5 0.3 0.2 0.0 0.0 0.3 0.0 0.0 0.0 0.0 0.0 0.0 Kao Anal 1.0 1.5 0.0 0.0 0.0 1.0 0.0 1.5 2.7 1.0 0.0 0.0 1.5 0.3 1.5 0.0 0.3 0.0 0.0 Tot Cal 0.7 0.0 0.5 50.8 0.2 8.2 48.2 2.0 1.4 23.3 33.4 48.3 0.0 0.5 19.0 7.4 47.2 0.0 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 0.0 0.5 2.0 FOG 1.0 0.5 1.5 0.5 1.0 1.0 0.0 1.0 1.0 0.5 1.0 0.2 1.0 0.0 0.5 0.2 0.0 8.0 6.5 OOG Tot I/S 0.5 23.4 31.1 0.0 5.5 2.0 0.0 2.0 3.1 1.0 1.5 0.5 7.5 11.1 6.5 11.4 0.0 9.0 0.5 Tot Chlor 2.0 0.2 0.3 0.0 4.5 3.5 0.0 6.0 0.7 1.0 1.5 1.0 1.5 1.5 7.5 5.5 0.0 1.5 5.5 Dep Clay 0.5 8.5 5.0 0.0 2.5 1.0 0.5 0.5 0.3 2.0 0.5 1.5 0.5 21.3 1.5 7.4 0.0 0.0 0.0 2nd Poros 3.0 1.0 0.0 0.5 2.5 2.5 0.5 3.0 2.1 2.5 1.2 0.2 3.5 1.5 1.5 3.0 0.8 3.5 3.5 sandstones. Prim Poros 23.8 3.0 5.0 1.0 19.5 17.8 0.0 27.9 24.0 10.4 6.5 1.5 19.5 5.1 10.0 4.0 1.0 14.5 17.5 70 MSW Tot Poros 26.8 4.0 5.0 1.5 21.9 20.3 0.5 30.8 26.0 12.9 7.7 1.7 23.1 6.6 11.5 6.9 1.8 18.0 21.1 of PCP 27.8 18.2 26.8 49.2 28.4 31.9 41.7 37.3 31.5 33.0 36.4 43.0 27.3 12.9 32.0 19.1 38.8 30.5 27.6 Diagenesis Grain Size 2.53 2.52 2.90 2.70 2.62 2.52 2.31 2.66 2.75 2.63 2.75 2.91 2.64 2.79 2.56 2.31 2.31 2.23 2.39 (cont.). Unit B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3 B2B2 4.11 Figure Sample 7307.0 7310.0 7312.0 7314.0 7317.0 7317.1 7317.1 7317.1 7318.0 7318.5 7320.0 7321.0 7322.0 7325.0 7327.0 7331.0 7331.0 7339.0 7342.0 174 Kao 0.0 1.0 4.5 0.5 3.5 5.0 0.0 1.0 0.0 1.0 1.5 0.2 0.3 1.5 0.0 0.2 0.0 0.0 0.0 Anal 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 .0.0 0.0 0.0 0.0 0.0 0.0 0.6 0.0 Tot Cal 0.0 0.0 0.0 3.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 17.0 29.9 44.1 0.0 0.0 FOG 2.5 0.2 0.2 0.2 0.0 0.0 1.5 1.0 0.5 1.0 0.2 0.2 1.0 0.2 0.0 0.0 0.0 0.0 0.3 OOG 5.0 2.0 2.5 0.0 0.3 2.0 3.0 2.0 1.0 4.5 2.0 2.0 1.0 4.0 0.5 1.5 1.5 0.6 2.6 Tot I/S 0.0 0.5 1.5 2.0 0.5 4.5 4.0 1.0 4.4 0.0 1.5 3.0 1.6 3.0 16.5 2.0 0.5 8.5 18.6 Tot Chlor 3.0 7.4 15.4 3.5 4.5 6.0 1.0 4.5 4.4 0.5 5.5 11.0 8.3 7.5 5.0 0.2 0.3 20.4 4.1 Dep Clay 0.0 10.9 1.5 0.0 5.0 8.0 0.0 0.5 0.0 0.5 0.0 0.5 7.3 0.0 4.0 9.0 0.5 6.8 1.0 2nd Poros 3.0 1.5 1.0 3.7 3.0 2.0 6.5 2.5 3.9 2.5 3.5 2.2 2.6 2.0 0.5 1.0 0.5 0.8 2.6 sandstones. Prim Poros 15.0 5.4 6.5 15.8 14.1 6.0 14.0 23.5 20.2 21.0 19.5 12.5 15.1 13.0 1.0 1.5 12.0 11.3 5.7 70 Tot Poros 18.0 6.9 7.4 19.6 17.1 8.0 20.5 26.0 24.1 23.5 22.9 14.7 17.7 15.0 1.5 2.5 12.5 12.2 8.3 MSW of PCP 23.5 14.6 25.1 20.5 21.4 21.1 21.0 31.0 28.1 28.0 27.2 24.4 21.6 24.7 33.0 29.4 37.3 37.4 28.7 Diagenesis Grain Size 2.25 2.22 2.34 1.92 2.06 2.27 1.84 1.83 1.63 1.62 1.90 1.97 2.08 2.01 2.52 2.46 2.53 2.51 2.62 (cont). Unit 4.11 B2B2B2B2 S2S2S2S2S2S2S2S2S2S2B1B1 B1B1B1 Figure Sample 7346.0 7347.0 7351.0 7354.0 7364.0 7365.0 7366.0 7368.0 7372.0 7376.0 7385.0 7390.0 7397.0 7398.0 7408.0 7409.0 7410.0 7411.0 7411.5 Kao 0.0 0.0 4.5 0.0 0.0 0.5 0.5 0.0 0.0 0.0 0.2 0.2 0.3 0.0 0.0 0.0 Anal 0.0 0.0 0.3 0.0 0.2 0.3 0.3 0.2 0.0 0.0 0.0 0.5 0.0 0.0 0.0 0.0 Tot Cal 0.0 0.0 0.0 0.0 0.5 0.0 0.0 4.0 53.7 3.2 3.5 12.9 3.0 31.4 3.5 0.0 FOG 0.0 0.3 0.3 0.5 0.2 0.3 0.8 0.0 0.3 0.2 0.0 0.0 0.0 0.0 0.0 0.0 OOG 0.5 2.0 0.5 2.0 1.5 0.5 0.5 1.0 0.5 1.0 0.2 0.2 0.3 0.0 0.3 0.3 Tot I/S 8.2 4.0 3.0 3.0 1.0 12.5 4.0 5.0 0.0 10.9 11.9 9.4 12.7 0.5 21.7 12.7 Tot Chlor 14.4 12.0 6.0 10.6 7.4 9.0 6.5 13.0 0.5 7.4 10.0 8.4 5.6 0.2 3.0 2.5 Dep Clay 19.5 2.0 0.5 0.0 0.0 1.0 1.5 0.0 0.0 0.5 11.9 0.0 7.6 14.0 9.1 13.2 2nd Poros 1.0 2.5 3.5 4.5 3.0 3.5 3.5 2.5 1.0 2.0 1.5 0.5 1.5 0.0 0.5 0.0 sandstones. Prim Poros 2.1 13.5 13.0 19.6 19.8 9.5 21.1 11.0 8.2 14.9 5.0 12.4 6.1 0.0 3.0 6.1 70 Tot Poros 3.1 16.0 16.5 24.1 22.8 13.0 24.6 13.5 9.2 16.8 6.5 12.9 7.6 0.0 3.5 6.1 MSW of PCP 21.5 29.3 24.1 34.2 30.2 24.5 30.2 24.7 46.8 29.7 21.9 31.2 19.3 18.2 18.9 19.6 Diagenesis Grain Size 2.53 2.33 2.32 2.44 2.40 1.68 1.91 2.39 2.39 2.54 2.55 2.50 2.61 2.89 2.66 2.84 (cont). Unit 4.11 B1B1B1B1B1B1B1B1B1SISISISISISISI Figure Sample 7414.0 7415.0 7418.0 7419.0 7421.0 7422.0 7422.5 7423.5 7423.5 7424.5 7427.0 7428.0 7430.0 7435.0 7440.0 7445.0 porosity in most of the samples from these sandstones. Porosity is minimized in laminated samples because these rocks have the highest detrital clay content. Recrystallized I/S and chlorite are the most abundant diagenetic minerals in S 2 sandstones. QOGs and FOGs are übiquitous but volumetrically insignificant, and there is no diagentic calcite. The lack of diagetic cements is texturally obvious in S 2 sandstones; while not "crumbly," these rocks are poorly indurated and are easily broken by hand. The contact between S 2 and B 2 is gradational. Clay-rich, chondrites-like burrows ,and the abundance of reactivation surfaces, indicate that the environment of deposition of B 2 is different from S2. B 2 sandstones probably were deposited in some kind of sand-flat, or tidal-inlet delta environment. B 2 sandstones have a mottled appearance (similar to Figure 4.8) due to the heterogeneous distribution of detrital and recrystallized clay. Relatively abundant QOGs and kaolinite suggests that the clay-free portions of these sandstones were exposed to a large amount of fluid flow. Figure 4.58 shows that, in general, QOGs are more abundant in coarser-grained sandstones, indicating their relationship to fluid flow (Mcßride, These 1984). Units S 2 and S 4 have the most QOGs in the MSW 70 core. sandstone units also have rather high QFL compositions (Table 4.1), indicating that a trigger for precipitation is also important. Unit B 3 is composed of sandstones from diverse back-barrier environments. The lower portion of the unit includes abundant oyster shells, wood pieces, very large clay intraclasts, reactivation surfaces, and is heavily bioturbated and mottled. Chlorite, recrystallized I/S matrix, syndepositional calcite, and analcime are the common diagenetic cements. Calcite concretions very commonly show ash-shard replacement textures (Figure 4.21). Analcime is almost always closely associated with syndepositional calcite concretions. Burial diagenetic calcite is rare in MSW 70 sandstones. It is most common in unit 83, where it precipitated near syndepositional calcite concretions. The core from Minnie S. Welder#7Oiswellinduratedinareasofcalcitecement. TheupperportionofB 3 is made up of many upward-fining thin (<1 ft), clay-rich sandstones. Trough cross-beds and ripples are common sedimentary structures. Reservoir quality is controlled by clay content. Preserved porosity in unit B 3 can be quite high, sandstone, shorezone indicates marker Closed %. Calcite and % QOG vs. Size Grain #7O Welder S. Minnie 4.58. Figure Calcite sandstones. shorezone coarser in common more QOG's Abundant sandstone. back-barrier indicates marker open size. grain to related not is and sandstones back-barrier in common more is syndepositional) (predominately 178 however, its distribution, even in thin section, is very heterogeneous. Figure 4.13 shows recrystallizing matrix in a lower B 3 sandstone. presence The of this material in the intergranular space severely restricts fluid flow. Middle-shoreface unit S 3 grades upward into back-barrier unit B 4 in a similar manner as unit B 2 is gradational with unit S2. S 3 is a salt-and-pepper sandstone (as are SI, S4, and S 5 (Figure 4.23)) with few sedimentary structures. B 4 is very similar to 82, clay-rich, chondrites-likt burrows and reactivation surfaces are common in these fine, white sandstones. Relatively abundant QOGs in both units are probably partly related to the high detrital quartz content of the sandstones (Table 4.1). Calcite is absent in these rocks and reservoir quality is controlled by the amount ofrecrystallized, originally detrital, matrix I/S. Unit S 4 is composed of relatively coarse-grained, middle-shoreface sandstones. Many of the sandstones preserve an upwards-fining texture (7281, 7278, & 7276, and, 7272, 7270, and 7268). Table 4.11 shows that the tops of these upward-fining sandstones contain more recrystallized I/S and consequently less preserved porosity than their bases. The presence of abundant QOGs in the upper S 4 sandstones indicates that fluid flow was focused towards the top of this unit. Evidence of strongly preferential fluid flow of this nature is relatively uncommon in these shorezone sandstones. The high sand content of the whole package (relative to the sand content of a shelf package) is probably responsible - for this relationship there is less impetus for focused flow if almost all of the rocks in a sandstone package are highly permeable. The composition and diagenesis of middle-shoreface unit S5, and back- barrier sandstones B 5 and 86, is similar to that found in rocks of the same facies. Reservoir quality is controlled by the presence ofrecrystallizing I/S matrix. Calcite is only present as an early concretion in S5. In conclusion, the diagenesis of Minnie S. Welder #7O sandstones follow the patterns developed in the previous cores. The occurrence of abundant QOGs and secondary porosity (Figure 4.57) is related to grain size, which, like PCP, is related to fluid flow. In these shorezone sandstones, permeability is maximized in thick, shoreface and higher energy (B 2 and B4) back-barrier sandstones. The detrital control on permeability is clay content, more detrital clay is incorporated 179 (through laminations or bioturbation) into the lower-energy sandstones of the back-barrier facies. Additionally, many of these lower-energy facies rocks contained significant amounts of detrital volcanic ash, which alters easily to syndepositional analcime and calcite. Figure 4.59 shows that porosity in MSW 70 sandstones is maximized in the presence This amount of of ~5% IGV filling clay. clay enhances reservoir quality by inhibiting later precipitation of QOGs and calcite. Copano TST 104 #7 The condition of this core prevented accurate logging, but the sandstones themselves, and the SP log, indicate that this core is from the middle­ shoreface depositional environment. Two cored intervals, ostensibly 6885-6917 ft, and 7153-7172 ft, were sampled. The sandstones are coarse-grained, and, where not filled with detrital clay, preserve significant amounts of porosity. Rock The fragments, including CRFs, are especially common (Tables 4.1 and 4.10). most common of the diagenetic mineral is calcite. Analcime is present in many samples. Its presence in these sandstones, as well as in sandstones from TST 49 #2, is probably related to the very saline nature of the associated formation water from this fault block. Discussion The major diagenetic "events" that have modified Frio Formation sandstones (QOG, burial diagenetic calcite and kaolinite precipitation, and secondary porosity development) can be correlated to either sandstone grain size available PCP. or Both of these parameters are related to permeability and have been shown to be correlatable to the extent of diagenetic modification in sandstones from many diverse depositional environments, geologic ages, and locations (Bucke and Mankin, 1971; Blanche and Whitaker, 1978; Hawkins, 1978 Taylor, 1978; Dutton and Land, 1978; Irwin and Hurst, 1983; Winn et al., 1984; Mack, 1984; Blackboum, 1984; Haszeldine et al., 1984; Figure 4.59. Total Porosity % vs. IGV Clay % Closed markers indicate shorezone sandstones, open markers indicate back-barrier sandstones. Total porosity is maximized in the presence of ~5% IGV Filling clay (diagenetic and detrital) that prevents precipitation of later IGV filling QOG's and calcite. 181 Whitney and Northrop; 1987; Mcßride et al., 1988; Scotchman and Johnes, 1990; Gardiner et al., 1990; Cowan and Shaw, 1991; Howard, 1992; Barnes et al., 1992; Hartkamp et al., 1993). These relationships strongly suggest that preferential fluid flow is the major control on the diagenesis of sandstones, including those investigated here. Sandstone diagenesis occurs as a sequential series ofmore oi less discrete events (Taylor, 1978). Cement precipitating at time A will lower the porosity and permeability of sandstone that before time A had been the most porous and permeable pathway available for fluid flow. At a later time B, sandstones less modified by permeability-reducing cements precipitated at time A become the dominant fluid migration pathway and become the focus of precipitation of time B cements. This switching of preferential flow paths occurs different scales. at many Segregation of cements into separate laminations (Figure 4.11) is evidence of this process occurring on the thin section scale. Alternation of QOG and calcite cemented bands within individual sandstones (i.e. TST 49 #2 samples #47-#5O,Figure 3.5) is evidence ofpreferential fluid flow on the individual bed scale. Segregation of QOG dominated sandstones from calcite dominated sandstones in TST 346 #1 shows that the process operates on the sandstone package scale. On all these scales in the Frio, the original control on permeability is clay content, which is ultimately a function of depositional environment. This relationship between detrital clay content and extent of diagenetic modification (and reservoir quality) in sandstones has been identified from many different depositional environments and geologic ages (Taylor, 1978; Colter and Ebbem, 1978; Winn et al., 1983; Mcßride et al., 1987; Mcßride et al., 1988; Gardiner et al., 1990; Scotchman and Jones, 1990; Houseknecht and Ross, 1992; Howard, 1992). Application of this idea on an even larger scale can explain the differences in QOG content of TST 49-2 shelf sandstones. If we consider the whole interval from 8,500 to 9,200 ft as being composed of sand-rich/permeable packages separated by sand-poor/impermeable packages, then the QOG distribution seen in Figure 4.52 can simply be interpreted as having been developed by preferential fluid flow through the package with the coarsest grain-size. Another important factor affecting preferential fluid flow is the relative position of a sandstone in relation to the other members of the sandstone package. The presence of heavily cemented "stand-alone" sandstones in TST 392-4 and TST 51-1 (and other units (Shew and Gamer, 1990)) indicate that fluids preferentially flow through the first permeable pathway possible, even if more permeable units exist further inside the sandstone package. This effect also operates on many scales and is probably partially responsible for the QOG distribution seen in Figure 4.52. Ascending fluids leaving the sand-poor section below -9,200 ft first encounter, and will preferentially flow through the lowermost sand-rich package. At this scale, however, the focusing of flow is not complete and significant fluid still travels upwards and can be "caught" and transmitted laterally by more shallow sand-rich packages. The distribution of analcime and kaolinite in TST 49-2 sandstones (Figure 4.53) indicates that flow can completely bypass some sand-rich packages, for reasons not obvious from the sandstones themselves. Sandstones within shales do not only act as preferential pathways for fluid flow but actually attract fluid. Glezen and Lerch (19§5) and Rostron and Toth (1989) have shown that permeable sandstones act as lenses to focus flow lines and produce lateral migration of fluid flow into the sandstones from two to seven times the radius of the sandstone body itself. In other words, sandstones surrounded by shales effectively draw fluid to themselves. Figure 4.60 shows that spatialdistribution ofTST49-2 distal-shoreface/inner-shelf sandstones. Thelarge lateral extent of these sandstones allows for a great deal of fluid to be concentrated This into these more permeable rocks instead of their less permeable neighbors. focusing effect, which is ultimately a product of permeability differences, does not function in very sand-rich sections where the permeability of any one sandstone or sandstone package is approximately the same as any other (Glezen and Lerche, 1985). This is probably the reason why the diagenesis of thick shorezone sandstones is more homogeneous than the diagenesis of more distal sandstones. 183 184 #2 49 #2. TST of block fault permeability (1988). reservoirs, Morton average in and sandstone variations Galloway after Spatial modified 4.60. distal-shoreface/inner-shelf Figure Slightly Chapter 5 CALCITE Calcite is the most abundant diagenetic mineral in Frio sandstones in the Corpus Christi area, averaging 9% of the rock volume (Table 4.3). In shelf sandstonesthe abundance ofburial diageneticcalcite ispositively correlated to permeability (Chapter 4). In shorezone sandstones syndepositional and burial diagenetic calcite is abundant in back-barrier environments. This section investigates the petrography, trace element and isotopic chemistry of diagenetic calcite. Petrography One of the earliest diagenetic minerals to form in Frio sandstones is calcite (Figure 4.8). High preserved PCP's (Figure 4.20), floating grain textures, micritic crystal size, association with ash-shard textures (Figure 4.21), isopachous rimming texture, association with soil textures (calcite and kaolinite root cast and crack fill), and location in back-barrier and upper-shoreface/foreshore facies sandstones indicates that this calcite precipitated soon after deposition (c.f. Matthews, 1971; Bathurst, 1975; Longman, 1980; Machette, 1985; Blodgett, 1988). Syndepositional calcite in MSW 67 and MSW 70 sandstones has undergone extensive dissolution (Figures 4.8 and 4.21). Burial diagenetic calcite precipitates relatively late in the paragenetic sequence ofthe sandstones (Figure 4.6). In shorezone sandstones, burial diagenetic calcite is most common in back-barrier facies rocks, the pre-existing syndepositional calcite acting as a nucleus for precipitation of the later phase (Figure 4.8). In shelf sandstones the abundance ofburial diagenetic calcite, on the unit or laminae scale, can often be related to permeability (Chapter 4). The distribution of burial diagenetic calcite can range from patchy to completely filling all available intergranular space. Most samples of burial diagenetic calcite show some evidence of dissolution, though never demonstrably more abundant than one or two percent (Figures 4.8, 4.17, and 4.21). Feldspar dissolution continues past 185 the time of burial diagenetic calcite precipitation, and calcite erosional remnants can be found inside patches of kaolinite. With the exception of rare euhedral crystals that occur within secondary porosity, there is no petrographic evidence for an episode of post-dissolution calcite formation. Cathodoluminescence petrography shows that all burial diagenetic calcite is dully luminescent and unzoned. Trace Element Chemistry Based on stained thin sections, previous workers (Lindquist, 1976, 1977; Loucks et al., 1986; Diggs, 1992, Diggs and Land, 1993) have identified a time of Fe-poor burial diagenetic calcite formation (which stains pink) followed by an episode of Fe-rich burial diagenetic calcite precipitation (which stains purple). Staining suggests that both types of calcite can occur as both grain replacements and IGV fill, though calcite-replacing feldspar is more commonly Fe-poor (Figures 4.16 and 4.21). One or both types of calcite is present in individual thin sections. WDS analyses were performed on 21 diagenetic calcite samples using a JEOL 733 supermicroprobe with a 12 pm spot and 10 nA sample current. Analyzed elements and count times were Ca (10 sec.), Mg (20 sec.), Fe (40 sec.), and Mn (40 sec.). Chemically distinct calcite types are identifiable by electron microprobe analyses. Figure 5.1 shows how burial diagenetic calcite chemistry differs from syndepositional calcite chemistry in one shorezone thin section. In this sample the syndepositional calcite stained bright pink, the burial diagenetic calcite stained purple and the color difference supported the obvious textural differences between the two generations of calcite. Examples of microprobe analyses of burial diagenetic calcite are shown in Figures 5.2 through 5.4. MSW 70 7,314 (Figure 5.2) stained uniform bright pink, TST 10/2 10,432 (Figure 5.3) and TST 346/1 (Figure 5.4) stained purple in some areas, pink in others. The microprobe analyses, however, do not show separate The chemical populations, rather, they indicate a large range of compositions. 187 calcite syndepositional indicate circles Closed ft. 8053 67 MSW from calcite of chemistry element Trace 5.1. Figure clear are There replacements. shard ash indicate squares open cement, calcite "diagenetic" indicate circles open cement, cements. calcite differentiated easily three these of chemistry the in differences 188 showed section thin this in cement calcite The 7314. 70 MSW from calcite of chemistry element Trace 5.2. Figure color. pink uniform a stained calcite the though even chemistry element trace in variation significant thin this stained in parts chemistry other while element pink trace stained in calcite variation the The of ft parts 10,432 some 10/2 sample TST this in in calcite however, of chemistry 5.2, Figure in element that Trace to similar 5.3. is Figure section that supportchemistry. not water does data formation to microprobe Therelated is 3.1) calcites. (Figure 2 different block fault chemically from of calcite of generation one content than Mn more high implyingThe purple, hypothesis. 189 vs. for area random to method is stained due is calcite, pink unreliable chemistry is of diagenetic color distribution an element burial stain The of trace ft. samples calcite calcite 14,521all in 346/1 sandstones, and rangethis, The TST in these in that fromreplacement, precipitation. calcite indicates of grain calcite vs. This of chemistry Fill IGV analyses. episodesburial. element and during area, multiple Trace microprobe 5.4. stained the recrystallization differentiating Figure purplewitnin 190 distribution of calcite type (pink area vs. purple area, IGV vs. replacement) in the data is random; areas that stained pink are no more likely to be relatively low-Fe than relatively high-Fe. Grain replacement calcite has the same range in composition as IGV calcite. This indicates that subtle differences in stain color, such as the difference between pink and purple, do not indicate distinctly different calcite chemistry. Carbonate stain color is very procedure specific, the intensity of the stain is affected by etch time, stain time, stain strength, and less quantifiable parameters such as whether the sections were "swished" in the stain at the same velocity, whetherthesectionswererinsedinrunning waterorjustdipped,andwhetherall All these factors the sections equally clean at the start of the staining procedure. effect the calcite surface that the stain is supposed to affect and the amount of stain supplied to that surface. It is therefore proposed that in these siliclastic rocks subtle carbonate stain color differences are due to sample surface effects or other of two factors stemming from the staining process and do not indicate the presence chemically distinct generations of calcite. The microprobe analyses support the petrographic evidence for only one episode of burial diagenetic calcite formation. This hypothesis is further supported by the relationship between PCP and depth for sandstones with >5% ofPCP values in burial diagenetic calcite cement (Figure 4.7). The large range any one depth interval is due to variable matrix content (Figure 4.5) and the heterogeneous distribution of matrix and calcite in the sandstones. The PCP data show that significant compaction has not occurred deeper than -8,000 ft. This indicates that the cements that prevented compaction below this depth (burial diagenetic calcite being the most volumetrically important) formed at or above 8,000 ft. Sandstones containing only "late Fe-rich calcite" should have smaller PCP’s than sandstones containing "early Fe-poor calcite" (Boles and Ramsayer (1988) used this relationship as evidence of two generations of of calcite cement in Stevens and North Coles levee sandstones). The PCP range pink-stained calcite samples is the same as the PCP range of purple-stained calcite samples, indicating that they both formed at essentially the same burial depth. Considered along with the chemical and petrographic data, this indicates that there is only one episode of burial diagenetic calcite precipitation in Frio sandstones. The variation in calcite trace element composition is shown in Figures 5.5 The through 5.7. None of the elements show a significant trend with depth. exception is Mg: its loss with depth can be explained by the observation that the shallow, relatively high Mg calcites are syndepositional cements originally precipitated as aragonite or high-Mg calcite that have preserved a hint of their original chemistry through burial. Isotopes 8 13C and 8 180 were determined on 93 diagenetic calcite samples. The analytical procedures and fractionation factors used are from Land (1984). Tables 5.1 and 5.2 list the analyses. d13C The average 5 13C value for diagenetic calcite in Frio sandstones is -6.2 o/oo (PDB) (Figure 5.8). The 8 13C of most of the burial diagenetic calcite samples fall in a narrow range of between —4 to —9 o/00. This supports the hypothesis of Milliken et al. (1981) and Land (1984) that approximately 25% of the carbon reservoir has been derived from depleted organic carbon. The very depleted 5 some syndepositional calcites indicates a somewhat values of larger proportion of organic carbon, produced by bacterial oxidation of organic matter or sulfate reduction in the shallow burial environment. Enriched 5 13C values in syndepositional calcite are due to incorporation of carbon produced by bacterial fermentation (methanogenesis), which is also active in the shallow subsurface (Hudson, 1977; Curtis, 1978). Figure 5.9 shows that the average 8 13C of calcite is unchanged with depth The relatively enriched and depleted values at less than ~8500 ft are from syndepositional calcites, which only formed in shorezone facies sandstones. 192 Key to symbols: Fault block I=o, fault block 2= ., fault block 3= +, fault block 4= Q fault block 5=X (see Figure 3.1). Figure5.5. Mgcontentofcalcitecement. ThehighMgcontentofcalcitefromfault block 1 is due to the presence of syndepositional calcite that was originally high-Mg calcite or aragonite and has preserved a hint of that chemistry through recrystallization 193 Key to symbols: Fault block 1= O, fault block 2= fault block 3= +, fault block 4=o, fault block 5=X (see Figure 3.1). Figure 5.6. Fe content of calcite cement. 194 Key to symbols: Fault block I=o, fault block 2= ., fault block 3= +, fault block 4= ., fault block 5=X (see Figure 3.1). Figure 5.7. Mn content of calcite cement Fault block 2 is characterized by Mn-rich diagenetic calcite and allochthonous, Mn-rich brines. Differences in water chemistry and calcite trace element composition from adjacent fault blocks indicates that the fault blocks have acted as hydrologically separate systems. 195 Table 5.1 Isotopic analyses of diagenetic calcite. Data from MSW 67 and MSW 70 (Portilla field) are give in Table 5.2. del 13C Well Depth del 18 0 87Sr/86Sr Mutchler 8785.0 -8.6 -8.3 8792.0 -8.3 -5.5 Cecelia Kelly 11824.0 -4.1 -7.6 12185.0 -4.0 -8.0 0.707272 12191.0 -3.5 -8.0 12198.0 -4.0 -7.8 Shell 392-4 12263.0 -6.5 -10.9 12270.2 -5.0 -9.5 12270.5 -5.4 -10.2 12271.0 -4.6 -8.6 0.708314 12272.5 -5.9 -10.4 0.707469 12593.5 -6.1 -10.7 12595.0 -6.1 -9.0 Shell 346-1 14521.0 -8.4 -8.5 0.707112 14538.0 -4.6 -9.2 14550.5 -12.3 -9.4 14551.0 0.707450 14554.0 -11.7 -10.2 14555.0 -9.5 -9.7 Arco 430-5 10071.0 -7.6 -8.8 0.707916 10077.0 -8.5 -8.8 10095.2 -5.9 -7.2 Arco 470-4 9674.0 -5.0 -5.5 0.707850 9748.0 -1.6 -6.3 9757.0 -4.2 -7.1 9805.0 -20.8 -2.0 9965.0 -6.8 -7.0 10570.0 -4.6 -7.6 0.707740 10717.0 -6.0 -7.4 10832.0 -5.7 -8.1 10900.0 -2.9 -9.0 0.707497 11070.0 -6.9 -7.9 11115.0 -8.6 11322.0 -7.0 -8.0 0.707513 11463.0 -5.8 -4.7 Table 5.1 (cont.). Isotopic analyses of diagenetic calcite. Well DeDth del 13 C del 18 0 87Sr/86Sr Cities 51-1 10454.0 -7.4 -6.4 10463.0 -6.6 -6.4 10502.0 -8.2 -6.7 10507.0 -8.4 -6.7 Cities 49-2 8040.0 -3.8 -6.7 8040.5 -4.0 -5.4 8049.0 -4.3 -6.5 8539.0 -6.2 -7.0 8553.0 -4.8 -6.2 0.707827 8712.0 -4.7 -5.9 8722.0 -6.4 -5.9 8733.0 -6.5 -6.3 0.708643 8999.0 -6.8 -6.6 9026.0 -7.4 -6.9 9107.0 -6.6 -6.7 9107.5 0.6 -6.4 9119.0 -6.5 -6.0 9124.0 -8.4 -6.7 9126.0 -7.2 -7.2 0.710009 Cities 10-2 10399.0 -3.9 -6.0 10411.0 -4.7 -8.1 0.710216 10432.0 -5.1 -6.5 0.711388 10438.0 -3.2 -5.9 Cities 4-2 10398.0 -8.3 -7.2 10416.0 -11.3 -6.6 10429.0 -6.6 -8.1 Copano 104-7 7163.0 -4.6 -6.2 7163.5 -2.5 -7.2 197 Table 5.2: Isotopic composition of calcite from MSW 67 and MSW 70. MSW 67 Sample 8053 8064 8070 8075 B 8075 A 8075 P del 13C 9.8 0.1 2.4 -6 -3 -7.3 del 180 -6.2 -6.3 -6.6 -4.1 -4.2 -7.5 87Sr/86Sr Petrography Early mottles w/shards Early caliche mottle Early caliche texture Early caliche concretion Early caliche concretion Late IGV fill 8080 -6.6 -8 Late IGV fill 8090 8095 P 8095 C -14.2 -5.7 -6.2 -4.1 -5.5 -7.8 Early large poiks, later dissolved Early IGV fill Late IGV fill 8102 W 8102 B 8103 C 8103 P -17.7 -8.5 -4 -7.7 -3.1 -6.2 -6.3 -7.7 0.707818 0.70807 0.707488 0.708487 Early IGV fill Late spar w/early remnants Mostly late w/some early shard + grain rep Late IGV fill w/some dissolution 8104 MSW 70 -7.7 -6.8 Early IGV fill Sample 7235 C 7238 7314 C 7314 7318.5 del 13C -18.8 -2.2 -9.8 8 -8.3 del 180 -3.8 -6.1 -6 -7.1 -7.8 87Sr/86Sr 0.707364 0.707951 0.707852 Petrography Early concretion w/o dissolution Early rep and IGV fill Early shard & micritic concretion Early IGV fill Late IGV fill 7320 C 7320 7327 7409 7410 7423.5 7428 7435 -4.3 4.9 -19.2 -7.1 -8.5 -22.4 -6.5 4.6 -6.6 -7.2 -6.7 -4.9 -3.9 -4.7 -5.8 -7.1 0.707684 0.707359 Early shard centered concretion Late IGV fill w/shell frags Early shard centered concretion and later spar Early caliche texture Early large poiks, minor shard rep Early isopachous bladed rims Early isopachous rims, w/minor later spar Early calcihe texture and poiks 198 a carbon burial diagenetic indicates the average of o/oo burial 25% = The >~O o C 8 calcite, 13 diagenesis. approximatelyC, for early that source during syndepositional the indicates active • =oxidation value are cements. This microbial processes (PDB). a calcite Both o/oo for indicates C. source. -6.2 C the is 13 o/oo 8 for 13C organic 8 vs. <~-10 0 source an 8 18 calcite from 8 5.8 Figure calcite. fermentation diagenetic derived is 199 Figure 5.9. 8 vs. depth for calcite cement Enriched (8 >0 o/oo) and very depleted (8 <-10 o/oo) syndepositional calcite is found at shallow depth. 200 d18O Unlike 8 the 8 180 of diagenetic calcite in Frio sandstones has a large range ofvalues (Figure 5.8). With the exception of the syndepositional calcites, the variability of 8 180 in samples from similar depths in the same well is approximatley 2 o/oo (Figure 5.10). Syndepositional calcites have 8 180 between -2 and -8 o/00. These values are within the range of modem syndepositional calcites and caliches (Margaritz et al., 1981; James and Choquette, 1984; Hays and Grossman, 1991). The large range in 8 180 of syndepositional calcites is due to low temperature precipitation from solutions that are mixtures of depleted ~ meteoric water (8 180 ~-4 o/oo) and seawater (8 180 0 o/oo) and by subsequent modification. There is compelling evidence that there is only one episode ofburial diagenetic calcite in Frio sandstones (see above) and that the relative time of calcite cementation in all the sandstones was the same. Figure 5.10 shows that 8 180 in calcite becomes depleted with depth. (This trend is also seen in the data of Milliken et al. (1981) and Land (1984)). For both of these observations to be true requires either that the 8 180 of formation water has become enriched over time, or the 8 180 of the calcite has reequilibrated during burial. Several lines of chemical evidence favor recrystallization as being responsible for the change in 5 180 of calcite during burial. Precipitation of 5 180 = -11 o/oo calcite at -80° C (< 7,000 ft burial depth at present day temperature) requires a formation water with 5 180 of ~-1 o/00. Reaction of formation water with silicate phases during diagenesis results in 180 enrichment of the water (Land et al., 1988); 5 180 of formation water in Tertiary sandstones is almost always >0 o/oo (Kharaka et al., 1977, 1978; Suchecki and Land, 1983; Land and Macpherson, 1992). Therefore, it is unlikely that formation water that had already been involved with the precipitation of quartz overgrowths, chlorite, and I/S, had a 5 180 of -10/oo at the time of burial diagenetic calcite formation. A more reasonable explanation for the decrease in 6 180 with depth is that the calcite recrystallizes and reequilibrates during burial. Figure 5.10 shows how some of the Figure 5.10. 8 vs. depth for calcite cement The depletion in calcite 8 180 with depth is best explained (by petrographic, trace element, and evidence) by progressive and continuous recrystallization of calcite during burial. Most burial diagenetic calcite is not in equilibrium with present temperature and formation water 8 composition (lines). 202 deeper calcite is nearly in equilibrium with formation water with reasonable 5 180's and the present temperature. Emery (1987) interpreted linear increases in Fe and Mn in diagenetic limestones as evidence of progressive addition of trace element-rich fluid to the autocthonous formation water. Similarly, the large range in trace element composition in burial diagenetic calcites in this study (Figures 5.5 through 5.7) can be explained as evidence of dissolution and reprecipitation in chemically changing formation water. However, changes in the trace element content of calcite can also be due to the effects of temperature (Veizer, 1983; Mucci, 1987), crystal size and growth rate (Lorens, 1981; Veizer, 1983; Land, 1985, Mucci, 1988; Dromgoole and Walter, 1990), water chemistry (Ichikuni, 1973, Farr, 1988, Moorse and Bender, 1990), pH and PCO2 (Burton and Walter, 1991) on distribution coefficients, so the support provided by the trace element variation is less convincing. Milliken et al. (1981)proposedthatifrecrystallization wascontinuous during burial there would be no variability in 5 180 among closely associated samples. This is only true ifthe factors causing recrystallization (primarily temperature and formation water chemistry) of the calcite are supplied equally to all the samples. Figure 5.11 shows the relationship between calcite 5 180 and sandstone grain size. Calcite within the coarser, more permeable sandstones is more 180 depleted, implying that calcite reequilibration is (at least partly) a function of fluid flow. 87Sr/86Sr Twenty-foursandstonesampleswerecleanedandanalyzedforcalcite 87Sr The / 86Sr according to the procedures of Mack (1990) and Awwiller (1992). results ofthe analyses are listed in Tables 5.1 and 5.2 and shown in Figure 5.12. Like most of the diagenetic Frio calcite previously analyzed, most ofthese lower than coeval seawater. Ratios < -0.7077 sampleshave 87Sr/86Srequaltoor indicate that these calcites precipitated (or reequilibrated) after reaction of silicates (predominately VRF's and volcanic plagioclase) had added unradiogenic Sr to the Figure 5.11. 8 I*ocalcite vs. sandstone grain size. Within individual sandstone suites, depleted calcite is associated with the coarser-grained samples, indicating that fluid flow is an important factor effecting calcite recrystallization. This also implies that chemical reequilibration between calcite and formation water is the driving force behind recrystallization. 204 Key to symbols: Fault block I=o, fault block 2= . fault block 3= +, fault block 4= Q fault block 5=X (see Figure 3.1). Figure5.12. /86§rratjo0fcalcitecement. The / 0fcalcitecementis relatedtothe87Sr/865roftheassociatedformationwater. Valueslessthancoevalsea­water are due to the addition of unradiogenic Sr from VRFs and CRFs. Allochthonous formationwaterinfaultblock2has /86srratiossimilartotheassociatedcalcite. 205 formation water (Mack, 1990). The persisting decrease in 87Sr/86Sr with depth is consistent with calcite recrystallization and reequilibration in a fluid-buffered system dominated by VRF derived Sr, the net result of which is to alter the 87Sr/86Srofthecalcitefromitsstartingcomposition(coeval seawater87Sr/86Sr ranges from 0.70775 to 0.70795) to a value closer to a volcanic silicate signature (~0.706 (Mack, 1990)). The decrease in 87Sr/ with depth is seen in calcite from fault blocks 1,3, and 4. Radiogenic 87Sr/86SrvaluesinFrioformationwaterfromthenorthTexas gulf coast are due to addition of radiogenic Sr from reacting older cratonic silicates coupled with the absence of unradiogenic Sr-rich VRF's and carbonate rock fragments in the associated sandstones (Mack, 1990). Tables 4.1 and 4.2 show that the detrital composition of sandstones in fault block 2 approximates the detrital composition of the entire data base, yet calcite cement in sandstones from The source of the fault block 2 is considerably more radiogenic than the others. radiogenic Sr must be external to the sandstone. Formation Water Most formation water in the Gulf coast Cenozoic section can be interpreted mixtures of three fundamental chemical types. Dilute, acetate-rich water as originated as seawater and has been modified by early diagenetic reactions and the addition of shale clay inter-layer water. The composition of NaCl water is largely due to dissolution of diapiric salt. This is the most common water type. Ca-rich brine, the least common, originates through extensive albitization of plagioclase and the injection ofallochthonous formation water (Morton et al., 1981; Morton et al., 1982; Morton and Land, 1987; Land, 1987; Land et al., 1988; Land and Macpherson, 1989; Land and Macpherson, 1992). Table 5.3 B gives examples of the three types of formation water found in the Frio Formation. Tables5.3Aand5.4showthecompositionofformationwaterin thestudy area. Acetate-type water is found in fault blocks 3 and 5 (based on coeval to unradiogenic 87Sr/86Sr,relativelylowTDSandCa,andrelativelyhighalkalinity). Water in fault block 1 is best characterized as NaCl type. Albitization of Table 5.3. Formation water chemistry. Concentrations in mg/L. A. Formation water composition from study area (Morton et al., 1983). Table 5.4 lists formation water chemistry from Portilla Field, fault block 1. B. Characteristic Frio water types (Land and Macpherson, 1992). Silica, boron oxygen and strontium isotopic values for Ca-rich water are from TST 49-2. A Fault Block 2 Fault Blocks 3 & 5 Fault Block 4 Field Encinal Channel Mustang Island Red Fish Bay TDS 213,000 241,000 48,000 60,000 22,000 95,000 Ca 21,000-31,000 300 2,000 400 6,000 - Na 53,000 58,000 18,000 22,000 7,000 29,000 Alkalinity 41 -123 354 -1,315 403 898 - B South Texas Na-Acetate Ca-Rich Water "Shale" Water NaCl Water Cl 80,353 16,700 62,490 Sulfate 6 46 2 Bicarbonate 119 495 Organic acids 349 Na 24,086 10,600 37,820 K 887 264 198 Li 24 165 Ca 23,672 491 1,930 Mg 198 45 475 Sr 1,476 46 176 Ba 501 83 Mn 0.64 0.8 Fe 2.6 18 Z 0.1 Silica 54 173 81 B 34 47 40 del 180 6.0 4.8 4.3 87Sr/86Sr 0.7111 0.7077 0.7082 207 del C 18 4.3 4.6 4.3 6.0 4.9 4.6 4.7 4.7 4.8 5.4 4.4 4.6 5.0 Sr 86 Sr/ 0.70974 0.70891 0.71054 0.70932 0.70968 0.70942 87 TDS 80986 85409 83778 83429 76490 81657 29944 73446 85796 85330 86937 94299 65688 88658 75608 82924 66325 0 Bi 227 307 269 270 265 ND 231 265 288 288 325 210 275 225 260 230 HC03 560 628 662 758 799 774 1418 1019 906 733 585 775 1030 523 258 545 922 mg/L. a in 49396 52116 50964 49714 46160 49436 17263 44750 52187 50945 52702 57437 39142 53863 45852 50885 39747 Si 32.29 28.87 32.05 34.69 39.38 29.37 36.80 36.37 39.15 25.96 35.34 28.76 28.79 36.49 25.69 26.58 50.28 Concentrations Li 6.47 6.85 6.24 7.12 6.45 6.26 4.05 5.97 6.17 6.53 6.87 7.83 6.22 7.02 5.14 6.68 6.46 Mn 2.01 2.43 2.36 2.44 1.73 2.39 0.20 1.51 2.45 2.56 2.63 3.84 0.64 3.06 1.92 2.43 1.19 Fe 3.96 1.76 1.72 1.06 1.38 1.30 3.18 0.34 0.50 0.71 3.44 2.78 0.56 2.56 3.30 24.08 18.78 Zn 1.68 1.53 1.96 1.83 1.34 1.47 0.56 1.26 2.02 1.60 1.94 1.82 1.06 1.64 1.27 2.25 1.26 Ba 150 160 161 149 150 169 17 141 178 185 172 201 116 190 21 154 103 chemistry. K 231 215 226 186 232 214 190 236 242 238 233 250 219 219 114 220 223 water Sr 478 477 490 444 428 485 65 412 495 508 501 562 348 511 195 473 340 M& 219 232 222 221 237 228 53 204 222 244 245 299 242 266 219 214 218 formation Ca 4881 5135 4877 4244 4099 4751 383 3934 5171 5404 5254 6151 2975 5225 1696 5061 3030 field Na 24798 26097 26129 27395 24064 25294 10511 22473 26078 26748 26907 28254 21368 27535 26990 25051 21434 A! 3.08 0.14 1.92 Portilla m Pb Depth 7700 7450 7300 7400 7600 8000 8500 7600 7400 7400 7400 8100 7300 5700 7500 8500 0.14 0.10 5.4. LUL LULU LU LUU U UUU 19 50 6870 74 19 9 Table Sample Sample449101215 27274949 6767 274967 208 unradiogenic, volcanic, plagioclase can explain the relatively high Ca content of thebrine, however,theradiogenic 87Sr/86Srimplies that atleast someoftheCa is derived from the addition of underlying Mesozoic waters. Fault block 4 water is a mixture of Na-acetate and NaCl waters (slightly radiogenic 87Sr/86Sr, intermediate TDS, Ca, and alkalinity). Very high TDS and Ca concentrations characterize fault block 2 waters. With the exception of the offshore Miocene Picaroon field, EncinalChannelfieldwaterhasthemostradiogenic 87Sr/86Srvalues(0.7111) reported in literature. This value is even higher than the ratio in South Texas brines(thetype-section)andisverysimilartothe 87Sr/86SrofMesozoicbrines (Land and Macpherson, 1992). It is interesting that the two fault blocks that contain water with evidence of allochthonous brines are also underlain by shale diapirs (Galloway and Morton, 1988). Discussion Examination of Figure 5.12 and Tables 5.3 A and 5.4 indicates that there is astrongrelationship betweenthe87Sr/86Srofcalciteinsandstonesandthe 87Sr/86Sroftheformationwaterinthefaultblock. Unradiogenic87Sr/86Sr characterizes both the calcite and the formation water from fault blocks 3, 4 and 5. Highly radiogenic 87Sr/86Sr is found in both the calcite and formation water in fault block 2. This implies that the fault blocks have behaved as chemically separate systems at least since the time of calcite precipitation. A supporting observation that the fault blocks act as hydrologically separate entities and that there is at least some Mesozoic connection to the Ca-rich Cenozoic brines, is the high Mn content of the calcite in fault block 2. Burial diagenetic calcite in fault blocks 1,3, 4, and 5 have low Mn content as do the associated NaCl and acetate-rich formation waters. The only published Gulf coast water analysis with high Mn concentration (>2OO mg/L) is from a Cretaceous carbonate reservoir (Land and Macpherson, 1992). The high Mn content of calcite cement in fault block 2 suggests that Mn is being remobilized at depth and transported, along with abundant Ca and radiogenic Sr, into the Frio Formation. 209 A third line of evidence for the hydrologic separation of the fault blocks is the relative abundance of burial diagenetic analcime in fault block 2. Analcime precipitation requires a high Na/H ratio. Formation water from fault block 2 has the highest Na/H in the study area and must have been similarly saline at least as long ago as analcime precipitation. The position of the analcime 639 peak is a function of Si content (Figure 4.24). Figure 4.25 shows that there is an decrease in the position of this peak with depth. This relationship, and the interesting zoned nature of this anisotropic analcime, may also be evidence of progressive mineral recrystallization with depth. The change in calcite 87Sr/86Sr with depth supports recrystallization of calcite. Infaultblockscharacterizedbyunradiogenic87Sr/86Srformationwater, calcite becomes further depleted in 87Sr with depth. In fault block 2, recrystallization occurs in a radiogenic environment so that deeper calcite, which has experienced more dissolution and reprecipitation (i.e. recrystallization), has a more radiogenic character than shallow calcite. Shallow burial diagenetic calcite in fault block 1 has slightly radiogenic 87Sr/86Srratios(Table5.2). Table5.4showsthatthe offormationwater This from Portilla field can be quite a bit more radiogenic than the calcite. difference is due to the fact that the calcite, because of its shallow depth and only limited time in the "recrystallization window" (depth relationships imply that burial diagenetic calcite precipitates at -7,000 ft) has yet to significantly equilibrate with the formation water, the composition of which is more rapidly being modified and converted to a water-buffered condition by mineralogic reactions and by mixing with allochthonous brines. as evidence of Itismoredifficulttointerpretthe 87Sr/86Srtrendwithdepth preservation of original calcite chemistry. In fault blocks 3,4, and 5, the change in calcite 87Sr/86Sr would mean that a water-dominated, unradiogenic system of an would have had to become more radiogenic over time without the presence autocthonous or allochthonous source. In fault block 2, the formation water 87Sr/86Sr would have had to evolve from radiogenic to unradiogenic to radiogenic again to account for the relative changes in calcite and the composition of the present water. Neither scenario is supported by the mass-balance calculations of Mack (1990). The 2 o/oo variability in 5 180 of neighboring samples is due to the presenceofcrystalsthathave undergonevariousamountsofrecrystallization (as hypothesized by Land (1980), and Milliken et al. (1981)). The relationship between sample 5 180 and permeability (Figure 5.11) implies that the thermodynamic drive toward chemical equilibration between the calcite and the The absolute formation water is an important driving force for recrystallization. values and large range in calcite 5 180 and trace element chemistry at any depth indicates that equilibration has not yet been achieved. Reequilibration ofcalcite cement in sandstones has also been shown to occur in the meteoric environment (Longstaffe and Ayalon, 1987; Ayalon and Longstaffe, 1988). In some of the samples there are weak correlations between total calcite and depleted 5 180. This may indicate that in addition to reequilibration ofthe pre-existing calcite, there may indeed be a small addition (undetectable by That would make the petrography) of calcite during the recrystallization process. results of the recrystallization of calcite during burial diagenesis very similar to the changes seen to occur in the quantity and chemical and isotopic composition of mixed-layer I/S (Chapter 2; this study, Suchecki and Land, 1983). Both calcite and I/S continuously seek equilibrium with new chemical and temperature environments, attainment of that goal being limited by system (such as fluid flow) and kinetic (Ostwald step law-like neoformation of diagenetic products not quite in equilibrium with their environment) controls on the reactions. The differences in calcite and formation water chemistry in separate fault blocks, and especially the implications to the geohydrology of the system, must be considered when developing process models to explain the diagenesis of both the rocks and the brines. Chapter 6 CLAY MINERALS IN FRIO SANDSTONES Tables 4.1 and 4.2 and Figure 4.5 show that detrital clay matrix is a significant component of most shelf and many shorezone sandstones. Some of the earliest diagenetic reactions in the sandstones involves recrystallization of detrital clays and precipitation of I/S and chlorite. Chapter 4 showed that these detrital and early diagenetic clays are important in controlling fluid flow and the distribution of QOGs and calcite cement in sandstones. Later precipitation of chlorite and kaolinite further modified the intergranular volume of the sandstones. The purpose of the section is to characterize the clay minerals in Frio sandstones and especially to investigate the mineralogy and diagenesis of "matrix" clay. The sandstone clay suite is then compared to the clay mineralogy of shale core and shale cuttings. Detrital Clay Matrix Clay matrix is either an original component of a sandstone (concentrated as laminations or as clay rip-up clasts) or is introduced soon after deposition by bioturbation or infiltration (Figures 4.9, 4.10, and 4.11). Compaction and recrystallization of this material are two ofthe earliest, and most important, diagenetic processes operating in sandstones (Chapter 4). Pseudomatrix is formed when ductile clay clasts are deformed between more rigid grains during compaction (Figures 4.11 and 4.13). The results of XRD analyses on the <2 pm fraction of 118 sandstones are listed in Appendice D and shown in Figures 6.1 through 6.3. Significant differences exist between the fine grained clay mineralogy of sandstones and shales. In general there is more I/S and chlorite but less discrete illite and kaolinite in sandstones than in shales. Before assigning any significance to these differences we must first answer two questions. Is the sampling philosophy the same in the two data sets to allow comparison? And do sample preparation techniques allow comparison between the two data sets? 212 Figure 6.1. %IinI/S ofclayfractionfromsandstones andshales. • = sandstone <2(im, o= shales 2 pm) clay. The relative clay mineralogy of this coarse fraction which is dominated very different from the clay mineralogy in the < 2 pm by I/S (chlorite rosettes are commonly >lO pm in size, kaolinite crystals even larger). Differences in the clay mineralogy of samples determined by XRD and thin section and SEM petrography are real and should be expected. In this XRD study both the shale and sandstone clay data bases are heavily biased towards the fine grained, I/S dominated fraction (which in sandstones The differences occurs as detrital matrix and the products of its recrystallization). real in the clay mineralogy of the fine fraction of both sandstones and shales are and should have a geologic explanation. Mixed-Laver I/S Mixed-layer I/S is by far the most abundant component of detrital clay matrix; chlorite, discrete illite, and kaolinite are present in subequal, much lower abundances. The %I in I/S increases with depth. There is a large variability in %I in sandstones from the same depth; the %I in matrix I/S can differ greatly from the %I in I/S in shales from the same depth (Figure 6.1). 216 The extent of the smectite to illite reaction is primarily a function of temperature, though secondary factors such as water/rock ratio, fluid composition, mineral composition and time are also important (see Chapter 2). Sample set A in Figure 6.1 is from MSW 70. I/S from this -200 ft core ranges from 0%1 (i.e. end-member smectite) to 65%1. Figure 6.4 shows that the %I in MSW 70 I/S is related to grain size. In facies B 3 and 85, higher %I in I/S is found in samples with more total porosity; in facies S 1 and S2, the higher %I I/S is found in samples with more abundant potassium feldspar dissolution and feldspar overgrowths; in facies S2, 82, 83, and B 6 sandstones high %I in I/S is associated with abundant QOG's. All of these relationships indicate that in these sandstones fluid flow is an important factor influencing the conversion of smectite to illite. Sample sets B and C are from different TST 49-2 cores. The relationship between grain size, total porosity, and %I in core 9079 (sample set C) again shows the importance of fluid flow on diagenesis (Figure 6.4). In this thick distal­shoreface / inner-shelf sandstone package, focused flow has actually permitted the smectite to illite reaction to proceed further than in shales of the same depth. In core TST 49-2 8521 (Figure 6.1, set B) I/S from three separate sandstones have greatly different %I. The most smectite rich sample (Figure 3.7 sample#l5) comes from a thin, isolated, sandstone with abundant matrix and diagenetic calcite. The samples with higher %I I/S (Figure 3.7 samples #l4 and #24) are from sandstones with evidence of greater flow; the difference between these two samples is due to QOG and calcite precipitation "shutting off" the reaction (by filling the intergranular space and restricting further fluid flow) in sample #24 sooner than chlorite precipitation shut it off in sample #l4. Sample sets D 1 and D 2 are from TST 4-2 and TST 10-2 respectively, both from similar depths in Corpus Channel field. Sand percent maps (Galloway and Morton, 1988), electric logs, and core logs all show that TST 10-2 is significantly more sandstone rich than TST 4-2. I/S in TST 4-2 is 61 %I and 74%1. I/S in TST 10-2 is all >BO%I indicating that more fluid has traveled through the sandier sections of this area and allowed the I/S reaction to proceed further than in the shalier sections, in which the I/S %I is very close to the shale "baseline." The very 217 sandstone (and cores individual Within I/S. in %l and porosity, total size, grain between Relationship 6.4. Figure grain both to related is Permeability sandstones. porous more and coarser-grained in highest is I/S in %\ the packages), temperature, by controlled only not is reaction illite to smectite the of extent the that shows data This porosity. and size flow. fluid by also but package, sandstone individual an from samples the all for same the essentially is which 218 high %I in sample set E (TST 346-1) is also due to the fluid focusing effect of sandstones surrounded by low permeability shales and siltstones. Sample set F (TST 430/5) shows the importance of "effective" fluid flow. Porosity, grain size, and total clay content of the samples are similar, however, in some sandstones the detrital clay is present mostly as uncompacted clay clasts while in others the detrital clay is mostly pseudomatrix or burrowed matrix. The %I in the clay clast I/S is -50%, the %I in the pseudomatrix I/S matrix is >75%. Deformation during compaction effectively breaks open relatively impermeable clay clasts which allows fluid greater access to reactive sites in the clay minerals and permits further conversion of smectite to illite. All these examples, and the rest of the data, support the conclusion in Chapter 4 that effective fluid flow is a very important factor influencing clastic diagenesis. Preferential flow through permeable conduits allows more dissolved ions to be delivered to areas undergoing precipitation or away from areas undergoing dissolution. In this case, I/S in more permeable sandstones is exposed to more K and Al so more smectite layers are converted to illite layers than in I/S from less permeable units. Preferential fluid flow in sandstones relative to shales allows the I/S reaction to go further in some sandstones than the shale "baseline." Chemical analyses (Milliken et al., 1994) indicate that Frio sandstones lose K2O and shales gain K2O during burial. Sandstone I/S is exposed to potassium-rich fluids derived from autocthonous potassium feldspar dissolution before shale I/S. If smectite is being converted to illite in a relatively potassium starved system, then potassium availability is probably an important factor in controlling the extent of the reaction. In restricted or isolated sandstones the lack of "average" fluid flow is responsible for the preservation of anomolously low %I at depth. Texture Under high magnification, most of the non-descript brown matrix material seen in Figures 4.9, 4.10, 4.11, and 4.13 appears to be composed of variable proportions of clays (predominately I/S) exhibiting two different textures. Figures 4.18 B and 6.5 are SEM photographs of the characteristic "sheet" matrix texture. The material appears to be made up of thin (~0.1 |im) sheets commonly showing bulbous edge extensions and protrusions. This texture appears more detrital than diagenetic. Its presence over the entire depth and %I in I/S range, however, indicates that some diagenetic changes (in this case K and Al for Si chemical substitution and the accompanying crystallographic reorganization) can occur in clays without greatly disrupting the mineral's macrostructure. The other matrix texture, which is also seen over the complete depth range, is the "cornflake" fabric common for smectite and I/S (Figure 6.6). This texture is very clearly diagenetic. In these sandstones (as well as in others) it develops through the coalescence of thin wisps and filaments into the fully developed Frio sandstone boxwork (Pittman et al., 1992; Folk et al., 1994). Literally every examined in the SEM has some cornflake I/S even samples that show no - evidence of matrix clay or diagenetic clay rims by standard thin section petrography. Most matrix-rich sandstones show both textures though in various relative amounts. No relationship is observed between I/S texture and %I. Figure 4.14 shows arecrystallizing detrital matrix mass in the intergranular space between quartz grains. The matrix appears to be composed of both sheet and cornflake texture I/S and is serving as a nucleus for the cornflake I/S growing on the surrounding quartz grains. Thin section petrography does not differentiate the sheet from the cornflake textures in matrix rich Frio sandstones. Small (lO pm) and euhedrism ofthe crystals in all the samples. Keller and Hanson (1975) and Keller (1976, 1988) proposed that dickite is more likely to form equidimensional pseudo-hexagonal crystals than kaolinite, the crystals of which can be surprisingly minerals from these elongate. It was impossible to divide the kaolin group sandstones into separate euhedral and subhedral samples to test this hypothesis. HF dissolution calorimetry shows that the enthalpy of dickite and kaolinite is effectively the same (Barany and Kelley, 1961; Robie et al., 1978). The ordered nature of dickite relative to kaolinite imparts a residual entropy that can be measured(Pauling,1935,KingandWeller, 1961;UlbrichandWaldbaum,1976). The difference in entropy between the minerals should result in different free 224 Reynolds, or separate.and kaolinite Moore are dickite 1980; peaksand Brown, other all kaolinite and Q, ofBrindley labelled dickite. pattern (from peaks and XRD peaks quartzkaolinite Random dickite D, both 6.7. to labelled Figure Diagnostic 1989) common 225 Q 18 assuming Kaolinite 20.78 22.08 20.26 20.91 20.32 20.03 19.22 20.31 20.09 £. Q I8 calculated 20.40 21.57 20.13 20.40 19.92 19.61 18.91 19.68 ' 19.53 Sample §_ 0 18 8 10 104 13 12 14 1419 18 Kaolinite Ouartz % XRD. 9090 96 87 88 8686 8182 by Kaoliniteproportions % 0.80 Turn) +/­quartz Size 2-20 2-20 2-20 2-20 2-20 2-20 2-20 2-20 20.44 and = Fraction <1 18 8 Kaolinite 0 7347 7347 7351 8035 8095 8103 8103 9119 9124 kaolinite 18 0. Sample 8 70707067676767 17. 49-2 49-2 0 18 Kaolinite = MSW MSW MSW MSW MSW MSW MSW TST TST Average 8 quartz 6.1. Table detrital energy of formation values, though it is possible (and debatable) that this difference is of lesser magnitude than the error in measurement of the enthalpy (King and Weller, 1961;Kittrick, 1966). Other kaolinite synthesis and equilibrium experiments support the standard free energy values derived through calorimetry (Kittrick, 1970; Eberl and Hower, 1975; La Iglesia and Van Oosterwyck-Gastuche, 1978; May et al., 1986). Application of water chemistry modeling programs that thermodynamically differentiate between kaolinite and dickite are probably unrealistic due to the questionable nature of the thermodynamic data. Reevaluation of previous data (of Ewell and Insley (1935) by this study and Eberl and Hower (1975)) indicates that dickite has not been produced in the laboratory. Successful kaolinite synthesis experiments, though relatively rare, indicate that very dilute solutions are required for the mineral to form, as opposed to gibbsite or an Al-gel, and that the degree of required oversaturation remains the same regardless of temperature. The most successful experiments, especially at low temperatures, have generated kaolinite in the presence of high concentrations of organic acids or by using pre-organized Al gels as starting material (Kittrick, 1970; Linares and Huertas, 1971; Van Oosterwyck-Gastuche and La Iglesia, 1978). (A cynical person would find the results of most of these experiments suspect since the papers only list the phases produced and do not include the XRD patterns used for the ID.) Is there any significance to the occurrence of dickite in these sandstones? Crystalline, strongly bonded, disordered metastable minerals (such as kaolinite), which can grow from small "embryo" crystals and are favored by relatively rapid precipitation, encounter great difficulty converting to the thermodynamically more account for stable form (Goldsmith, 1953; Carpenter and Putnis, 1985). This may the presence of both polymorphs in these and other sandstones. In Australia, kaolinite lines pores subsequently filled by dickite. Dickite is concentrated in the coarser, more permeable sandstones, and is itself often > 40 pm in size (Bates, 1952; Bayliss et al., 1965; Loughnan and Roberts, 1986; Plancon et al., 1988). It can occur as a clast replacement when the associated pore-filling material is 227 kaolinite (Keller, 1976). Bates (1952) suggests that brine foreign-ion chemistry affects which mineral will form. All these facts hint at the importance of dilute fluids and slow precipitation for the formation of dickite (or even kaolinite). These conditions are all met in the sandstone burial diagenetic regime, so the presence of dickite, while exciting, may be neither so rare or unexpected. The occurrence of dickite in Frio sandstones may only be significant in so much as it forces us to evaluate what we mean by "equilibrium" in rocks that are composed of unstable and metastable detrital and diagenetic minerals that form and are variably preserved in environments as dissimilar as cold-temperature meteoric lenses and high temperature, almost greenschist facies, burial. Chlorite Chlorite in Frio sandstones is present as discrete rosettes (Figures 4.11, 4.16, and 4.17), as an intimate mixture with recrystallizing matrix I/S (Figure 4.14), and as radial rims (Figure 6.8). Chlorite precipitation overlaps QOG formation (Figures 4.13 and 4.17) andcontinues at leastinto thetimeofburial diagenetic calcite precipitation. The ability of chlorite to inhibit QOG precipitation and preserve porosity is well documented (Heald and Larese, 1974; Hastings, 1990; Winn et al., 1990; Warren et al., 1990; Pittman et al., 1992; Winn et al., 1993; Ehrenberg, 1993). In Frio sandstones chlorite also can inhibit calcite precipitation, though Figure 4.16 clearly shows that this is not always the case. Figure 4.15 shows that significant chlorite is often associated with abundant detrital matrix. Figure 4.14 shows that chlorite rosettes are a component of the recrystallizing, predominately I/S, matrix. The relationship between well diagenetic chlorite and recrystallizing matrix is seen in other sandstones as (Glennie et al., 1978; Barnes et al., 1992; Crossey and Larsen, 1992). In many Frio sandstones, chlorite is associated with matrix rich laminations (Figure 4.11) and burrows. These relationships imply that the I/S matrix is the source of the elements used in chlorite formation. Figures 4.13 and 6.6 appear to catch the conversion of I/S to chlorite in the act. Figure 4.13 shows large, green, Figure6.8.Vicksburg-likeGreenStripes. Greyhostrock(Figures4.18and4.23)is changed to green-stripesduring diagenesisby dissolutionofVRFsand detritalfeldspar and by recrystallization of I/S to chlorite and precipitation of albite. The minerals found in the green stripes (quartz, albite, chlorite, and illite) are the greenschist facies assembledge. pseudohexagonal chlorite crystals forming at the expense of brown I/S matrix. Even better evidence is seen in Figure 6.6, which shows "cornflake" texture I/S coalescing into chlorite proto-rosettes. Depth trends for chlorite abundance are complicated by the fact that it is extremely difficult to differentiate the clay minerals in sandstones by thin section petrography (when they can be as intimately associated'as in Figures 4.13 and 4.14) or XRD (because chlorite can be significantly coarser than the standard <2 pm "clay fraction"), however, Figure 6.3 does shows that there is an increase in the chlorite content of the clay fraction of sandstones with depth. The figure also shows that the amount of kaolinite in sandstones decreases with depth. The formation of chlorite (albeit Mg-rich, not Fe-rich) at the expense of kaolinite and smectite with increasing temperature is predicted by the experimental work of Aja et al. (1991 a & b). Burton et al. (1987) have shown SEM's of the conversion of kaolinite to chlorite in shales. There is no evidence in this study that the conversion involves a mixed-layered chlorite/smectite intermediate as proposed by Chang et al. (1986). Jahren and Aagaard (1989 & 1992) and Hillier and Velde (1991) propose that chlorite chemistry continuously reequilibrates with its diagenetic environment throughout its burial history, which is similar to the hypothesis concerning I/S chemistry presented in Chapter 2. An unusual occurrence of chlorite in Frio sandstones supports the formation of chlorite at the expense of I/S and also gives a clue as to the potential "end-products" of burial diagenesis. Figure 4.9 shows the strange "green-stripe" texture in a Frio sandstone that is quite common in Vicksburg sandstones (Langford and Lynch, 1990; Genuise, 1991). Figures 4.18, 6.9, and 6.10, and Table 6.2 show that the IGV in the beige mother-rock is filled with I/S matrix (incorrectly identified as chlorite by J. Grigsby in Langford and Lynch (1990)). Figures 6.8, 6.9, and 6.10, and Table 6.2 also show that the IGV in the green- stripes consists of tangential chlorite grain rims, chlorite replaced rock fragments (not visible in Figure 6.8), and abundant secondary porosity. Unpublished point count analyses of Vicksburg green-stripe sandstones by this author indicate that the secondary porosity was produced by dissolution of both feldspar and rock fragments. Figures 6.8, 6.9, and 6.10 also show that euhedral albite crystals have 230 Figure 6.9. XRD pattern of ’’Vicksburg-like" Green Stripe, <2 pm. A: B: Grey background pattern. Green stripe pattern. Q = quartz, A = albite, K = potassium feldspar, C = chlorite, I/S = illite/smectite. 231 Figure 6.10. XRD pattern of"Vicksburg-like” Green Stripe, 2-20 pm. A: B:= Grey background pattern. Green stripe pattern. Q quartz, A== = albite, K potassium feldspar, C chlorite, I/S = illite/smectite. 232 Table 6.2. Clay mineralogy ofVicksburg-like green stripes. All samples are from TST 470-4. Sample Fraction % Chlorite 11,247 <2 |im Green 67 33 11,247 <2 |im Grey 37 63 11,247 2-20 Jim Green 100 tr 11,247 2-20 Jim Grey 100 tr 11,250 <2 (im Green 62 38 11,250 <2 |im Grey 57 43 45 55 11,226 <2 |im 37 11,232 <2 |im Green 63 11,232 <2 |im Grey 42 58 233 formed in the green-stripes and the detrital feldspar suite of potassium feldspar (and plagioclase?) present in the beige mother-rock has been completely removed. The net result of the reactions that produced the green-stripes is a rock that consists of quartz, albite, and chlorite the greenschist metamorphic facies, - assemblage (minus illite and calcite). Figure 4.28 shows how detrital I/S matrix, chlorite, and kaolinite effect the amount ofpreserved porosity in Frio sandstones. Figure 4.59 supports the hypothesis of Houseknecht and Ross (1992) that sandstone porosity is maximized by thepresence of a small amountofclay, whichpresumably actsas aninhibiting agent for later QOG and calcite cement. MineralogyofShale Core Two shale core suites were studied in order to determine differences in the clay mineralogy ofsandstone and shale core and shale cuttings. 12 shale samples were taken from Cecelia Kelley #2. These shales are interbedded with the distal­ shoreface/inner-shelf sandstones that compose most of the core. Six samples (between 11,826ftand 11,862ft)comefromtheuppercore,six(between12,186 ft to 12,204 ft) from the lower. The second suite samples 15 shales between 9,650 ftand11,306ftfromArcoTST470-4. Thesesamplesarenormalbasinalshales associated with a series of distal-shoreface/inner-shelf sandstone packages. The results of the analyses are listed in Appendice D. Clay Mineralogy Figures 6.11 through 6.15 show the relative clay mineralogy of the <2 pm fraction of the shale samples, the <2 |im fraction clays from sandstones from the same wells, and the "shale baseline," <1 pm clays from shale cuttings (Chapter 2). Figure 6.11 shows that, in general, the %I in I/S from shale cores is more like the %I in associated sandstones than the shale baseline. Additionally, figures 6.12 through 6.15 show that the relative abundances of the clay minerals in the fine 234 fraction are more like the proportions in the associated sandstones than in the shale cuttings. Whole Rock Abundances Figures 6.16 through 6.19 show the whole rock abundances for the shale core samples. Comparison with Figures 2.10 through 2.13, which show the shale cuttings-based whole rock abundances for two wells, indicates that the shale cores are siltier than the shale cuttings. Plagioclase and albite, and calcite are more abundant in Cecelia Kelley samples, otherwise all the mineral abundances in all the cores are subequal. Discussion Why is the clay mineralogy of sandstone and closely associated shale core different from the clay mineralogy assumed to be characteristic of basinal shales? There are only two possible answers: either the original detrital clay composition of the three different rock types was different or they have been differentially modified by diagenesis. The siltier nature of the shale core vs. the shale cuttings might imply that there is a detrital difference. If this is the case, then the clay minerals associated with the siltier shale cores should reflect a more proximal composition (relatively abundant in kaolinite vs. VS as shown by Ramsayer and Boles (1986) and Chapter 2). In fact the opposite is true, the < 2 pm fraction of sandstones and shale core is dominated by I/S, and kaolinite is less abundant than in the shale cuttings. In this study, I/S in sandstone and shale core is generally more illlitic than the VS in shale cuttings from the same depth. These results are at odds with Boles and Franks (1979), Howard (1981), Ramsayer and Boles (1986), and Wood and Boles (1991) who found most VS in sandstones is more expandable than shale VS at the same depth. Low %I in Frio sandstones is found in isolated, impermeable sandstones or at depths where the smectite to illite reaction is only getting underway (possibly 235 236 is I/S core shale in %I the that shows data The cuttings. shale and core, shale and sandstone in I/S in %I 6.11. Figure cuttings. shale baseline the to than sandstones associated in %I the to similar more core shale and than suite sandstone clay text. both and of proximal 2.9 content more Figure I/S a See The contain I/S. core finer cuttings. shale of shale and instead and kaolinite core, sandstone shale If coarser in and cuttings. shale enriched sandstone of be of content shouldcontent I/S it I/S the then 6.12. than cuttings greater Figure shale is diagenesis. during consumed is Illite cuttings. shale and core, shale and sandstone of content illite Discrete 6.13. Figure flow. fluid in differences to related be can distribution This most. the cutting shale illite, of amount least the has clay Sandstone 238 239 product a as well as reactant a Kaolinite, cuttings. shale and core, shale and sandstone of content Kaolinite 6.14. Figure types. rock permeable least th« in abundant more is diagenesis, of often is sandstones in chlorite Because cuttings. sandstone and core, shale and sandstone of content Chlorite 6.15. Figure analyses. these in underestimated is abundance its fim, than larger much 240 241 of deviation standard one show bars Error #2. Kelley Cecelia from core shale of content quartz and Clay 6.16. Figure more contain shales These shales. spaced closely very between exist differences Significant analyses. independant three location. proximal more their to due probably is and cuttings shale than quartz 242 due are mineralogy clay in Differences #2. Kelley Cecelia from cores shale of content calcite and Feldspar 6.17. Figure cuttings shale to compared rich, calcite and albite are shales Kelley Cecelia factors. diagenetic and detrital both to 243 of deviation standard one show bars Error #4. 470 TST from core shale of content quartz and Clay 6.18. Figure was sample indicates marker open shale, as logged was sample indicates marker Filled analyses. independant three packages sandstone distal-shoreface/inner-shelf with associated are which shales, These siltstone. as logged cuttings. shale than clay less contain standard one show bars Error #4. 470 TST from core shale of content calcite and Feldspar 6.19 Figure indicates marker open shale, as logged was sample indicates marker Filled analyses. independant three of deviation cuttings. shale of that to similar is shales these of content calcite and feldspar The siltstone. as logged was sample 244 due to an effect similar to the residence time hypothesis of Ramsayer and Boles (1986)). Cecelia Kelley sandstones have lower %I than associated shale core or cuttings because burial diagenetic calcite precipitation isolated the clay from reactive fluid flow. Calcite cementation has also restricted the smectite to illite reaction at North Coles Levee (Ramsayer and Boles, 1986). It is important to notice that the reaction has slowed but not stopped in the approximately 4,000 ft of continued burial since the time of calcite precipitation. The relationship between the highly-cemented Cecelia Kelley sandstones and the calcite rich interbedded shales supports a shale source for the carbonate. It is significant that the clay minerals in abundance in the sandstones and shalecoresarethehypothesizedproductsofburialdiagenesis(I/S andchlorite), and the clay minerals that are uncommon are the hypothesized reactants (discrete illite and kaolinite) (Lynch, 1985; Awwiller, 1992; this study Chapter 2; Figure 6.6). This chapter has shown how %I in I/S in individual sandstones is effected by permeability and fluid flow. It is proposed that the differences in the clay mineralogy of sandstones, shale core, and shale cuttings are at least in part due to increased diagenetic modification of an originally similar clay suite because of increased fluid flow through sandstones and sandy shale packages vs. basinal, sand-poor shales, as described in Chapter 4. The anomolously high smectite content in some sandstone and shale packages (Figure 6.1 sample set X (TST 470/4)) may indicate that these packages have been isolated from fluid flow for a significant amount of time. Howard (1985, 1991) has determined that the permeability of Frio shales increases with sand content (as laminations) and in a general sense supports the hypothesis that the sandy shales associated with sandstone packages have been exposed to more fluid flow than sand-poor basinal shales. 245 Chapter 7 DIAGENETIC MODEL AND CONCLUSIONS The modelfor theburial diagenesisofFrio sandstones developedin this section is based on the paragenetic and isotopic relationships determined in this for the study plus the previous work (especially the depths offirst occurrence diagenetic minerals) discussed in Chapter 3. Oxygen Isotope Geochronology 5 180 analyses were obtained for calcite (Chapter 5), quartz overgrowths (Chapter 4), and kaolinite/dickite separates (Chapter 6). Figure 5.8 shows that 8 180 values >-6 o/oo PDB belong to syndepositional calcites, therefore the most The enriched 8 180 for burial diagenetic calcite is assumed to be -6 o/oo PDB. data from two quartz isolate suites indicates a diagenetic quartz 8 180 of -25 o/oo SMOW (Figure 4.26). The average 8 180 of the kaolinite/dickite separates is -21 o/oo SMOW (Table 6.1). The locus of equilibrium values of formation water 8 180, temperature, and the three diagenetic minerals are shown in Figure 7.1. Could the three minerals have formed in isotopic equilibrium at the depths of their first occurrences? Land and Macpherson (1992) show that the 8 180 of formation water from most Gulfcoast Cenozoic reservoirs is >0 o/oo and becomes moreenrichedwith depth, as ispredictedfor a systembecomingbuffered by illitization of smectitic shales (Kharaka et al., 1977, 1978; Suchecki and Land, 1985; Land and Fisher, 1987). There is a great range in 8 lsO at any depth (~4 o/oo).The"average"water8 180at6,000ftis~3o/00,and~5o/ooat10,000ft. Cenozoic formation water more 180 enriched than ~6 o/oo is relatively rare and only occurs in the deepest reservoirs. Quartz is the first burial diagenetic mineral to form in the sandstones. The first significant occurrence of QOGs is at -6,000 ft, which has a present day temperature of 75°. Precipitation of QOGs with 5 180 =25 o/oo at 75° requires a water 5 180 < 0 o/00, which is uncommon in the Cenozoic and almost unknown in the Frio. This implies that either the formation water has become -3 o/oo more for and rapidly calcite ft temperature*8 hot, reasonable 8,500 which at and from with between depth temperature. water water precipitated found the burial 8 of formation QOGs at current values are temperature that its from temperatures thewith than equilibrium produced implies These This ofbe cooler equilibrium can field). locus 6,000. the o/oo (grey at 25 temperaturesin are of 115°C ~ precipitated curves and precipitatedrecords have a Bold 8 to with 100° have field) appears Quartz between to (blue appear field) Calcite geothermometry. (red kaoliuitc. temperatures however, water. Isotopeand )Gs, kaolinite at Q( 7.1. calcite, +5.5) formation li. )Gs, to Figure (~+4 10,000rising precipitates, Q( 247 enriched since the time of quartz formation, or that the QOGs precipitated from waters greatly out of equilibrium with the present temperature and isotopic conditions. Evidence Against Deep Meteoric Water Invasion Is it likely that Frio formation water at 6,000 ft had a 5 180 < 0 o/oo when QOGs were forming? Depleted meteoric water has been the proposed explanation for the relatively light 8 180 values of QOGs in the Travis Peak Formation (Dutton and Land, 1978; Dutton and Diggs, 1990), the Frontier Formation (Dutton, 1993), the Norphlet Formation (Mcßride et al., 1987), and the Pennsylvanian sandstones from the Anadarko basin (Dutton and Land, 1985). In each of these cases the QOGs were precipitated from the ascending limb of a deep The meteoric recharge system generated by the presence of significant relief. relief necessary to drive a deep meteoric system of this type did not exist along the TexasGulfcoastduringFrio time. Hancock and Taylor (1978), Blanche and Whitaker (1978), Blackboum (1984), Longstaffe (1984), Longstaffe and Ayalon (1987), Kantorowicz et al. (1987), Saigal and Bjorlykke (1987), Ayalon and Longstaffe (1988), Glassman et al. (1989 a,b), Longstaffe (1989), Longstaffe and Ayalon (1991), Bjorlykke and Aagaard (1992), Fallick et al. (1993), Hathon and Houseknecht (1992), Longstaffe et al. (1992) and others have shown how direct meteoric influx into sandstones can in greatly affect their diagenesis. However, the rapid generation of overpressure sediments such as the Frio (Bredehoeft and Hanshaw, 1968; Sharp and Domenico, 1976; Galloway, 1984; Sharp and Mcßride, 1986; Bethke et al., 1988; Sharp et al., 1988) can effectively stop significant meteoric water influx (Harrison and Summa, 1991; Bjorlykke and Aagaard, 1992). In the Frio, a relationship does exist between depositional facies and early The diagenesis involving meteoric water. presence of syndepositional calcite and kaolinite in back-barrier and exposed shoreface and foreshore sandstones is clear evidence for early meteoric diagenesis. The isotopic composition of syndepositional calcite cement supports the role of meteoric and mixed-meteoric 248 water in their precipitation (c.f. Dominico and Robbins, 1985; Hays and Grossman, 1991). The occurrence of feldspar overgrowths, which are restricted to shorezoneand themostproximal distal-shoreface/inner-shelffacies sandstones, has also been related to early meteoric, or mixed-meteoric water (Galloway, 1984; Loucks et al., 1986; Bjorlykke et al., 1986; Glassman et al., 1989 a,b; Milliken, 1990). There is no petrographic evidence that either syndpepositional calcite or FOGs ever existed in the more distal shelf sandstones, and, in fact, the existence of a relationship between early, meteoric diagenesis and depositional facies implies thattheinfluxoffreshwaterwasrelatively smallscaleandnotlikelytoeffect sandstones any significant distance from the recharge area (Harrison and Summa, 1991; Primmer and Shaw, 1991; Longstaffe et al., 1992). There is no evidence in the rocks, the regional tectonics, or the water chemistry (Morton and Land, 1987; Donnelly, 1989; Macpherson and Land, 1992), to indicate that meteoric water played a significant role in the burial diagenesis of these sandstones. This implies that it is reasonable to assume that the 5 180 composition of formation water at the time of Frio diagenesis was similar to the 5 180 composition of formation water today. With that assumption, it is possible to use the 5 180 composition of diagenetic minerals to derive their temperatures of formation. Diagenetic Model Figure 7.1 shows that only kaolinite, which is the last important burial diagenetic mineral to form, appears to have precipitated in equilibrium with isotopically reasonable water at the depth (and present temperature) of its first significant occurrence (red field). Calcite (blue field) and quartz (grey field) first occur at depths (and temperatures) difficult to reconcile with the measured isotopic compositions of the mineral and the present formation water. The depth to abnormal fluid pressure in the study area is ~5,000 ft (T. McKenna, pers. com., 1994); the depth to hard overpressure is between 8,000 and 9,000 ft (Bebout et al., 1981). The first occurrence of all three diagenetic minerals is within this "transitition zone" between hydropressure and hard geopressure. Figure 7.2 is a cartoon representing the proposed diagenetic model for Frio Formation sandstones in the Corpus Christi area. Before entering the overpressured regime (time A) shelf sandstone diagenesis consisted entirely of compaction and recrystallization reactions involving depositional I/S matrix. Neoformed cornflake-texture I/S and chlorite precipitated and drastically reduced sandstone permeability and porosity. Before time A, shorezone sandstones may also have been modified by syndepositional diagenetic reactions, primarily calcite precipitation, which also effected porosity and permeability. At time B, in the abnormally pressured regime, the sandstones enter a circulation system developed in the first few thousand feet of The overpressure. isotopic analyses of QOGs (Fig. 7.1 grey field) indicate that they precipitated from waterwithreasonable8 180valuesattemperaturesbetween-100°and115°. = Assuming an average water 8 180 ~4.7 o/00, and the present geothermal gradient, the source of the QOGs precipitating at 6,000 ft appears to be hot water originating at approximately 9,000 ft. This hot water must have travelled the intervening3,000ftratherquicklyinordertoremain at thetemperaturerecorded by the oxygen isotopes. The QOG 8 180 from the TST 346 #1 sample suite is 19.4 o/00, significantly more depleted than the other two suites (Figure 4.26). Assuming this value is real (and not somehow related to the difficulty determining QOG vs. detrital quartz in this very fine-grained, very quartz-rich suite), Figure 7.1 shows that the water involved with the precipitation of these overgrowths was derived at great depth (14,000 ft or deeper). Chapter 4 showed that the abundance of diagenetic quartz in a sandstone is related to permeability and fluid flow. Rapid upward movement of hot Sio2 saturated fluids also helps with the "water problem" identified by Land (1984) because not only will each pore volume lose the amount of SiC>2 it was oversaturated with at depth (assumed to be 50 ppm by Land (1984), his figure 6, though the amount of quartz oversaturation may itself vary with depth (Land and Macpherson, 1989)), it will also lose the ~30 ppm difference between quartz solubility at 105° and 75°. The 8 180 of diagenetic quartz in Frio sandstones determined by Milliken et al. (1981) and Land (1984) is ~32 o/oo (Figure 7.1). Land (1984) interpreted this value to indicate quartz precipitation at -60° from ascending water with a Figure 7.2. Diagenetic model for Frio Formation sandstones. The paragenetic sequenceforthismodelisdevelopedinChapter4. Theexistanceofrising,hot,water is inferred from oxygen isotope geothermometry (Figure 7.1). The process for recycling formation water, required by solubility constraints (Table 7.1), is unclear. 251 8 ISO between 4 and 5 0/00. In the Corpus Christi area, 60° is reached at -4,500 ft. Significant quartz cementation does not occur until -6,000 ft (75°). Most of the samples used by Milliken et al. (1981) and Land (1984) come from the detrital quartz and QOG-rich Houston Embayment. That, and the differences in sample preparation techniques, may be the source of the discrepencies between the quartz 8 180 determined in this study and the previous work. Mcßride (1989) lists 23 proposed sources for the SiC>2 cement in sandstones. Figure 2.2 shows that a significant proportion of smectite conversion to illite in Frio shales occurs over the same depth and temperature range as the The mass balance precipitation of quartz, calcite, and kaolinite in Frio sandstones. calculations presented in Chapter 2 showed that clay-dominated reactions that occur in Frio Formation shales can supply a significant amount of SiC>2 to the associated sandstones, and that there are several lines of evidence suggesting that the shales do indeed act as chemically open systems during diagenesis. Burial diagenetic calcite precipitated at time C. Chapter 4 showed that the distribution of burial diagenetic calcite is also related to sandstone permeability and fluid flow. The controls on calcite solubility in a multi-component system are complicated by the buffering potentials of various acids (possibly) present in the formation water (Surdam et al., 1984; Hutcheon, 1989; Hutcheon and Abercrombie, 1990; Milliken and Land, 1991; Land and Macpherson, 1992). The calculations of Lundegard (1985) and Lundegard and Land (1986) show that significant calcite precipitation can occur due to CO2 loss from ascending water. Land (1984) proposed this mechanism for precipitation of 5 180 =—7 0/00 calcite from formation water with 5 180 =4or 5 at 80°, which is just slightly cooler than the temperature at the depth of calcite occurance in the Corpus Christi area. The data of Milliken et al. (1981) support the burial diagenetic calcite 5 180 =-6 o/oo value of this study. Figure 7.1 shows that even using 8 180 =+6 formation water it is difficult to precipitate 5 180 =-6 o/oo calcite at 7,000 ft and the present temperature. The equilibrium temperature calculated for burial = 4.7 diagenetic calcite with 8 180 =-6 o/00, and formation water with 8 180 o/00, is -75°, which occurs at -6,000 and is coincident with the first occurence of QOGs. The isotopic data are consistant with calcite forming in the down-side of a convection cell of the type proposed by Wood and Hewitt (1984). SiC>2-saturated The water derived at 9,000 rapidly ascends to -6,000 ft and precipitates QOGs. water quickly cools and reequilibrates at 75°, and is reentrained in the down-side ofthe circulation system. Relatively rapid downward fluid movement rates are implied by the isotopic composition of the calcite that forms at 7,000 with a temperature signature of 6,000 ft. Precipitation of calcite may be triggered by the inverse relationship of calcite solubility and temperature. Figure 7.2 shows that at time D kaolinite is formed roughly in equilibrium with reasonable water compositions at the depth of its occurrence. The kaolinite oxygen isotope precipitation temperatures support the petrographic observation Just that the times of formation of burial diagenetic calcite and kaolinite overlap. like the other burial diagenetic cements, the abundance of kaolinite can be related to sandstone permeability (Chapter 4). At time E the sandstone leaves the active circulation system developed in the transition zone. Secondary porosity, which was generated during time D, continues to form. Recrystallization reactions involving clays, calcite, and feldspar continue to take place; the extent of these reactions controlled to some degree by fluid flow. Convection? The petrographic and isotopic data used to construct this model imply that much of the diagenesis of Frio sandstones takes place in the -3,000 ft transition The burial curve ofMilliken zone between hydropressure and hard geopressure. et al. (1981) shows that it takes -2.5 m.y. for a sandstone to pass through this depth interval. During the first 1.25 m.y. quartz precipitation is accompanied only Haszeldine et al. (1984) have by I/S recrystallization and chlorite neoformation. also noted the relatively short duration of quartz cementation in North Sea Land's (1984) calculations indicate that thousands of pore volumes of sandstones. formation water are required to precipitate the amount of quartz cement seen in the Frio. Similar calculations, Table 7.1, show that almost 100,000 pore volumes of formation water are required to have passed through some of the distal Frio 253 / flow abundan Dischar^ (cc 0.0054 0.0045 0.0090 0.0080 0.0066 0.0170 0.0032 0.0707 0.0042 pore precipitation. yeai with fluid 4,000 amount sandstones requires quartz Volumes Required 6,784 5,653 11,307 9,964 8,303 21,200 4,038 88,333 5,300 of of great shelf Case episode Pore a % Distal Best m.y. 253025 253025 3026 25 PCP requires Sandstone even a sandstones. fluid, 1.25 needed rock) oversaturation in assuming 1,696 2,827 5,300 1,211 22,967 cc Frio / 1,696 2,491 2,491 1,325 Water (cc (aq) Sio2-oversaturated calculated Sio2 observed rate (aq) (ppm) 505030 505050 7030 50 Reasonable overgrowths slightly flow Si02 oversat g/cc, of quartz rock) volumes 2.65 added cc 858585 125 125 265 85 689 66 of pore density Si02 amount /(mg Requirements. = 80,000 Sio2 the % for over 4.3, 3.2 3.2 3.2 4.7 4.7 10 3.2 26 2.5 QOG Volume Table account require (aq) to Pore from % PCP Case), Si02QOG sandstone QOG average high low PCP QOG 7.1: of (Worst average high high Case (1984) facies, facies, facies, Case cc Table per QOGs volumes. All All All Shelf, Shelf, Shelf, Best Worst Land 254 sandstones to account for the large amount of IGV filling quartz found in those rocks. Evidently fluid flow in the high pressure-gradient transition zone is a very active process. On the smaller scale, several lines of evidence indicate that fluid flow is an extremely important component of sandstone diagenesis. The relationships between permeability and QOG and calcite abundance (Chapter 4), permeability and calcite oxygen isotopic recrystallization (Chapter 5), and permeability and %I in I/S (Chapter 6), all show how diagenetic reaction extent is effected by fluid flow. Convectional flow has been postulated as the solution posed by the large number of volumes pore necessary for the observed diagenesis in the rocks (Wood and Surdam, 1979; Cassan et al., 1981; Wood and Hewitt, 1984; Hazeldine et al., 1984; Land, 1984; Davis et al., 1985; Blanchard and Sharp, 1985; Sharp et al., 1988; Sharp and Mcßride, 1989) though its existence is not universally accepted (Bjorlykke, 1979; Bjorlykke et al., 1988; Caritat, 1989; Bjorlykke and Egeberg, 1993) nor has it been demonstrated. A major problem with convection in an overpressured environment is the difficulty in returning the water against the pressure gradient for recycling. The 5 180 value of diagenetic quartz implies that the upward component of any circulation system operating in the transition zone is probably focused through faults or other conduits that allow rapid movement. Bodner et al. (1985), Bodner and Sharp (1988), and Pfeiffer (1988) have presented data that supports upward fluidflow alongGulfcoastgrowthfaults,however, there isnodirect evidenceof downward fluid flow. The problem becomes even more difficult because of the coincidenceofthedepthofdiagenesis in the sandstones and the depthofthe highest pore-pressure gradient. Ifconvection is not operating, another process that can account for the mineralogic changes in the rock while remaining within the constraints set by water chemistry equilibrium and kinetic relations must be identified (Land, 1987; Hutcheon, 1989). Seismic pumping (Sibson et al., 1975) is an attractive hypothesis because it eliminates the difficulty in returning the water to depth for recycling. Ulrich et al. (1984), Kyle and Price (1986), and Kyle and Agee (1988) 255 have proposed that episodic fluid flow, produced or enhanced by seismic pumping, is responsible for salt dome mineralization in the Gulf coast. Wood and Boles (1991) envoke seismic pumping along a fault and connecting permeable sandstone pathway for long-distance material transport in sandstones (Boles and Ramsayer, 1987). This mechanism is enticing because it provides a way of moving a large quantity of water quickly, however, it does not identify the ultimate source of the water, which still remains a mystery (Bjorlykke, 1979; Land and Dutton, 1979). If the water involved with the diagenesis of the sandstones is not derived from diagenesis of the shales, where does it come from? Relationship Between Diagenesis and Formation Water Chemistry Surdam et al. (1984), Surdam and Crossey (1985), Crossey et al. (1986), Surdam and MacGowan (1987), Surdam et al. (1989), MacGowan and Surdam (1990), and Helgeson et al. (1993) have proposed that secondary porosity generation, pH, calcite precipitation, and sandstone diagenesis as a whole is controlled by reactions involving organic acids. Mass balance calculations by Lundegard (1985) and Lundegard and Land (1987) indicate that insufficient oxygen is available in the kerogen in Frio shales to produce enough organic acid to account for the diagenesis in the rocks. The importance of organic acids in formation water chemistry is also debatable (Lundegard, 1985; Lundegard and Land, 1987; Hanor et al., 1993; Land and Macpherson, 1992). Tables 5.3 and 5.4 show that fault block 1 is characterized by NaCaCl water, fault block 2 by CaCl water, and fault blocks 3,4, and 5 by modified acetate water. Fault blocks 1 and 2 have both shorezone and distal­ shoreface/inner-shelf sandstones. The one core from fault block 5 is a distal­shoreface/inner-shelf sandstone and the cores in fault block 4 are shelf sandstones. Figures 7.3 and 7.4 show the variability in sandstone diagenesis in the different fault blocks. Figure 7.3 shows that higher total porosity is associated with the coarser, more proximal sandstones in fault blocks 1 and 2. Secondary porosity is not Burial enhanced in the fault blocks associated with high organic alkalinity. diagenetic calcite from all five fault blocks has approximately the same average 5 13C, -6.2 o/00, which indicates an approximate 25% contribution of 13C depleted organic carbon to a predominately inorganic carbon reservoir. Many syndepositionalcalciteshave5 13Cvalueswhichindicateasignificantorganic carbon contribution. The early diagenetic environment is where the greatest amount of oxygen, including carboxalic acids, is liberated from the maturing organic matter (Tissot and Welte, 1984; Rashid, 1985). There is no chemical or petrographic evidence that organic acids played an important role in the development of secondary porosity in these sandstones. Figure 7.4 shows the abundance of QOGs and calcite in the different fault blocks. The calcite content of all the shelf sandstones is approximately the same; the high calcite content in fault block 1 is due to syndepositional calcite precipitation. Presumably, if organic acids were forming soluble complexes with Si there would be less diagenetic quartz precipitated in sandstones associated with organic rich waters. In fact, the sandstones from fault block 4 have the most QOGs observed in this study, due to the highly focused nature offluid flow in the distal shelf (Chapter 4). There is no evidence that sandstones associated with organic acid-rich formation water have undergone grossly, or even subtly, different diagenesis than sandstones associated with other water types. The petrographic evidence shown here and in Chapter 4 indicates that there are some differences in the diagenesis of sandstones from separate fault blocks but these differences are easily explained by facies-related effects. Chemical analyses of calcite (Chapter 5) show that formation water chemistry has been different in but these differences have only separate fault blocks for perhaps as long as 26 m.y. been manifested as unique trace element signatures and have had no significant effect of the diagenesis of the sandstones. 257 Figure 7.3. Fault block control on diagenesis: porosity. Secondary porosity is not enhanced in fault blocks 4 and 5, which are characterized by organic acid-rich formation water. Total porosity is greater in fault blocks 1 and 2 which contain more proximal sandstones. Diagenesis 'Tills up" more IGV in distal sandstones because more fluid is focused through these sandstones than in the proximal facies 258 Figure 7.4. Fault block control on diagenesis: QOGs and calcite. QOGs are most abundant in the distal shelf sandstones from fault block 4. If organic acids were important Si complexers, then it would be reasonable to assume that sandstones associated with organic acid-rich water would have the least amount of diagenetic quartz, because precipitation would have been be inhibited. This is not the case. Burial diagenetic calcite content is similar in most fault blocks, abundant syndepositional calcite is found in sandstones from fault block 1. 259 Summary and Unanswered Questions The analyses presented in Chapter 2 show that shale diagenesis is an active process throughout burial. The conversion of smectite-rich I/S to illite-rich I/S is accompanied by an increase in the abundance of the mixed-layer phase and chlorite, and a decrease in the abundance of discrete illite and kaolinite. The data do not support an aluminum-conservative smectite to illite conversion mechanism. Combining whole shale chemistry with the quantitative mineralogy determined in this study indicates that shales are chemically open systems; the mineralogic changes observed in Frio shales require the import of significant amounts of potassium. The source of the needed K2O appears to be external to the Tertiary section. Mass balance calculations imply that the amount of SiC>2 needed for QOGs in Frio sandstones can be derived from the shales. Further investigations require a better understanding of the chemical changes in Frio VS during diagenesis, since the data from Hower et al. (1976) have been shown to be inaccurate. Diagenesis of Frio Formation sandstones can include compaction, precipitation of syndepositional calcite, kaolinite, and analcime, syndepositional dissolution of feldspars and rock fragments, recrystallization of detrital clay matrix (which apparently continues throughout burial and includes the conversion of smectitetoilliteand theneoformationofchlorite),precipitationofauthigenic quartz, calcite, kaolinite (and dickite), chlorite, analcime, and albite, and further The net result of sandstone development of secondary porosity (Chapter 4). diagenesis is toproduce anincredibly heterogeneousdistributionofauthigenic phases, altered detrital phases, porosity, and consequently, reservoir quality. Many of the burial diagenetic events are related to fluid flow. Sandstones, or parts of sandstones, that are exposed to more flow are the most altered. Factors that affect fluid flow include framework grain size, available PCP, position within the sandstone package, and abundance of detrital clay matrix. It is impossible at this time to predict, with any confidence, the specific differences (at least quantitatively) in the diagenesis of one sandstone to another, or to foresee the preeminent control on the diagenesis of a particular sandstone (E.F. Mcßride’s "vagaries of diagenesis," or, "every theory has its holes when real life steps in," (Biafra, 1986)). Authigenic calcite is the most abundant diagenetic mineral present in Frio Formation sandstones from the Corpus Christi area. Syndepositional calcite is restricted to shorezone facies rocks. The carbon isotopic signature of syndepositional calcite indicates that organic matter was a significant source of carbon for these early cements. 8 180 values of burial diagenetic calcite decrease with depth, the result of continual recrystallization of the calcite with its chemically and thermally changing environment. The trace element content and a function of the formation water causing recrystallization. Differences in the chemistry of diagenetic calcite and the associated formation water from adjacent growth fault blocks indicates 87Sr/86Sr values of burial diagenetic calcite are largely that the fault blocks have been hydrologically separate since the time of calcite precipitation (Chapter 5). The association of 180-depleted calcite with coarse grain-size indicates that fluid flow is an important control on recrystallization of calcite. The data presented in Chapter 6 show that the conversion of semctiteto illite in I/S is also effected by fluid flow. I/S from more permeable sandstones is more illitic than I/S from flow-restricted sandstones. The clay mineral suite in sandstones and associated shale core is enriched in the products of diagenesis It (high %I I/S and chlorite) relative to the shale baseline developed in Chapter 2. is proposed that this relationship is also predominately due to fluid flow effects; flow is concentrated in sandier sections rather than shalier sections. Originally similar clay suites are thus differentially modified. Detrital I/S matrix clay is a volumetrically and diagenetically important component of Frio shelf sandstones. How does the clay component of Frio sandstones from other depositional environments differ from the marine rocks of this study? Are there regional changes in the clay content of Frio sandstones (as there is in Frio shales)? What is the relationship between sandstone clays and shale clays from other units? The model proposed in this chapter, constrained by the isotopic analyses of QOGs, calcite, and kaolinite cements, calls for rapid, upward movement of diagenesis-causing formation water. Calculations constrained by geochemical and 261 petrographic data indicate that great quantities of water are required to have passed through the sandstones in order to produce the identified diagenetic changes. What process is operating that allows for almost 100,000 pore volumes of formation water to have passed through some Frio sandstones? The amount of water in the Frio Formation, made available from the diagenesis of shales, is much less than is required to produce the diagenetic changes observed in the sandstones, therefore it is necessary to identify a process that allows for the recycling and reuse ofreactive formation water. It is especially difficult to understand how Unlessconvection can operate in the high pore-pressure gradient transition zone. some mechanism can be identified that allows for water recycling, we may be forced to admit that we are missing something fundamental about sandstone diagenesis. There is no evidence that organic acids play an important role in the diagenesis ofFrio Formation sandstones. There's lots we don't understand and plenty more to do. 262 Appendix A. XRD analyses of shale cuttings. <1 um fraction w?n Pepth (fp Order Type %L IHite % Chlor. % Kaol. % Temp, (C) Kenedy 7145 R=0 13 48 28 7 17 87 Fig. 2.1 7560 R=0 32 64 13 5 18 91 - #2 8565 R=0 4769 J7 3 11 101 9045 R=0 6166 22 6 5 105 9525 R=0 4676 12 6 7 110 10035 R=0 63 6919 56115 10515 R=0 5774 14 4 8 119 11055 R>0 8184 9 3 3 124 11670 R>0 7372 17 8 3 130 12135 R>0 7465 22 6 7 135 13065 R>0 8179 12 4 5 144 14055 R>0 8174 18 5 3 153 14265 R>0 8168 24 5 3 155 14715 R>0 8283 12 3 2 159 15135 R>0 8379 10 8 3 163 15645 R>0 8172 17 4 6 168 16215 R>0 7777 16 3 4 174 16725 R>0 8274 13 6 6 178 17145 R>0 8368 14 9 9 182 17685 R>0 8482 9 6 3 188 18085 R>0 8480 10 5 5 191 Cameron 3005 R=0 0 70 9 4 16 49 Miocene 3586 R=0 0 42 26 8 23 54 4192 R=0 018 35 16 31 59 4768 R=0 959 21 7 12 65 Fig. 2.1 5347 R=0 31 54 24 8 13 70 #1 5876 R=0 5066 21 10 2 75 6568 R=0 4863 25 9 4 81 7085 R=0 5955 26 14 4 86 7662 R=0 5670 19 6 4 91 8180 R=0 4649 33 11 6 95 8740 R=0 5859 29 7 5 101 Frio 9377 R=0 5369 17 6 8 106 9931 R=0 5678 13 5 4 111 10482 R=0 6979 7 5 9 116 11107 R=0 55 66 20 1 13 122 11703 R=0 4680 11 1 8 127 12285 R=0 69 75 13 2 10 133 Appendix A. XRD analyses of shale cuttings (cont.). Ml Depth (ft) Order Type %\ I/S % Illite % Chlor, % Kaol. % Temp. (Q 14484 R>0 84 45 22 6 26 152 14984 R=0 73 64 17 14 5 157 MSW27 8507 R=0 10 46 23 6 26 103 Fig 2.1 9090 R=0 46 59 10 2 29 108 #3 9936 R>0 7558 7 1 35 116 10517 R>0 7380 7 5 9 121 11119 R>0 81 50 12 3 35 127 11693 R>0 72 58 11 4 27 132 12270 R>0 8067 8 7 18 137 12787 R>0 79 68 11 2 18 142 13271 R>0 74 55 10 6 29 146 13844 R>0 8266 8 5 21 151 13844 R>0 8269 7 4 20 151 14419 R>0 7770 7 3 20 157 14967 R>0 8261 8 5 26 161 15506 R>0 72 61 13 12 14 166 16052 R>0 7147 8 5 40 171 16531 R>0 72 70 12 7 11 176 17182 R>0 71 56 17 7 20 182 17752 R>0 66 64 13 3 20 187 TST36#1 5910 R=0 4 58 21 0 21 75 Fig2.1 6450 R=0 21 44 13 1 42 80 #4 7080 R=0 1852 18 j 28 86 7710 R=0 3161 15 1 22 92 8270 R=0 3442 19 3 36 98 8810 R=0 4558 13 2 27 103 9350 R=0 3167 27 0 6 108 9950 R=0 6975 14 0 11 114 10700 R>0 8164 9 2 25 121 11420 R>0 74 56 22 2 21 128 12620 R>0 8864 8 1 28 139 13320 R>0 86 60 10 0 30 146 13830 R>0 77 49 21 3 27 151 14490 R>0 8366 7 5 23 157 Mustang 12671 R>0 79 72 11 3 14 146 Isle. 13376 R>0 79 77 7 4 12 153 13960 R>0 75 65 11 4 19 159 Appendix A. XRD analyses of shale cuttings (cont.). w0 78 77 7 5 12 165 #5 15230 R>0 75 69 10 4 16 172 15837 R>0 7780 9 4 7 179 16434 R>0 7779 7 4 10 185 17034 R>0 8190 3 3 4 191 Calhoun 8010 R=0 8 54 10 1 35 92 Fig 2.1 10026 R=0 23 58 19 1 22 111 #6 11053 R=0 5670 8 2 20 120 12050 R>0 6564 6 2 28 130 13650 R>0 6968 6 3 23 145 14062 R=0 4558 7 0 35 149 Brazoria 6972 R=0 23 72 10 0 18 85 Fig2.1 7472 R=0 27 78 8 0 14 89 #7 7965 R=03668 9 0 23 94 8460 R=0 4169 10 0 21 98 9018 R=0 2859 19 0 22 104 9579 R=0 3367 10 0 24 109 10135 R=0 5571 9 0 20 114 10624 R=0 42 66 15 0 18 118 11120 R>0 66 66 16 0 18 123 11654 R>0 5971 8 1 21 128 13000 R>0 8489 3 0 8 140 13600 R>0 8779 7 2 11 146 14035 R>0 8891 4 0 5 150 14605 R>0 8176 9 1 14 155 15175 R>0 74 64 12 2 21 160 15704 R>0 7873 9 2 17 165 265 Combine with Appendix B. Framework grain petrography. "Diagenesis" tables from Chapter 4 for total sample petrography. Wsii Depth Otz Ksoar Plag RF Well Depth Otz Kspar Plag RF 10 10399 35.5 2.5 8.5 20.0 51 10454.0 34.5 2.5 7.5 17.5 #2 10411.0 28.5 1.5 6.5 20.5 #1 10463.0 36.0 1.5 6.5 11.5 10418 28.5 4.0 12.0 21.0 10466.0 38.0 2.0 7.5 12.5 10432 31.0 2.5 6.5 19.5 10471.0 31.5 2.0 8.0 8.5 10438 35.5 4.5 8.0 27.5 10476.0 30.5 1.5 13.5 16.5 10448 32.5 3.0 4.0 12.5 10484.5 26.5 4.0 13.5 22.0 10449 30.5 3.0 4.0 15.5 10489.0 30.0 2.5 11.5 22.0 10496.0 28.5 1.5 8.0 17.5 346 14501.0 29.5 0.5 13.0 18.5 10499.5 34.5 2.0 6.0 30.0 #1 14502.0 25.5 0.5 16.0 16.0 10501.0 28.5 3.0 8.0 15.5 14506.0 25.0 1.0 17.5 13.5 10502.0 34.0 2.0 8.0 15.0 14507.0 24.0 0.5 13.5 12.0 10507.0 32.5 0.5 6.5 22.0 14508.0 29.0 0.0 13.0 18.0 14509.0 23.0 0.1 13.0 16.0 Cecelia 11824.0 23.0 0.5 4.0 30.5 14513.5 28.0 0.1 16.0 11.5 Kelley 12181 24.0 0.0 11.5 22.0 14515.0 33.0 1.5 11.0 18.5 12185 20.5 0.0 9.0 30.0 14518.5 28.5 0.5 15.0 13.0 12188 19.0 0.0 8.5 28.0 14519.7 27.5 0.5 19.0 11.0 12191 23.0 0.0 4.5 26.5 14521.0 25.5 0.5 11.0 13.0 12196 21.0 0.0 9.5 25.0 14522.5 27.0 1.0 14.0 15.5 12198 20.5 0.0 8.0 32.5 14523.0 29.0 0.5 13.5 14.0 14529.0 27.5 0.5 12.5 18.0 4 10390 26.5 2.0 4.5 6.5 14530.0 28.5 2.5 16.5 13.0 #2 10398 25.0 2.0 5.5 18.5 14533.7 29.5 3.0 8.0 14.5 10416 32.5 2.0 3.6 7.6 14534.2 30.0 0.5 11.5 17.0 10427 19.2 3.1 4.2 6.1 14534.5 29.5 2.0 16.5 15.5 10429 21.5 1.0 4.0 12.0 14535.5 33.0 1.0 14.0 16.0 14538.0 24.5 1.0 13.0 13.0 Copano 6908.0 45.0 5.5 6.5 6.0 14542.0 28.0 2.0 7.5 16.5 104#7 6917.0 32.0 4.5 5.5 11.0 14543.0 28.5 2.5 10.5 20.5 7154.0 20.0 3.5 9.0 31.5 14550.5 21.5 1.0 9.5 26.0 7158.0 30.5 1.5 9.5 24.0 14552.0 28.0 2.0 8.0 19.0 7163.0 22.5 2.5 9.5 29.0 14553.0 30.5 0.5 12.5 15.5 7163.5 18.0 4.0 10.5 31.0 14554.0 28.5 2.0 9.5 19.5 7171.0 16.0 2.5 11.5 31.5 14555.0 31.0 1.0 10.5 17.5 14564.0 17.5 0.5 9.0 21.5 MSW67 8048 29.5 5.5 10.5 13.5 8053 23.0 4.0 0.5 11.0 392 12259.0 35.5 0.0 4.5 14.0 8070 31.0 3.0 2.5 6.5 #4 12263.0 37.5 0.0 6.0 14.5 8072 37.5 5.0 2.0 3.5 12668.5 42.0 0.0 6.5 11.0 8073 27.5 4.0 0.5 6.0 Appendix B. Framework grain petrography (cont.). 3Kfill Depth Otz KsDar Plag RF Well Depth Otz KsDar Plag RF 392 12270.0 36.5 0.0 3.5 7.0 MSW67 8075 19.5 4.0 3.0 4.5 #4 12270.2 37.5 0.0 5.5 7.5 8075 37.0 2.0 5.0 7.5 12270.5 37.5 0.0 6.5 11.0 8080 45.0 7.0 3.5 10.5 12271.0 34.0 0.0 3.5 14.0 8086 36.5 3.5 10.5 12.5 12271.5 33.5 0.0 3.0 12.0 8088 44.5 6.5 5.0 12.5 12272.0 31.0 0.0 4.0 11.5 8090 23.0 5.0 4.5 5.0 12272.5 24.0 0.0 7.0 13.0 8094 48.5 7.0 4.0 12.0 12288.0 38.5 0.0 7.5 9.0 8095 36.5 2.5 7.5 14.5 12589.0 39.0 0.0 8.5 12.0 8102 26.0 11.0 3.0 7.0 12591.5 35.5 0.0 10.5 10.5 8102 36.5 5.0 4.0 9.0 12593.5 38.0 0.0 6.5 9.5 8103 38.0 8.0 5.0 7.5 12595.0 39.5 0.0 14.0 10.5 8103 33.5 7.5 2.0 13.5 12598.0 41.0 0.0 9.0 10.0 8103 41.5 7.5 6.5 13.5 12599.0 33.5 0.0 3.0 9.5 8104 27.5 7.5 5.5 6.5 12599.5 31.5 0.0 9.5 11.5 12601.0 33.5 0.0 12.0 11.5 MSW70 7214 40.6 5.7 5.3 11.3 12602.0 31.0 0.0 5.0 8.5 7218 38.3 4.3 8.3 16.3 12607.0 30.0 0.0 8.5 10.0 7223 30.0 7.3 6.0 30.3 7228 28.4 5.0 12.0 23.9 430 10071.0 36.5 1.0 6.0 22.0 7231 27.9 5.0 16.0 26.9 #5 10077 30.5 0.5 7.0 16.0 7235 23.5 6.0 6.0 23.5 10095.2 29.5 1.0 7.0 18.5 7235 26.0 3.0 4.0 23.0 10095.8 29.0 0.1 3.5 14.0 7235 30.5 7.9 4.9 29.6 10096 31.0 2.0 4.0 18.0 7238 27.4 3.0 10.0 24.4 10107 23.5 0.1 3.0 21.0 7245 27.0 4.7 5.7 15.0 10113 29.5 1.5 6.0 24.5 7255 27.9 6.5 5.0 21.4 10119 23.5 0.5 3.0 14.5 7261 34.4 7.8 8.3 21.1 10121 21.5 0.5 3.5 15.5 7265 31.3 3.9 3.9 20.1 7266 36.1 4.2 7.9 15.2 49 8029.0 36.0 2.5 6.0 18.5 7267 34.1 5.8 8.7 21.4 #2 8035.0 25.0 2.5 7.0 27.0 7268 39.3 6.3 5.0 14.0 8036.0 17.0 1.5 6.0 29.5 7270 35.3 7.0 9.0 18.9 8040.0 23.5 4.0 7.0 31.0 7272 44.1 6.0 9.0 12.0 8040.5 16.0 2.0 5.0 33.0 7276 34.9 6.6 7.0 18.3 8043.0 15.0 1.0 2.0 53.0 7278 26.5 9.0 7.5 20.0 8047.0 18.5 2.5 8.0 37.5 7281 32.3 5.0 12.3 18.0 8048.0 19.5 3.0 5.0 43.5 7282 28.5 7.5 9.0 15.5 8049.0 17.5 4.0 5.0 38.0 7285 35.0 5.5 5.5 22.0 8051.0 26.5 3.0 6.5 31.5 7286 44.4 7.0 1.0 8.0 8056.0 28.5 3.0 10.0 34.0 7287 44.3 10.4 2.0 14.4 8535.5 37.5 4.0 3.0 17.0 7290 45.3 7.9 4.2 14.7 267 268 269 Appendix B. Framework grain petrography (cont.). w?n Depth Otz KsDar Plag RF Well Depth Otz Kspar Plag RF 49 8539.0 39.5 5.0 3.5 16.5 MSW70 7292 45.9 7.0 3.0 8.5 #2 8546.0 40.0 4.0 4.5 15.0 7295 38.4 10.5 7.0 9.0 8553.0 31.0 4.0 3.5 16.0 7297 38.5 8.0 6.5 15.5 8560.0 42.0 4.5 5.5 13.0 7300 28.5 9.0 6.0 24.5 8561.0 40.5 2.5 4.0 16.0 7301 38.7 8.0 5.0 20.1 8562.0 41.5 6.0 5.5 17.0 7302 29.4 7.5 7.0 18.0 8562.5 41.0 6.0 4.5 20.5 7304 32.7 7.9 8.9 19.8 8577.0 29.5 3.0 5.5 17.0 7305.5 29.0 7.5 5.0 25.5 8580.0 30.5 6.5 3.0 21.5 7306 28.7 8.1 4.5 24.7 8712.0 29.5 5.5 5.5 15.0 7307 30.8 9.4 6.0 20.3 8722.0 36.0 5.5 3.5 9.0 7310 31.9 9.0 8.0 12.5 8730.0 40.0 1.5 7.5 12.5 7312 34.6 7.5 5.5 8.5 8732.0 38.0 0.5 5.0 14.0 7314 23.6 9.5 2.5 11.6 8733.0 39.0 3.5 5.0 10.5 7317 34.4 6.5 4.5 18.5 8738.0 33.5 4.0 4.5 17.5 7317.1 40.1 5.4 2.0 14.4 8977.0 38.0 9.0 2.0 15.0 7317.1 32.2 5.0 4.0 9.5 8981.0 34.5 3.5 6.5 13.5 7317.1 28.4 5.5 4.0 16.9 8997.0 42.0 4.5 2.5 12.0 7318 37.7 1.7 3.8 20.2 8999.0 33.0 4.5 5.5 15.5 7318.5 28.3 5.5 4.5 19.4 9002.0 36.5 3.5 5.0 14.0 7320 32.4 3.0 2.5 15.5 9006.0 34.0 2.0 5.5 13.0 7321 28.9 5.0 2.0 10.9 9015.0 38.0 5.0 6.5 12.0 7322 29.1 4.0 3.0 28.1 9017.0 36.0 3.5 6.5 18.0 7325 34.9 5.6 3.5 14.2 9023.0 39.0 4.5 3.5 14.0 7327 30.5 3.0 1.0 16.0 9026.0 33.5 3.0 5.0 10.5 7331 40.2 6.5 2.0 9.4 9079.0 40.5 4.0 4.5 10.0 7331 34.5 7.6 2.0 6.1 9081.0 41.5 1.5 3.5 14.0 7339 45.0 6.0 2.0 10.0 9086.0 39.0 5.0 5.5 17.0 7342 50.6 5.5 1.5 6.5 9106.5 42.0 2.5 3.5 14.5 7346 57.0 5.5 0.5 8.5 9107.0 33.5 4.5 3.5 11.5 7347 59.4 5.0 1.0 5.0 9107.2 39.5 1.5 3.0 13.5 7351 52.6 4.0 2.5 7.9 9107.5 28.5 4.0 3.0 9.0 7354 50.5 6.9 1.5 12.4 9112.0 41.0 2.5 6.0 8.0 7364 37.8 6.0 4.0 21.2 9114.0 40.0 3.5 8.0 10.5 7365 47.7 6.0 2.0 9.0 9117.0 39.5 4.5 8.5 13.0 7366 44.5 9.0 3.5 10.0 9119.0 40.5 2.0 3.0 11.5 7368 44.5 5.0 2.5 11.5 9120.5 40.0 3.0 11.0 16.5 7372 40.9 8.9 3.4 11.3 9124.0 36.5 1.0 3.5 24.0 7376 49.0 6.0 3.0 10.5 9126.0 36.5 3.0 3.5 15.5 7385 43.4 6.5 4.5 12.0 9127.0 42.5 1.5 9.0 15.5 7390 53.4 4.5 1.5 8.5 9132.0 41.5 2.5 7.0 15.5 7397 43.1 7.8 3.1 8.3 Appendix B. Framework grain petrography (cont.). Weil Depth Otz Ksnar Plag RF Well Depth Otz Kspar Plag RF 49 9137.0 44.5 4.5 7.0 14.5 MSW70 7398 41.9 9.0 5.5 12.5 #2 9138.0 45.5 3.5 7.0 13.5 7408 38.0 4.0 4.5 8.5 9138.5 41.0 5.0 6.0 14.5 7409 39.8 5.0 5.0 5.0 9142.0 40.0 0.0 7.5 12.5 7410 27.1 3.5 2.0 8.0 9143.0 45.0 3.5 7.0 14.5 7411 44.8 1.1 0.6 4.0 9144.0 38.5 4.0 7.0 13.0 7411.5 42.4 5.7 1.0 5.7 9145.0 41.0 7.0 6.5 16.0 7414 35.4 4.6 4.1 9.2 7415 36.6 5.0 5.5 16.5 7418 44.6 7.0 5.5 11.0 7419 33.7 8.0 6.5 11.1 7421 44.6 5.9 4.5 10.9 7422 35.0 8.5 2.5 16.5 7422.5 35.7 8.5 3.5 13.1 7423.5 39.9 2.0 3.5 16.5 7423.5 21.5 2.0 4.1 5.6 7424.5 32.7 3.5 4.5 16.8 7427 29.9 3.5 7.0 14.9 7428 33.7 3.5 4.5 12.9 7430 41.6 5.1 5.1 10.7 7435 31.4 5.0 2.5 12.5 7440 32.2 4.5 4.0 17.1 7445 33.6 7.1 3.1 21.4 Mutchler 8785 43.9 3.7 5.5 15.2 8789 21.5 1.5 2.6 5.6 8792.8 41.4 2.0 2.0 13.2 8793 39.6 1.9 7.1 10.4 8798 39.3 4.2 4.2 20.2 8802 39.3 5.2 4.7 19.9 8810 40.8 4.0 5.0 3.5 8816 42.2 5.5 4.0 11.6 Appendix C. Electron microprobe analyses of diagenetic calcite. Mg ppm Fe ppm Mn ppm Mg ppm Fe ppm Mn ppm Shell 392-4 760 3,039 4,159 Shell 346-1 151 933 3,501 989 3,171 3,276 229 1,042 9,279 12271 1,194 3,599 3,880 14521 121 1,524 5,677 621 3,778 9,945 241 1,547 5,793 1,230 4,415 5,305 513 1,617 2,138 1,954 5,379 5,321 627 2,161 17,488 1,574 6,218 7,613 531 2,759 5,886 1,260 7,112 13,980 501 3,451 3,206 2,087 11,403 14,948 706 4,431 6.142 742 5,161 3,315 Shell 392-4 772 3,078 5,050 1,110 5,713 4,570 784 3,280 5,693 2,219 6,615 1,750 12272.5 808 3,311 5,576 2,497 6,638 3,710 826 3,863 8,109 1,176 7,540 4,724 1,007 3,918 4,167 1,435 9,110 4,763 688 3,956 7,451 1,460 10,206 5,576 862 4,431 8,303 1,948 12,281 4,732 989 5,255 13,368 1,616 12,911 3,648 1,194 5,635 13,693 2,159 13,245 4,825 1,677 5,643 13,019 1,864 14,295 3,973 929 5,698 13,856 1,773 15,359 4,082 959 6,063 12,067 1,369 6,078 12,748 Shell 346-1 742 2,977 22,135 1,230 6,755 11,796 1,037 4,539 9,573 1,761 6,832 14,259 14554 1,279 6,529 9,240 1,285 7,042 12,833 1,815 8,729 5,150 1,254 7,182 13,283 2,099 9,491 11,656 1,405 7,439 13,848 1,990 9,600 7,551 1,242 7,501 12,415 2,069 9,631 5,786 1,279 7,548 12,694 2,418 9,786 9,302 1,212 7,897 12,880 2,515 10,299 10,402 2,328 10,649 10,742 Shell 346-1 1,616 13,735 8,597 2,382 10,804 10,030 1,918 14,225 8,721 1,417 10,836 11,308 14554 1,767 14,232 9,674 1,357 12,375 10,169 (com.) 2,418 19,246 7,869 1,508 12,382 12,229 1,496 12,584 11,091 270 Appendix C cont. Electron microprobe analyses of diagenetic calcite. Mg ppm Fe ppm Mn ppm Mg ppm Fe ppm Mn ppm MSW70 7,370 0 170 MSW 70 615 1,516 3,361 6,737 0 1,309 495 1,593 3,555 7234.5 6,809 0 1,317 7318.5 663 1,695 4,035 7,213 0 356 621 1,710 3,493 7,629 0 798 591 1,811 3,749 5,567 16 2,138 700 2,091 3,570 6,912 47 775 712 2,332 4,267 6,188 62 1,588 814 2,363 3,710 6,725 70 1,100 766 2,402 4,159 7,092 101 782 856 2,721 4,748 5,150 101 217 911 2,767 3,609 8,051 109 914 814 2,837 4,229 7,111 132 1,355 874 3,078 3,834 6,658 140 867 947 3,148 3,694 6,339 17,233 1,247 706 3,257 3,369 820 3,280 3,950 MSW 70 3,094 11,030 163 820 3,381 4,221 1,098 12,888 3,315 814 3,576 4,144 7314 1,399 14,940 3,694 941 3,599 3,958 1,393 15,157 3,973 899 4,197 5,065 1,580 16,331 4,771 1,447 16,347 4,175 1,472 16,502 4,802 MSW 70 11,827 0 860 1,791 16,774 4,980 9,306 0 2,804 1,544 17,000 5,181 7410 10,054 0 1,247 1,725 17,085 4,748 8,474 0 1,913 1,514 17,318 3,787 9,541 0 1,642 1,526 17,334 5,205 9,993 0 759 1,610 17,839 3,934 13,871 0 310 1,550 19,689 5,693 5,772 0 465 1,852 19,961 4,647 8,661 8 2,409 1,894 22,394 6,227 9,698 8 1,634 12,219 54 372 MSW 70 953 9,483 2,378 8,504 202 1,348 1,086 10,501 2,533 2,829 295 5,391 7410 1,381 11,729 2,362 8,419 319 1,836 (corn.) 1,423 14,302 2,471 645 6,731 2,021 271 Appendix C cont. Electron microprobe analyses of diagenetic calcite. Mg ppm Fe ppm Mn ppm Mg ppm Fe ppm Mn ppm MSW 70 5,826 3,669 922 MSW 70 2,931 0 472 5,735 3,560 914 3,052 194 209 7317 5,633 2,822 829 7423.5 2,720 4,065 775 6,803 4,166 914 2,853 124 325 6,755 3,700 604 1,544 6,203 349 6,483 5,410 953 3,088 878 805 4,843 3,008 682 3,009 241 821 5,530 3,272 627 3,534 140 341 4,849 3,793 813 3,178 109 503 6,664 4,127 999 3,070 311 558 7,243 6,257 1,286 2,925 0 1,015 5,965 3,638 658 2,268 684 1,100 5,910 3,879 651 3,070 62 674 5,947 3,498 527 3,124 16 1,053 5,862 3,964 891 2,129 350 1,139 5,090 3,164 782 MSW 67 15,855 303 108 6,767 4,625 1,278 9,445 428 186 7,454 4,190 1,177 8053 11,097 700 0 7,068 4,780 1,015 10,036 1,018 194 6,073 2,977 720 9,861 1,049 0 4,403 2,946 790 11,477 5,760 2,866 2,810 6,366 945 MSW 67 17,176 497 0 4,131 6,397 682 15,940 979 635 2,853 6,638 867 8075 905 1,205 1,115 1,779 6,786 937 820 1,290 1,084 1,665 7,198 1,975 1,411 1,508 984 1,954 7,345 705 12,080 1,640 1,239 1,948 7,579 1,727 1,520 1,850 991 1,797 7,757 1,293 881 2,386 1,929 7,575 8,737 519 1,182 3,319 3,191 7,092 8,745 465 16,754 5,115 1,719 1,900 8,791 2,006 13,178 5,387 2,633 1,767 9,056 1,766 6,881 6,972 4,136 2,515 9,475 1,433 38,496 7,229 1,859 2,678 9,662 1,131 1,574 8,830 5,375 2,382 11,092 7,311 MSW 67 2,792 11,216 6,909 120,451 11,146 1,595 8075 cont. 5,398 55,756 3,059 272 Appendix C cont. Electron microprobe analyses of diagenetic calcite. Mn Mg ppm Fe ppm Mn ppm Mg ppm Fe ppm ppm MSW67 1,870 3,202 2,068 MSW67 21,392 0 728 13,883 0 46 10,241 482 201 8102 15,705 0 116 8103 10,717 567 0 13,540 2,068 410 9,692 1,290 155 11,580 202 0 6,875 1,640 132 11,766 233 604 1,833 11,108 3,021 9,613 16 287 1,665 10,470 3,741 1,821 10,727 2,393 1,749 10,385 3,570 8,884 692 170 1,821 11,799 3,113 17,219 0 0 3,251 5,348 852 16,326 23 356 8,896 1,524 380 15,162 54 310 11,248 948 325 9,221 23 472 1,640 1,749 984 1,586 9,040 1,673 1,622 10,424 3,377 1,912 10,058 1,812 25,626 334 139 1,025 1,663 2,169 1,646 11,139 3,547 17,116 16 0 1,839 11,683 3,152 1,248 2,526 1,998 1,206 2,114 2,153 1,303 2,456 2,014 1,502 5,138 3,206 1,267 2,868 1,743 1,110 1,586 2,401 965 995 2,114 1,212 2,044 2,324 11,990 0 0 11,423 1,251 403 14,679 171 0 11,230 1,858 163 1,737 3,031 2,657 6,387 1,648 372 1,098 2,402 2,200 18,696 1,477 798 4,023 1,609 302 1,652 11,045 3,431 17,978 1,213 310 1,429 9,265 3,532 19,631 832 170 1,007 1,632 1,882 1,116 1,555 2,455 2,045 5,705 8,403 1,453 3,754 2,385 Arco 430-5 1,435 4,011 6,978 1,200 2,977 2,052 1,646 4,990 6,599 10071 2,503 6,755 9,255 Cecelia Kelley #2 1,460 4,936 2,192 2,129 5,573 8,450 1,665 6,483 2,610 2,334 7,345 9,503 12185 6,719 12,211 2,533 2,286 6,444 9,890 1,140 3,591 1,743 1,104 3,319 6,010 8,986 23 0 1,683 5.286 7,993 1,369 6,327 2,943 868 3,171 4,632 953 3,646 1,650 273 Appendix C cont. Electron microprobe analyses of diagenetic calcite. Mg ppm Fe ppm Mn ppm Mg ppm Fe ppm Mn ppm Arco 430-5 1,912 6,234 7,977 CeceliaKelley #2 1,677 6,553 3,113 965 3,296 5,375 2,286 171 0 10071 1,689 5,418 7.822 12185 1,918 7,524 3,276 (corn.) 1,532 4,096 6,668 (corn.) 1,833 5,884 2,169 2,231 5,799 7,606 1,707 6,498 2,819 2,123 5,845 8,248 1,876 7,338 3,702 1,918 5,511 8,442 1,375 5,410 2,718 2,141 5,806 8,465 1,839 5,814 2,362 2,328 5,923 7,342 1,363 4,905 1,588 1,496 4,236 6,366 2,497 7,540 2,649 850 3,063 3,934 2,075 5,907 8,891 Cities 49-2 2,123 8,644 12,098 2,551 3,521 4,066 Cities 49-2 1,128 9,786 3,578 8553 1,966 7,905 12,005 929 8,434 2,331 2,099 8,535 13,112 8040.5 778 8,030 3,098 2,195 8,441 11,951 452 4,811 1,626 1,984 8,504 12,679 856 7,361 3,230 2,274 9,483 11,966 2,238 187 922 1,870 7,991 11,625 754 8,278 2,788 2,219 8,418 12,245 2,244 23 0 1,659 7,415 13,066 820 8,535 2,695 2,081 7,967 12,245 911 8,146 3,121 2,032 8,154 11,811 253 2,612 945 2,063 8,861 12,175 3,534 0 0 1,966 8,162 12,826 718 6,972 2,083 2,533 9,810 11,850 754 8,962 2,835 1,900 8,177 11,517 947 8,892 2,928 2,051 8,247 11,408 826 7,579 3,632 2,123 8,154 12,353 1,888 1,897 852 2,026 8,372 12,516 2,599 497 349 1,592 6,599 12,291 694 7,967 2,169 953 9,328 3,067 274 Appendix C cont. Electron microprobe analyses of diagenetic calcite. Mg ppm Fe ppm Mn ppm Mg ppm Fe ppm Mn ppm Cities 49-2 1,671 16,766 2,246 Cities 49-2 1,399 10,097 10,587 1,116 12,390 1,944 1,206 10,299 11,083 8577 1,622 16,665 2,478 8733 1,049 9,631 10,347 1,538 14,248 2,145 1,321 9,949 10,928 1,538 10,167 380 947 8,698 9,952 1,031 5,457 4,996 1,007 9,631 10,022 1,429 9,141 5,700 1,297 9,289 11,625 2,111 16,992 2,525 1,025 9,234 10,371 1,568 11,675 3,090 935 8,581 10,123 1,900 15,554 2,052 1,025 9,662 10,363 1,550 16,720 2,246 941 8,838 10,394 1,146 9,094 3,385 1,309 11,411 10,177 1,484 16,012 2,215 1,049 9,631 9,898 874 7,089 5,631 1,484 10,906 11,664 1,827 18,585 2,711 1,230 9,405 10,990 2,153 21,430 2,664 1,785 13,758 10,433 1,803 17,279 2,618 1,110 9,763 10,347 1,912 19,642 4,624 856 8,659 10,138 302 995 6,126 8,250 4,050 1,100 Cities 10-2 832 1,135 10,107 1,260 12,025 3,942 2,081 3,420 17,186 1,466 15,935 5,150 10411 1,001 1,928 12,346 5,639 140 488 874 1,306 10,301 947 1,492 10,657 Cities 49-2 2,768 8,931 25,690 1,067 1,391 10,440 3,414 11,022 27,472 881 1,461 9,960 9126 3,365 10,323 28,657 995 1,174 10,409 3,136 9,623 25,946 531 1,337 8,241 2,961 9,942 28,215 1,291 2,068 12,857 2,376 9,009 19,393 3,233 10,206 28.207 3,094 11,411 23,189 2,726 9,141 26,232 Cities 49-2 2,660 8,519 23,111 2,871 10,509 24,257 2,786 10,439 28,912 2,322 8,480 23,297 9126 3,329 10,540 28,440 3,432 11,255 27,162 (corn.) 3,221 9,981 22,786 3,040 9,879 26,240 3,118 9,242 25,535 275 Appendix C cont. Electron microprobe analyses of diagenetic calcite. Mg ppm Fe ppm Mn ppm Cities 10-2 3,329 7,112 31,452 2,455 5,387 26,589 10432 2,672 5,977 26,674 3,257 7,011 30,260 2,762 6,078 27,533 3,034 6,607 30,848 3,184 7,035 31,150 3,148 7,112 30,105 2,865 6,420 28,920 2,569 4,998 25,125 2,841 6,078 28,323 3,178 7,260 31,979 3,100 7,003 30,097 1,972 4,703 23,405 2,430 5.394 26.945 2,829 6,483 26,984 276 Appendix D. XRD analyses of <2 urn fraction of sandstone and shale core. w