Copyright 
by 
Carlos Alberto Amaya 
1996 

ARCHITECTURE OF ESTUARINE RESERVOIRS OF THE 
CRETACEOUS-CABALLOSFORMATION 
ORITO FIELD, PUTUMA YO BASIN, COLOMBIA 
by 
Carlos Alberto Amaya (BS) 
THESIS 
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 

MASTER OF ARTS 
The University of Texas at Austin 
August, 1996 

ARCHITECTURE OF ESTUARINE RESERVOIRS OF THE 
CRETACEOUS-CABALLOS FORMATION 
ORITO FIELD, PUTUMAYO BASIN, COLOMBIA 

Approved by Supervising Commitee: 
Dedication 
To my parents, Amayita and Leito, from whom 
I have always received love, prayers 
and guidance to build a beautiful family. 

ACKNOWLEDGMENTS 

The author would like to thank Dr. Noel Tyler for his friendship, valuable guidance and advice during all phases of this work. Thanks to Dr. William Fisher for teaching me all about sequence stratigraphy and my thanks to Dr. William Galloway for his suggestions and constructive criticism of the depositional model and the manuscript. 
Special thanks to Dr. Ray Levey for allowing me to use the computer facilities at the Bureau of Economic Geology. Thanks to my professor in Sedimentary Petrology Dr. Earle McBride and thanks to Drs. Shirley Dutton and Robert Folk for their discussions about diagenesis. 
Ecopetrol, the Colombian State Oil Company provided cores, well logs, file reports and financial support for this thesis. Special thanks to my friends Cesar Vasquez, supervisor of the Production Geology Department; Eduardo Ariza, Ernesto Calderon, Ramiro Amaya and Jose Urueta (Ecopetrol, Bogota), all of whom were most helpful in obtaining data. 
Students from Geology Department provided valuable help and support during my studies in Austin. In particular, Mulugeta Fesecha, Martha Beltran and Richard Weiland for their friendship and guidance. Pedro Leon, Feliz Diaz, and Jorge Nieto were helpful in the management of workstations and software necessary for this work. 
v 
Finally, my deepest appreciation is to my wife Amparo and my daughters Adriana Marcela and Maria Alejandra, who patiently and quietly sacrificed during the protracted completion of my studies. 
VI 
ARCHITECTURE OF ESTUARINE RESERVOIRS OF THE 
CRETACEOUS-CABALLOSFORMATION 
ORITO FIELD, PUTUMA YO BASIN, COLOMBIA 

Carlos Alberto Amaya, MA 
The University of Texas at Austin, 1996 

Supervisor: Noel Tyler 

ABSTRACT 
Orito Field occupies an area of 31 mi 2 (80 km 2) in the west-central portion of the Putumayo Basin, Colombia and forms part of an extensive littoral system that dominated sedimentation during Albian-Aptian time. The Caballos Formation represents the oldest Cretaceous unit, and was deposited at the beginning of a retrogradational episode immediately above the eroded Triassic-Jurassic surface. The Caballos Formation has an average thickness of 240 ft (73 m) and is largely composed of fine grained, highly compacted quartzarenites, cemented by quartz and kaolinite. 
A geologic model integrating all the available information allows the definition of four depositional events in the Caballos. The lowest depositional unit is composed of fluvial deposits with minor tidal influence. These fluvial sands grade upward into estuarine deposits formed in tidal channels and tidal flats, that are in turn 
vii 
overlain by tidal channel deposits, and are finally eroded and overlain by tidal mouth bars deposits. 
The vertical facies association is the product of a retrogradational episode and represents deposition in a tide-modified estuary, inside which diagenetic processes acted differently modifying the petrophysical properties of the facies that compose the Caballos reservoir in Orito field. 
Historical production trends of the Caballos reservoir correlate with the major depositional axes defined in this study and allow to delineation of high potential areas for future development, by means of targeted infill drilling and workovers. 
viii 
Table of Contents 
List of Figures........................................................................................................ x1 

INTRODUCTION .... .. ....... .. .. .......... .............. .... ............................ ........... .... ........ 1 

GENERAL INFORMATION .................................................................... ............ 3 
Location.................................................................................................... 3 
Previous Work .. . . . . . . .. . . . . . .. . . . .. ... . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . .. . . . . . . . . . . . .. . 3 
Data.......................................................................................................... 6 
Methods.................................................................................................... 8 

REGIONAL SETTING OF THE ORITO FIELD ..................... .............. .............. 11 Tectonic Framework . . .. ... ... .. .. .. .. .. .. .. ... .. .. ... .. .. ... . . . .. .. . . . .. ... ... . . . . . . . .. . . . . . . . .. ... . 11 Stratigraphic Framework . .. .. .. ... .. ... .. ... .. . . .. .. . . .. .. . . ... .. . . .. ... .. . . . . .. . . . . . . . .. . . .. .... . . 14 
STRUCTURAL SETTING AND FACIES ARCHITECTURE OF THE CABALLOS RESERVOIR IN ORITO FIELD ...................................... 22 Structural Model . . . . . . ... . . . . . .. . . . .. .. . . . . . . ... . . . . . . . . ... . . . . . . . . .. . . . . ... . . . . . .. . . . . .. . . . . . . . . . . .. . . 22 Ori to Anticline . . . . .. . . . . . .. . . . . . . .. . . . . . . . . . . ... . . . . . . . .. .. . . . .. . . . . . .. . . . . . . . . . . .. . . . . . . . . . . 23 Orito Fault..................................................................................... 25 Secondary Faulting........................................................................ 25 Stratigraphic Model . . . . . ... . . . .. ... . . . . . .. . . . . . . . ............ .. . . . . . .. . . . . . .. . . . .. . . . . . . . . . . . . . . . . . . .. 27 Major Facies .......... ........................... ......... ........ ............. ....... ...... .. 33 Pluvial Channel Facies .......... ......... ..... .............. ..... ............. 33 Tidal Flat and Marsh Facies . . .. ... . . . ... . . . . .. . . .. . . . . . . .. .. .. ... .. . . .. . . .. 39 Lower Tidal Channel Facies .............................................. 47 Tidal Mouth Bar Facies ......... .............. ...... ................... ...... 55 Summary of Depositional History ...... ............ ................... ...... ....... 58 
PETROGRAPHIC AND DIAGENETIC CHARACTER CABALLOS RESERVOIRS ....................................... ........... ... ....... ....... ..... ........ 65 Petrography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Diagenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Quartz Cement . .. . . . . . . . . .. . . .. .. . . . .. . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . .. . . . . . . . . . . . . . . . . . . . . . . 69 Kaolinite Cement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Calcite Cement . . . .. ... . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 72 Siderite Cement............................................................................. 74 Porosity ..................................................................................................... 74 
IX 
IMPLICATIONS FOR EXTENDED FIELD DEVELOPMENT . . ...... ................ .. 78 
CONCLUSIONS ................ ..... ............................ ............... ....... .. ...... ..... ...... ........ 83 
APPENDIX : PETROGRAPHIC DESCRIPTIONS .. ..... .. ................ .... ........ ........ 84 
REFERENCES CITED........................................................................................ 101 
VITA.................................................................................................................... 106 

x 
List of Figures 
Figure 1. Regional geologic setting of northwestern South America...................... 4 Figure 2. Information used in this study ....... .............................................. .......... 7 Figure 3. Synthetic seismogram of the Orito-9 well .............................................. 10 Figure 4. Regional cross section of the Putumayo Basin . .. ....... .. . . . . .. . . .... . . . .. .. . . . .. .. . 13 Figure 5. Structural trends and oil Fields in Putumayo Basin ................................ 15 Figure 6. Stratigraphic column of Orito field........................................................ 16 Figure 7. Isopach map of the Caballos Formation in Putumayo Basin ................... 18 Figure 8. Paleogeographic map of western South America during Aptian-Albian time..................................................................... 19 Figure 9. Structural map of the Caballos Formation in Orito field ......................... 24 Figure 10. Seismic expression of the Ori to fault . ... .... . . ..... . . . ..... . . . ....... .. .. . . .. .. . . .. .. ... 26 Figure 11. Time slice map of the 3D survey at 1000 milliseconds .......................... 28 Figure 12. Seismic line from the 3D survey showing secondary faulting ................ 29 Figure 13. Isopach map of the Caballos Formation in Orito Field .......................... 31 Figure 14 Type log of the Caballos Formation in Orito field ................................. 32 Figure 15. Core photo of the fluvial channel facies . . .. ... . . . .. .. . . . . .. .. . . . .. . . . . . . . . . . . . .. . . . ... . 34 Figure 16. Net sand map of the flu vial channel facies ............ ........ ...... ............. ..... 36 Figure 17. Facies map of the fluvial channel deposits ... ...... ... ..... ........................... 37 Figure 18. Stratigraphic cross section in Ori to field .. .. . .. . . .. . . .. . . .. . ... . . . . . .. . . .. .. . . . . . . . . . . 38 Figure 19. Amplitude map distribution of the flu vial channel facies ....................... 40 Figure 20. Core photo showing the character of tidal flat facies . . .. .. . . . . . .. . . .. .. . . . . .. . . . 41 Figure 21. Net sand map of unit 2C . . . . .. . . . . .. ... . . .. . . . .. .... . . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . .. . . . . .. 43 Figure 22. Net sand map of unit 2B ... ........................................ .......................... 44 Figure 23. Net sand map of unit 2A ..................................................................... 45 Figure 24. Facies map of unit 2C . . .. . . . . . .. . . . . .. .... . . . . . .. .......... .. . . . . . .. . .. .... . . . . .... . . . .. . . . . .. 46 Figure 25. Horizon slice at level of the tidal flat and marsh facies . . . . . . . . . . . . . . . . . . . .. . . . . 48 Figure 26. Core photo showing the character of lower tidal channel facies . . .. .. . . . . . 49 Figure 27. Net sand map unit 3B ..................... .............. ....... ........ ...... ..... ...... ...... 51 Figure 28. Net sand map unit 3A ... ....................................................... ............... 52 Figure 29. Facies map unit 3B ............................................................................. 53 Figure 30. Horizon slice at the level of the lower tidal channel facies . .................. 54 Figure 31. Core photo showing the character of tidal mouth bar facies . . .. . . . . .. . . .. .. 56 Figure 32. Net sand map of unit 4B ........ .................................... .... ...... ............... 57 Figure 33. Net sand map of unit 4A ........... .......... ........... ................... .................. 59 Figure 34. Facies map of unit 4A .................................................................... ..... 60 Figure 35 Horizon slice at the level of the tidal mouth bar facies . . . . . . .. . . . . .. .. . . .. . . . . .. 61 Figure 36 Distinction between tide dominated and wave dominated estuaries . . . .. .. 64 Figure 37. Mineralogic composition and cements for different sedimentary facies . 66 Figure 38. Generalized paragenetic sequence of Caballos Formation .................... 68 
xi 
Figure 39. Photomicrograph showing quartz overgrowths................................... 70 Figure 40. Photomicrograph showing kaolinite cement . .. . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . 71 Figure 41. Photomicrograph showing calcite cement .. .. . . . .. ......... . . . . . . .... . . . . . . . . .. . . . . . 73 Figure 42. Photomicrograph showing siderite cement.......................................... 75 Figure 43. Photomicrograph showing secondary porosity.................................... 76 Figure 44. Photomicrograph showing stylolites and quartz cement ...................... 77 Figure 45. Cumrnulative production in Orito field............................................... 79 Figure 46. Structural map of the Caballos reservoir showing high potential areas for future development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Figure 46. Thin section showing the character of fluvial channel facies . .. ...... ..... .. 88 Figure 47. Thin section showing the character of overbank deposits.................... 91 Figure 48. Thin section showing the character of tidal flat facies .......................... 94 Figure 49. Thin section showing the character of tidal channel facies . . . . . . . . . . . . . . . . . . . 97 Figure 50. Thin section showing the character of tidal mouth bar facies .. ...... ..... .. 100 
XII 
INTRODUCTION 
Fined-grained quartzarenites of Aptian-Albian age compose the main reservoir in Orito Field and the entire Putumayo Basin in Colombia. Despite the productive potential of Caballos Formation, little has been published concerning the depositional and diagenetic framework of this Cretaceous unit. The purpose of this thesis is to establish a detailed geologic model of the Caballos Formation in Orito field and to contribute to the general understanding of the depositional history during Albian­Aptian time in Putumayo Basin. 
In an attempt to develop this geological model, an analysis was undertaken involving the description of about 120 ft (40 m) of core, the interpretation of 31 linear miles (50 km) and 9 mi 2 (24 km 2 ) of 2D and 3D seismic respectively. Detailed stratigraphic cross sections were generated using well logs from 49 wells, which also provided information to generate net sand maps and facies maps. Petrographic descriptions support the diagenetic history established for each facies. 
On basis of this analysis, the Caballos Formation in Orito field has been divided in four stratigraphic units separated by shaly beds with some lateral continuity. In general these units present an average thickness of 50 ft (15 m), show a NNW paleodip direction and indicate a complexity of petrophysical properties in the Caballos Formation. These units were deposited in a tide-dominated estuary. 
Reservoir properties m these rocks are facies selective, thus, the facies distribution presented for each unit m this study provide the basis for the understanding of geological and engineering properties, and contribute to the definition of appropriate strategies for more efficient exploitation of the significant remaining oil resource. 
GENERAL INFORMATION Location 
Orito field occupies an area of 31 mi 2 (80 km 2 ) located in the southwest part of Colombia near the Ecuadorian border, at 0.65° north latitude and 74° west longitude (Figure 1). The field was discovered by Texaco in 1963 with the successful completion of Orito-1 at 800 barrels per day. Sixteen years later the field was bought by Ecopetrol, but the development of the field was performed basically by Texaco. 
A combination of gas cap expansion and active water influx constitute the drive mechanism of the field; however, the reservoir pressure has decreased from 3200 psi at time of discovery to 1600 psi in 1996 indicating the need of additional energy supply to produce a volume of 250 MMBLS of mobile oil, because only the equivalent volume of 28 percent of oil in place has been produced. Drilling through today has resulted in forty three oil wells, three gas wells and three dry holes. Only nineteen wells remain active and generate a daily production of 3800 bbl. The current well spacing including all wells is about 80 acres. Previous Work 
Published information specifically addressing the Caballos Formation in the study area does not exist. The lack of outcrops of this Cretaceous unit over most of the Basin, and the difficult access to the area are perhaps the main reasons for the lack of published information. The knowledge of Caballos Formation in Putumayo Basin 
3 

80° 78° 76° 74• 72° 70° 8" 

4• 
2· 
o· 
2· 
4• 

72° 70° 
Figure 1. Regional geologic setting ofnorthwest South America showing the location of Ori to field. (Modified from unpublished Ecopetrol reports) 
is basically supported by the information supplied by internal reports generated during the production history of the fields throughout the basin (Bueno, 1973; Valderrama, 1975; Beicip, 1988). While these reports constitute a bank of information, access is restricted. 
On basis of these reports, general studies have been published by a few authors, most of them workers for Ecopetrol, which constitute so far a unique source of information (Govea and Aguilera, 1980; Caceres and Teatin, 1985; Matthews and Portilla, 1994 ). Regional studies at a large scale provide analysis throughout the Sub­Andean basins which allow determination of the evolution of these basins and help to understand localized phenomena (Case, 1974; Macellary, 1988; Gallagher, 1989; Montgomery, 1992). 
The name of the Caballos Formation was initially defined by McArthur (1938; in Miley, 1945), a geologist from Texaco, to designate a sandy section located at Los Caballos hill, in Ortega, a town located in the Upper Magdalena Valley (Florez and Carrillo, 1994). In the Putumayo Basin the first reference belongs to Olsson (in Jenks, 1956), who presents the Caballos Formation as equivalent to the Hollin Formation in Ecuador, limited by the Mocoa Formation (top) and the Misahualli Formation (bottom), but he does not indicate either the description or origin of the name (Julivert, 1968). 
Reservoir studies and internal reports based on analysis supported by core description constitute a more recent attempt to improve the general understanding of the depositional features of Caballos Formation in Putumayo basin (Bejarano et al., 1990; Reyes and Ruiz, 1995). 
Data 
The data set includes 120 ft ( 40 m) of core, 31 mi ( 50 km) of 2D seismic lines, 9 mi 2 (24 km 2 ) of 3D seismic and 36 thin section samples (Figure 2). The cores include the most important sections of Caballos Formation and provided the samples for the petrographic description and the petrophysical analysis mentioned previously. The seismic data consisted of digital migrated seismic profiles with a vertical scale of 
7.5 inches per second, taken in three different surveys from 1988 -1992. The quality of these seismic profiles is adequate for structural interpretation but very limited for stratigraphic analysis. 
The 3D survey acquired in 1992, is formed by a regular grid of lines separated by 82 ft (25 m) from each other, which cover a total area of 9 mi 2 (24 km 2 ). Four seconds were recorded with a sample rate of two milliseconds. The well data set is composed by ascii files obtained from digitized logs. A typical well log set includes deep and medium induction, gamma ray, and sonic logs. 
The thin section samples selected from the Orito-4 core for petrographic analysis, include all significant rock types or facies. Thin sections were prepared by conventional methods and stained to assist in the identification of iron-rich carbonate cement and to distinguish between potassium feldspar and plagioclase feldspar. 

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Methods 
Basic lithological data were derived from description and interpretation of the Orito-4 core. Texture, sedimentary structures, basic rock types, contacts between different lithologies, and trace fossil types, were supported by petrographic description. The analysis of this information allowed the definition of four major units in the Caballos Formation in the study area, each separated by shale layers. A correspondence between core and logs was established, and this relationship was used to correlate the four units across the field using electrical logs. 
The initial correlations were improved in detailed sand-to-sand correlations. With these detailed correlations, net-sand maps and facies maps were generated for each individual sand body. The assemblage of these different maps resulted in the definition of the paleogeographic features that controlled the sedimentation of Caballos Formation in Orito field. The generated model was compared with ancient and modem examples found in the geological literature, and the depositional environments defined. 
The petrographic descriptions served to determine the effects of depositional and diagenetic processes on reservoir properties, and thereby assisted in calibration of geophysical logs to accurate lithologic determination in the rest of the field, where core data were not available. 
Seismic interpretation of the 30 survey involved a wide range of seismic technologies including both software and hardware. Programs and desk utilities provided by Openworks platform from Landmark® running on a Sun Spark 20 workstation, were used to accomplish the final interpretation. 
Electrical and acoustic logs were used to generate syntethic seismograms and calibrate the 3D volume (Figure 3). Structural interpretation of the 3D volume was extrapolated to the 2D seismic lines across the field, and a total coverage of the study area was derived. 
In addition to accurate structural definition, the 3D volume provided a wealth of physical properties useful in stratigraphic analysis (reflection strength, phase, amplitude, etc.). Amplitude extraction and horizon slices from mapped seismic occurrences were compared with the different maps derived from the depositional model and relationships were established. A detailed description and interpretation of these correlations is presented following the facies analysis for each unit, but a quantitative analysis of these relationships is beyond the scope of this study. 

Time 
Sec. 
= 
Figure 3. Synthetic seismogram of the Orito -9 well. 
REGIONAL SETTINGS OF THE ORITO FIELD Tectonic Framework 
The longitudinal fringe located between the Andes cordillera and the Guyana­Brazilian massifs in South America is occupied by an extensive system of basins that extends over 4000 mi (6400 km) from Venezuela to Argentina (Canfield et al., 1982). These basins are separated by cross-basinal arches that radiate from the massifs, and sedimentation characterized by dominance of elastic intervals (Macellari, 1988). 
Putumayo, one of these Sub-Andean basins, is located in the southwestern part of Colombia, occupies an extention of over 19,300 mi 2 (50000 km 2 ), and extends into Ecuador and Peru as the Oriental Basin and the Marafion Basin respectively (Figure 1). The basin has been classified as perisutural foredeep basin (St.John et al., 1987), is highly asymmetric with the depocenter just east of the Andes foothills where sediments have been downwarped in association with the upward thrusting of the Andes mountains (Shirley, 1985). 
The evolution of Putumayo Basin involves three successive stages throughout the Mesozoic-Cenozoic, namely rifting, followed by passive margin and foreland basin development as summarized by Matthews and Portilla (1994). According to these authors, red beds, volcanic flows and marine sediments represent the most important deposits formed during the rift phase that lasted from Triassic to early Cretaceous (Motema Formation). 
11 
Marine sandstones and shales deposited within Putumayo, Llanos and Upper Magdalena Basins in Colombia during Aptian-Campanian time are the result of deposition in a passive margin formed after the rift phase (Caballes and Villeta Formations). 
The Basin evolved into a foreland basin during the Campanian as consequence of the collision of the allochthonous Western Cordillera terrain against the Central Cordillera, causing uplift and thrusting of the Central Cordillera toward east. The Cretaceous sea was abandoned and sediments were deposited under continental and littoral conditions (Rumiyaco Formation). Two sources of sediments are observed for first time: an eastern source provided by the Guyana massif and a more restricted western source provided by the ancestral Cordillera Central (Macellari, 1988). 
The collision of the Panama -Baudo arc during Eocene forced the thrust front to advance to the eastern limit of the Central Cordillera. The thrust front shifted to the east during Miocene, causing uplif of the remainder of the Eastern Cordillera, thus separating the upper Magdalena valley and Putumayo Basins. 
Two major structural provinces have been defined in Putumayo Basin (Govea and Aguilera, 1980; Caceres and Teatin, 1985; Matthews and Portilla, 1994). The fold belt to the west, associated with the uplift of the eastern cordillera and characterized by NE-SW trending of asymmetrical hanging wall anticlines, related to major westerly dipping thrust faults such as Orito fault (Figure 4). 
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Figure 4 . Idealized structural and stratigraphic cross section across Putumayo Basin. Orito field is located at the eastern extreme that separates the two major provinces of the basin: fold belt and platform. Scales are approximate. Section location in figure 5. (Modified from Matthews and Portilla, 1994)  
-w  

This kind of thrusting extends across most of each basin, with structures becoming progressively younger to the east (Montgomery, 1992). 
The second province is represented by the area that extends east of Orito fault, where the intensity of structural deformation decreases resulting in a stable platform gently dipping toward northwest. North-south trends of reverse faults such as Puerto Asis fault, have been identified (Matthews and Portilla, 1994 and Figure 5). 
Stratigraphic Framework 
A sedimentary section of over 12.000 ft (3650 m) ranging in age from Triassic to Holocene is present in Putumayo Basin (Figure 6). This sedimentary volume thins eastward and is underlain by Precambrian crystalline igneous and metamorphic assemblage of the Guyana shield (Matthews and Portilla, 1994). 
The oldest sedimentary rocks are the Triassic-Jurassic Mote ma Formation, which is composed of arkosic sandstones, red mudstones and conglomerates deposited under continental conditions, and constitutes the economic basement in the study area. Little is known about the thickness of the pre-Cretaceous section, as only a few wells have partially penetrated it. Seismic information however, suggests, that in places several hundreds of feet of these rocks are present in the subsurface. Julivert, (1968), considers the Motema Formation equivalent to the Chapiza Formation in Ecuador. 

COLOMBIA 
Mansoya
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Figure 5.  Structural trends and location of the main oil fields in Putumayo Basin.  
Dashed line shows the approximate location of the cross section presented in figure 4.  
(Modified from Matthews and Portilla, 1994)  
- 
VI  


Unconformably overlying the Triassic-Jurassic continental section, is the retrogradational Cretaceous basal sandstone called the Caballos Formation. This widely distributed sandstone is usually gray in color, medium to fine grained, with local coarse-grained units. Sedimentary structures grade from cross bedding at the bottom, through continuous wavy lamination in the middle section, to massive at the top. The sediment is moderately to well sorted, mineralogically supermature, highly compacted, and is cemented with quartz overgrowths, kaolinite, and calcite. Porosity is mainly secondary through dissolution of framework grains and cements, and ranges from 12 to 19 % locally. 
The thickness of the Caballos ranges from 350 feet ( 107 m), in the extreme southwest part of the basin, to 0 feet in the eastern part (Fig. 7). The Caballos Formation is part of a sandstone belt derived from the Guyana massif during the late Aptian-Albian. These rocks were deposited in deltaic to shallow marine environments that covered Putumayo and Upper Magdalena Basins as well as the Santander massif and the Merida arch. The belt includes the Aguardiente, Une, and Caballos Formations (Macellari; 1988. and Figure 8). 
Conformably overlying the Caballos is a series of organic-rich marine shales, limestones, and thin sandstones that make up the Villeta Formation. This unit ranges in age from Albian to Coniacian (Matthews and Portilla, 1994 ), has a thickness of over 1200 ft (366 m) in Orito field, and is the primary hydrocarbon source rock for the Putumayo Basin. 
/l 
I ,,,,,.----­
EXPLANATION 
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Ul 0 10 20 \ 
0
~ § km \
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0 
m ECUADOR m
m 
Figure 7. Isopach map of the Caballos Formation in Putumayo basin. Values are in feet. Note the location of Orito field where the thickness is about 240 ft. (Modified from Bejarano et al., 1990) 
-00 
EXPLANATION  
MARINE  
~Chert/greywake/  
~ophiolites  
I:;::::::ILimestone {shale)  
-------­t--------1  Sandstone/  
shale  
~  
L__;__:__;_:j  Sandstone  
Volcanic arc  
APTIAN -ALBIAN  
0- -Km  500- CONTINENTAL ............ Sandstone/............ ............ shale  

Figure 8. Paleogeographic map of northwest South America during Aptian­Albian time. Note the extention of the retrogradational sandstones, named differently in each basin. (Modified from Macellari, 1988) 
According to Macellari, the Villeta Formation is part of interval that represents the maximum Cretaceous flooding of northern South America. To the north and east in the middle Magdalena area, the Villeta becomes the La Luna Formation (Montgomery, 1992). 
Maastrichtian-Paleocene continental red bed shales of the Rumiyaco Formation unconformably succeed the Villeta Formation, as consequence of regression. The slight angularity between the Villeta and the Rumiyaco Formations is difficult to observe at local scale, but it is quite apparent on regional cross sections. In Orito field, the Rumiyaco Formation is composed of a monotonous section of over 2250 feet (690 m) of varicolored but mostly red and brown mudstones that compose an excellent regional seal, with minor thin interbedding of siltstones and sandstones. 
The abrupt appearance of a conglomeratic interval marks the end of the Rumiyaco Formation and the beginning of the Pepino Formation. This conglomerate is located at the bottom of a reddish shaly section, indicates the clear dominance of continental conditions of deposition, created as consequence of the collision of the Panama-Baudo arc during the Eocene. 
The Pepino Formation presents an average thickness of 1200 ft (366 m) composed of two conglomeratic layers and coarse-grained sandstones with some interbedding of claystones, separated by a thicker layer of red mudstones. These sediments were detached from the Eastern Cordillera recently formed by that time at west part of Putumayo Basin. In areas where faulting has created migration pathways through the thick regional seal of Rumiyaco Formation (e.g. at Orito), the Pepino Formation becomes productive. 
The upper Pepino grades into the Oligocene Orteguaza Formation, composed of greenish finely laminated shales, locally silty or sandy. In Orito the Orteguaza Formation has an average thickness of 500 ft ( 150 m) that serves as seal to the upper conglomerates of Pepino Formation. 
Conformably overlying the Orteguaza are over 1500 ft (450 m) of Miocene to Pliocene deposits composed of fine-to coarse-grained elastics which were deposited under continental conditions and compose the outcrops present in the study area. 
STRUCTURAL SETTING AND FACIES ARCHITECTURE OF THE CABALLOS RESERVOIR IN ORITO FIELD 
Structural Model 
The structural interpretation of the Orito field is based upon integration of 3D seismic, 2D seismic, and wireline log data. The 3D seismic volume was interpreted using interactive Landmark® software. In addition to the top of the Caballos Formation, four horizons were interpreted; three above the Caballos and one below. These horizons represent geological markers calibrated with synthetic seismograms based upon data from three well logs. One of these seismograms is presented in Figure 3, and shows the tops of the Villeta Formations, Limestone A, the Motema Formation, and a reflector considered to be a maximum flooding surface near to the top of the Villeta, and called MFS 1 in this study. 
The seismic response of the Limestone A reflector is clear across the study area, and helps to define the position of the Caballos and Motema Formations where their seismic responses are not distinct. Limestone A is almost always characterized by high-amplitude reflections except in areas where amplitude is decreased by abundant faulting. This continuity, its nearness to the Caballos Formation and its brittle character are used to define the structural model of Orito field. 
Interpretation of the 3D volume was extrapolated to the 2D seismic lines, and the inferred time map was converted to a depth-structural map using a velocity function derived from well logs. Importantly, lateral velocity variation is not found in 
22 

the Orito field, so that the depth structural map of the Caballos Formation maintains the geometry identified in the time structural map (Figure 9). 
The Orito field is located on an asymmetric anticline about 8 miles (12.5 km) long and 5 miles (8 km) wide. The Orito structure is composed of three major components: the Orito anticline, the Orito fault, and secondary faulting. Orito Anticline 
The anticline has a maximum vertical closure of 1100 ft (335 m) measured between contour lines -4900' and -6000', with a generally north-south trending axis. The anticline's western limb gently dips 3-5°. The eastern limb is associated with Orito fault with dips ranging 10 -20°, increasing at the Orito fault shear zone. The anticline forms two structural highs, called the north dome and the south dome, separated by a narrow NE trending through. Ten wells perforated the north dome and 39 wells penetrated the south dome. These domes are considered independent reservoirs. 
Matthews and Portilla (1992) describe the fold Orito as a hanging wall anticline, apparently assuming that it developed as result of movement along a fault with alternating ramp-flat geometry. Seismic lines show a high angle fault zone (>45°), however, and the presence of the ramp-flat geometry is not clear in Orito field. 
1 N 
570000 N 
565000 N 
EXPLANATION 
e Oil Well

* 
Gas Well 
4-DryWell 
~Thrust 

~-5000'___../' Contour 
----,,,,-Oil-Water contact 

----_. ~ Oil-Gas contact 
-5052G1 Original oil-gas contact 
-5250G2 Current oil-gas contact 
km 
Figure 9. Structural map at the top of the Caballos Formation in Orito field. The fluid contacts information was provided by the Reservoir Division of Ecopetrol. 
The fold could also be fault propagation fold (Twiss and Moors; 1992, p.102), that accommodated deformation above the tip line of the Orito thrust fault. Orito Fault 
The Orito fault is one of the major fractures in Putumayo Basin, having a maximum vertical displacement of about 800 milliseconds or approximately 5900 ft (1800 m )(Portilla, 1991). Although the shear zone is crossed by only a few seismic lines the structural map indicates that it has a westward curving listric fault shape. The Orito fault zone forms a high-angle shear zone containing several fault planes. It has about 3000 ft (900 m) of heave at surface, and 1000 ft (300 m) at the level of the Caballos Formation. Seismic expression of the fault is clearly outlined in the western-eastern oriented seismic line presented in Figure 10. 
The fault trace indicated in Figure 9 probably represents combination of two or more individual planes, but the general Orito detachment zone has not been identified in the field, probably because the propagation wave energy was absorbed by the calcareous layers (Limestones A and B), of the Villeta Formation. Because the Orito fault affects the whole stratigraphic column, it was probably generated during the last stages of the Andean orogeny in the Miocene. Secondary Faulting 
Several faults with a NE direction crosscut strata in the field displacing blocks within the thrust sheet with displacements ranging between 20 and 100 ft (6 -30 m). 
w 

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<1) 
LL 
0
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These are generally reverse faults, and have been identified by repeated sections of the Villeta Formation in some wells. These faults controlled the initial accumulation of fluids in the Orito field, and have conditioned the subsequent fluid movement during the field's production history. 
The 3D seismic interpretation allows detection of secondary faulting, and establishes that the different oil-water contacts correspond to different structural blocks separated by secondary faulting. This contradicts earlier interpretations, based on electrical logs and production tests, which erroneously considered the oil-water contact to be tilted to the north based on their different depths throughout the field. Furtermore, production history indicates that the oil-gas contact has descended to different levels depending on the block, confirming the partial control of the above mentioned faulting. (Figure 9 shows the different positions of fluid contacts in the Ori to field.) 
Seismic expression of this faulting is presented in plan view in Figure 11, which is horizontal time slice map at 1000 milliseconds of the entire 3D survey. A cross section view of this faulting is indicated in Figure 12, corresponding to a north­south oriented seismic line, extracted from the 3D survey. 
Stratigraphic Model 
As a whole, the Caballos Formation represents a transgressive sequence documenting an upward transition from alluvial plain deposits at the base through 
Figure 11. Time slice of the 30 volume at 1.0 seconds. The stronger black reflection is the Limestone A. Note the alignment of secondary faulting, with a NE direction, indicated in yellow color. 
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tidal flat and tidal channel deposits, which are finally topped by tidal mouth bars. There is no indication of major shifting of the depositional environments as transgression advanced. 
Because the base of the Caballos Formation rests upon Triassic-Jurassic rocks, and represents a regional unconformity, isopach maps of the Caballos must reflect the paleotopography existing before Cretaceous deposition. This pre-Caballos palaeotopography which is documented by isopach thickness values ranging between 200 and 300 ft (60-90 m) is show in Figure 13. 
The isopach geometry is a generally south-north trending sinuous pattern. Higher values are located at the center of the field and gradually increase to the north, while lower values are located in the western and eastern extremes. This general geometry reflects an inferred paleovalley that serves as the Caballos Formation depocenter in this part of the basin. 
Based on the sedimentary features found in the core, as well as the vertical log patterns, four major depositional facies are identified within the Caballos Formation. Figure 14 outlines well log patterns of the defined facies, and a detailed description is presented in the following section. 

GR (API) ILD (Ohmm) (/) a>UNITS t----------t---------+·if A 4 B A 3 B A 2 B c 1 0 40 80 ..... . ....... . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .::::::: : ::: ::: :: ?~::............. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . 500  Permeability md (Logarithm} -1 3  
Figure 14. Type log of the Caballos Formation in Orito field. Note the good correlation between log pattern and petrophysical data. 


Porosity (%)
u-+-------+--""""--'-----1 
10 20 
Major Facies 
Pluvial Channel Facies 
The lower part of Caballos Formation is dominated by fluvial channel facies, characterized by medium to fine, and locally coarse-grained, sandstones. They display a blocky well log pattern, but the core reveals the presence of several (between 2 and 4) incomplete fining upward cycles, with individual bed thickness ranging from 5 to 10 ft ( 1.5-3 m). Internal sedimentary structures are simple, mainly comprising planar and low to moderate angle cross stratification. Some local scour surfaces are present, and wood fragments and stylolites are common (Figure 15). 
Basic petrophysical analysis of these rocks indicate porosity values around 15 percent and horizontal permeability between 20 and 800 millidarcies. Lower porosity values ( 12 % ) are observed in thin sections, and are mainly secondary by dissolution of framework grains and cements. The rocks are classified as quartzarenites composed of quartz (99%), and metamorphic rock fragments (1 %), that are cemented by quartz overgrowths and kaolinite. A more detailed description is presented in Appendix A. 
The presence of overbank deposits associated with this facies is indicated in some wells (0-4, 0-41, 0-16 and 0-29) These are composed of black siltstone, locally sandy, organic-rich material, and interbedded fine to medium-grained sandstones. Local paleosol and abundant wood fragments in subvertical position are common. 
Figure 15. Fluvial deposits formed by fine to medium-grained sandstones cemented by quartz, with planar cross bedding. This facies constitutes the lowest reservoir in Orito field. 
Because the fluvial facies constitutes the basal unit of the Caballos Formation, its geometry is controlled by the paleotopography existing prior to its deposition. Indeed, a net sand map of this unit (Figure 16), indicates a sinuous geometrical pattern, with a general north-south direction, that is quite similar to the Caballos Formation isopach map (Figure 14) discussed above. 
Net-sand values range from 0 to 105 ft (32 m), with higher values found in the west central portion of the south dome, and in the central part of north dome, along the depositional axis of this unit. This sand distribution was controlled by the facies architecture interpreted from the well log patterns outlined in Figure 17. The facies distribution shows a blocky pattern of stacked channels aligned with the major depositional axis. In some wells the vertical log pattern changes to a series of fining upward sections capped by local shale layers possibly representing channel plugs (Figure 18). This bedding geometry reflects vertical and lateral accretion in a sinuous sand-rich channel complex, formed by stacking and amalgamation of channel fills, and combined with point-bar accretion. The final result is a composite sand body that is much larger than the original channel (Galloway and Hobday; 1996, p. 80 ). 
This unit is bounded by the pre-Cretaceous unconformity at its base and by a relatively continuous conformable shale at its top. The fluvial channel fades form not only the thickest Caballos unit, but also one of the best reservoirs in the Orito field. 


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Thickness and petrophysical properties influence the seismic response of this unit (see Figure 19). The plan view generated from the 3D volume represents the expression of the rocks located immediately above the Motema Formation in terms of their amplitude content. 
Lower amplitude zones probably represent better sand development with higher porosity, forming a sinuous fringe of about 0.46 mi (750 m) wide, that is interpreted as a part of the main axis of the meanderbelt complex. Lateral stratigraphic changes in the unit are gradual and variations in amplitude are not dramatic. The meandering geometry observed in Figure 19 is probably influenced by the shape of the paleovalley and represents the location of the main channel at a specific period of time, while the isopach map (Figure 16) shows a less meandering geometry because it represents the overall trend. Tidal Flat and Marsh Facies 
Overlying the fluvial facies, is a shalier section of about 50 ft (15 m) thick, composed of interbedded very fine-grained sandstones and carbonaceous black, locally argillaceous siltstones. The main sedimentary structures in this sandstone are flaser laminations, ripples, and discontinuous wavy laminations, Rip-up clasts, wood fragments and stylolites are important accessories, and paleophycus is the most common biogenic structure (Figure 20). 
The sandstones are well sorted, composed of quartz (98 % ) and metamorphic rock fragments (2 % ), cemented by quartz overgrowths (20 % ), kaolinite (7 % ), 
Figure 19. Amplitude distribution at bottom of the Caballos Formation. Lower amplitude values probably represent better sand development with higher porosity, a result ofdeposition under fluvial conditions. 
Figure 20. Tidal flat deposits composed of very fine-grained sandstones and organic-rich siltstones, forming flaser and continuous wavy lamination. These rocks are highly cemented by quartz, kaolinite and calcite, occuding their porosity. 
calcite (6 %) and hematite (5 %). The complex diagenetic history of these rocks, a result of several dissolution and precipitation events, has almost totally destroyed their porosity, obviously affecting their storage capacity. Average porosity values in this section are less than 5 percent with a maximum of 10 percent. 
Detailed stratigraphic correlations of this unit resulted in the discrimination of three individual packages of about 20 ft (6 m) of thickness, named 2C, 2B and 2A from bottom to top respectively (Figure 14). Net sand maps of these individual sections are presented in figures 21 to 23. Because of their similar geometrical character and stratigraphic relationships, the description of one of these maps can be generalized to the others. 
All the maps show two major depositional axes whose position has evolved through time, but which have kept their general northwest direction. These major axes as marked by the 20 ft contour line, are about 0.6 mi (1 km) wide and are separated by a muddy zone more than 1.2 mi (2 km) wide occupying the central part of the field. While the southern axis does not usually cross the entire field, the more northerly axis is more continuous and usually extends across the field. 
The facies map of these three units are summarized in Figure 24, which shows fining-upward log responses in the sandy axes, and coarsening-upward log responses associated with the muddy areas. This facies assemblage, geometry, and the sedimentary structures are interpretted to have formed in the upper part of an estuarine bay. 


N 
~ 
570000 N • 570000 N 
. 2.1.
... 
• 
24 •38 
..... 
. . . . . . . 
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O' 
566000 N 
EXPLANATION 
23 
Oilwell
• 
23 
* Gas well 23 
-9-Dry well 
0 km 2 
0 mi 1.2 

m 

Figure 22. Net sand distribution of unit 2B illustrating the presence of two SE trending, major depositional axes, which do not extend across the entire field. 

m 

EXPLANATION 
23 
0 
Oilwell 
+Dry well * 23 
Gas well km 
2 
mi 1.2 
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21 36. 
Figure 24. Sedimentary facies interpretation for unit 2C, which may be also used to interpret units 2A and 2B. It shows tidal flat and marsh deposits cut by two tidal channels. 
The longitudinal sandy axes are associated with tidal channel deposits, while the muddy area is interpreted as tidal flat deposits (Figure 24 ). 
Considering the thickness of each individual unit, as well as the interbedded architecture of sand and shale, seismic expression of each individual package appears vague, and amplitude maps and horizon slide maps of this unit do not show sharp definition of their geometry. The best horizon map generated is presented in Figure 25, and generally shows the expression of tidal flat deposits, seen as an amorphous area of postive amplitude values. Negative amplitude values however, are forming a sinuous narrow pattern on the right side of the figure, and are interpreted as tidal channel deposits. Lower Tidal Channel Facies 
The appearance of a sand-rich interval marks the end of the tidal flat and marsh deposits and the beginning of tidal channel deposits. This unit, approximately 40 ft (12 m) thick typically has a scour surface at the bottom and a shale layer at the top. Because it is a result of very active sedimentary processes, primary structures include planar and low-angle cross bedding, ripples, and continuous wavy laminations developed in fine to coarse-grained sandstone (Figure 26) Towards the top the sediment becomes finer, and ripples, flaser lamination, stylolites, moderate to abundant bioturbation (Planolites, Teichnichnus, Thalassinoides), and many wood fragments are present. 
Figure 25. Horizon slice at the middle section of the Caballos Formation The central part ofthe area shows positive amplitude values that probably represent tidal flat and marsh deposits. Two possible tidal channels are located in the lateral sides of the figure, showing negative amplitudes. 
Figure 26. Tidal channel deposits composed of fining upward sandstones, massive at the bottom and thinly laminated toward the top. Dissolution processes have created high porosity values at the bottom of these channels. 
As with the previously described facies, the sandstone composition remains supermature, formed mainly by quartz grains and some metamorphic rock fragments, cemented by quartz overgrowths (18 %), kaolinite (2%) forming vermicular stacks of crystals, and hematite hosted in the finer grained units. The highest porosity values measured in the Caballes Formation in Orito field are in these rocks, with an average of 17 percent, and maximum values of 20 percent, a product of framework grain and authigenic mineral dissolution. Permeability in these rocks reaches values around 900 millidarcies. 
Porosity and permeability profiles show two decreasing upward patterns in this unit, associated with channelized deposition (Figure 14). This interpretation is extended to the rest of the field, given origin to the units 3A and 3B from top to bottom respectively. 
Net sand maps of these units include sand thicknesses ranging up to 30 ft (9 m), whose distribution forms two slightly sinuous elongate patterns about 0.6 mi (1 km) wide that cross the field in a NW-SE direction. (Figures 27 and 28). Cylindrical and bell-shaped electrical profiles are aligned with these axis, while shaly and funnel profiles are spread across the intervening area (Figure 29). 
The elongate axis is interpreted as a result of deposition m lower tidal channels, trending from NW to SE, cutting tidal flat deposits. The strong contrast between cleaner and porous sandstone and the surrounding muddy deposits, is found 


Q km 2 
o·~ 
mi 1.2 

S4 

Figure 30. Horizon slice of the Caballos Fm. at level of the lower tidal channel facies. Sinuous alignment of amplitude values is interpreted as channel axes, about 500 mwide. 
in the horizon map presented in the Figure 30, where a sinuous pattern 1250 ft (380 m) wide, defined by higher amplitude values, is clearly associated with the sandy distribution observed in this unit. Tidal Mouth Bar Facies 
Topping the Caballos Formation is about 30 ft (9 m) of greenish, massive, fine-grained sandstone, slightly bioturbated by palaeophycus and cylindrichnus. The rock is formed by quartz grains (83%) and detritial glauconite (17% ), cemented by flattened rhombic crystals of siderite (18 %) and carbonate patches (10%) (Figure 31). 
Porosity in these rocks ranges from 5 to 10 percent, and is mainly secondary through dissolution of glauconite and feldspar. Permeability is also very low, with maximum values of 8 millidarcies. Log response in these rocks typically shows two sandy bodies of very low resistivity. The low resistivity is caused by the high glauconite content, and has possibly caused inaccurate log calculations. These sandy bodies are named 4A and 4B from top to bottom respectively. 
Net sand map of the unit 4B shows values up to 30 ft (10 m), arranged in a north-south direction. Thickness decreases to the east, indicating for first time that sediment transport does not follow the topographic gradient, and suggesting the presence of strike dominated depositional processes. This general geometry is interrupted by a funnel shaped entrance, 0.6 mi ( 1 km) wide, that crosses the south part of the field and is projected (because of sparce data) in a west-east direction including higher net sand values (Figure 32 ). 
Figure 31. Core-photo showing glauconitic sandstone of the tidal mouth bar facies. The rock is massive and white spots correspond to patches of carbonate cement. 

2 
1.2 
566000N 
562000 N 
Figure 32. Net sand map of unit 4B. This unit represents the base ofthe tidal-mouth-bar facies and shows for first time a general change ofthe depositional axes direction, suggesting the influence of strike-oriented depositional processes. 
This geometry is interpreted as tidal flat and marsh deposits cut by estuary channels. The lower section of this sandstone is richer in glauconite, and has a sharp contact with the underlying shale that probably represents a local ravinement surface developed by the transgressive waves and tidal currents that also deposited these sands. 
The unit 4A marks the end of Caballos Formation and the beginning of Villela Formation. Net sand map of this unit indicates three elongate bars of about 1.2 mi (2 km) width and 3 mi (5 km) length, and oriented with a northeast-southwest direction. They display funnel shaped log pattern with maximum thickness of 20 feet (Figure 33). This morphology is interpreted to record tidal processes in a estuary mouth complex. The bars are separated by bell-shaped sandstones (on logs) interpreted as tidal inlet channels (Figure 34). 
Seismic response at the top of the Caballos Formation is presented in the horizon map of the Figure 35. Negative amplitude values form elongate bars whose shape and orientation correspond to those described above. Summary of Depositional History 
On basis of the facies architecture described above and considering the regional framework of the Caballos Formation, a revised model for the Aptian-Albian history of the section in the study area can be formulated. The model involves a funnel-shaped north-south trending river valley in which subaerial sedimentation was a consequence of vertical accretion in a coarse-grained meanderbelt (Unit 1). 

2
0 mi 1.2 
... 
I~ 
43 • ~ N
§ 
rn 
rn 

Figure 35. Horizon slice at top of the Caballos Formation. Alignment of negative amplitude values probably represent tidal mouth bar geometries. 
The valley was slowly drowned during the Albian transgression, forming an estuary , with resulting intertidal to subaqueous sedimentation of marshes, tidal flats and tidal sand bars (Units 2 and 3) As transgression continued, tidal processes partially eroded the previously deposited sediments, forming a ravinement surface over which estuarine mouth bars were deposited (Unit 4). Flooding reached its maximum, and open-water lithofacies on the muddy shoreface were deposited, marking the beginning of the Villeta Formation. 
A refined interpretation of the depositional setting for the Caballos Formation in the Orito field uses the tripartite zonation of estuaries based on physical processes described by Dalrymple (1992). The three end members of this scheme are: an outer zone dominated by marine processes such as waves and/or tidal currents; a relatively low-energy central zone, where marine energy (tidal currents) is approximately balanced by river currents; and, finally, an inner zone dominated by river processes. Facies architecture in each member is a result of the relative influence of wave and tidal energy and constitutes the basis of this estuarine classification. 
According to Dalrymple, barrier islands block the seaward end of wave­dominated estuaries and create a dip elongate lagoon on its landward side. The sediment in these lagoons comes from fluvial and marine sources; being filled by bayhead deltas on the landward side and by flood-tidal deltas and washovers on the barrier side (Figure 36). 
In contrast, although tide-dominated estuaries also receive sediment from the two sources, they are dominated by progradational deposits from the landward apex, and lack the tidal-flood deltas and washovers characteristic of wave-dominated estuaries. The progradational series of tide dominated estuaries includes deposits representing alluvial plains, marshes, tidal flats and tidal sand bars. 
This fundamental distinction considers the end-member cases of wave and tide dominance, but many complexities occur in cases of intermediate tidal and wave energy. 
The shape of the valley system being flooded also has an important role on the nature of the developed facies in the estuary. The development of wave dominated estuaries generally occurs in irregularly shaped valleys, where tide and wave amplification is less likely (Dalrymple, 1992). On the other hand, tide dominated estuaries usually occur where a funnel shaped geometry is initially present or subsequently developed. 
Application of Dalyrimple's classification to the Caballos Formation in the Orito field leads to its identification with the tripartite zonation as a tide-dominated estuary. This conclusion is based upon both the observed facies architecture of the Caballos (i.e., an inferred progradation from meandering channels, through tidal flats, tidal channels, and culminating in tidal mouth bars), and the funnel-shaped geometry of the paleovalley in which these sediments were deposited (Figure 36). 
ave Power 
Tidal Pow 
:-:-:-:-:-:-:-:-:-:-:-:-: I d ea I i zed I o cat i o n or:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-: -:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:·:-:-:-:·:-: Orito field are :-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-::;,:-:-:-:-:;z_ ~!~~~~~-~iY~'.'Y_~I!~~-~-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:-
Bayhead Delta 
Lagoon 
~c:;;:__g·· /: .~~a~ri~r.-:--"'.-:-~. ~~-::::::::::::::::~:-:-:--~ ~~· ~ 
d . .__...­
Tide-Dominated 
Wave-Dominated 
Estuary 

Estuary 

Figure 36. Morphological elements in plan view used to establish the fundamental distincion between tide-dominated and wave-dominated estuaries. The square shows the idealized location of Orito field area during Aptian -Albian time. (Modified from Galloway and Hobday, 1996) 
~ 
PETROGRAPHIC AND DIAGENETIC CHARACTER OF CABALLOS RESERVOIRS 
Petrography 
The basic scope of the petrographic description of the Caballos Formation was to investigate the effects of depositional and diagenetic processes on reservoir properties, and thereby assist in the calibration of geophysical logs for accurate lithologic determination in the rest of the field, where core data are not available. 
The petrographic descriptions indicate that most of the samples are fine-grained sandstones; sorting is generally fair to good. Grains in all samples are subangular to subrounded. The sandstones generally are highly compacted with concave-convex and linear contacts between grains. Intra and intergranular fractures are common. 
The sandstones are quartzarenites with a mean quartz, feldspar, rock fragments (Q : F : R) ratio of 98 : 0 : 2. Quartz grains are typically monocrystalline. Feldspars are rare or absent. Rock fragments are metamorphic, represented by poly­crystalline, internally oriented grains. Figure 37a summarizes the mineralogical composition for the different group of facies. 
The almost monomineralic composition very common in shore-zone systems (Galloway and Hobday; 1996 p.126), indicates the importance of current and wave action that removes unstable components by physical and chemical processes. 
65 



Quartz 
Feldspar Rock Fragments 
• Fluvial Channel o Lower Tidal Channel 
o Tidal Flats and Marsh o Tidal Mouth Bar 
b). CEMENTS 
Calcite 	Kaolinite Siderite* 
Figure 37. a.) Mineralogical composition and predominant cements for the different facies groups. Siderite cement is only present in the tidal mouth bar facies. b) Predominant cements in the different quartzarenites that composed the defined sedimentary facies. 
Only the samples from tidal mouth bar deposits contain less than 95 percent quartz; these deposits have a high content of detrital glauconite, which is here considered a mineral component instead of an accessory mineral. Diagenesis 
Sandstones of the Caballos Formation have undergone significant alteration since deposition resulting from complex episodes of diagenesis. The main diagenetic events are related to physical processes of lithification and deformation, and chemical processes of dissolution and precipitation of authigenic minerals. Indeed, high compaction in these Cretaceous rocks indicated by linear and concave-convex intergranular contacts, as well as grain fractures, suggest that the rocks were subjected to high pressures caused by deep burial conditions, where pressure solution and stylolitization processes became the main sources of silica. On the other hand, dissolution of framework minerals and precipitation of authigenic cements represent the most important porosity modifiers. 
Authigenic minerals comprise an average of 24 percent of the rock volume. Quartz overgrowths, kaolinite and calcite are the most abundant cements. In addition, reservoir bitumen (dead oil) a solid hydrocarbon residue that occurs as agglutinate element, was counted as cement, although it is not a mineral. The predominant cements in the different sedimentary facies are showed in Figure 37b. 
STAGE  
PROCESS :  EARLY  INTERMEDIATE  LATE  
Mechanical Compaction Quartz overgrowths Dissolution of feldspars and rock fragments Authigenic kaolinite Calcite cementation Fracturing by orogenic effects. Introduction of C02 -rich solutions Dissolution of calcite Oil migration  - -·­ -- 

Figure 38. Generalized paragenetic sequence of the Caballos Formation in Orito field. Note that siderite cementation has not been considered representative of Caballos paragenesis, because it occurs only in tidal mouth bar facies. O'I 


-
The order of occurrence of major diagenetic events in the Caballos Formation based on petrographic evidence is presented in a generalized paragenetic sequence in Figure 38. Some of these events overlapped in time, and others, such as siderite, hematite, pyrite and dead oil are facies selective. Siderite is only present in tidal mouth bars; hematite in tidal flats, and pyrite and dead oil are exclusively in fluvial channel facies. A brief description of the main authigenic minerals is presented in the following section. Quartz 
Quartz overgrowths are well developed in most of the facies of the Caballos Formation, having an average volume ranging from 13 to 22 percent of the total rock volume, except in the tidal mouth bar deposits where this type of cement is minor. Overgrowths usually completely surround each quartz grain, forming euhedral crystal faces (Figure 39). 
Facies associated with higher energy processes such as tidal channels and fluvial channels present higher quartz overgrowths volumes. Low energy facies such as tidal flats and marsh deposits present lower quartz cement volumes. These relationships were probably caused by the presence of clay grain coatings in the latter facies, which inhibited the nucleation of quartz overgrowths. Kaolinite 
Authigenic kaolinite is present in secondary pores, forming well developed crystals with vermicular texture among the framework grains (Figure 40). 
Figure 39. Thin-section photograph showing quartz grains (Q) with euhedral, bipyrarnidal outline produced by quartz overgrowths. The black material is dead oil associated with kaolinite. The sample is part of the fluvial channel facies . Cross-polarized light. Long dimension of photo 4.2 mm 
Figure 40. Photomicrograph showing vermicular texture developed by 
authigenic kaolinite (K) filling secondary pores among quartz grains (Q).. 
The black color is produced by the associated dead oil. Note also the blue 
stained calcite cement (C).Cross-polarized light. 
Long dimension of photo 4.2 mm. 

Kaolinite has probably been derived from dissolution of feldspars grains, but no partly dissolved feldspars were seen in any of the samples. Higher volumes of kaolinite are observed in fluvial sandstones associated with dead oil, occluding porosity and permeability. Important volumes of kaolinite are also observed in tidal flat and marsh deposits, where its generation was probably associated with bacterial oxidation of organic-rich fragments. Calcite 
Pervasive calcite cement filling primary and secondary pores and replacing some of the framework grains is present in tidal mouth bar, tidal flats and marsh deposits forming spherical patches with internal poikilotopic texture (Figure 41 ). In tidal mouth bar facies, authigenic calcite reaches 8 percent of the total rock volume, indicating the presence of important sources of calcite, such as skeletal particles and sea water. Precipitation of calcite in these rocks was probably caused by an increase of alkalinity associated with an increase in the rate of iron reduction. In tidal flat and marsh deposits calcite cement is about 15 percent of the total authigenic mineral volume, and was probably derived from bacterial or thermal decomposition of organic matter in these rocks. 
Figure 41. Pervasive calcite cement forming poikilotopic texture in which 
small quartz grains (Q) and glauconite grains (G) are irregularly scattered 
in an amorphous, blue stained calcite cement(C). 
Long dimension of photo 1.09 mm. 

Siderite 
Extensive siderite cement having volumes of about 18 percent of total rock volume, is observed exclusively in tidal mouth bar deposits of the Caballos Formation (Figure 42). Siderite cement composed of small flattened rhombic crystals has replaced calcite cement and silicate grains and filled primary and secondary pores. Reducing conditions provided by the maximum flooding observed at the beginning of the Villeta Formation were essential to favor siderite cementation. As with calcite cement in this facies, the sources of carbonate were probably skeletal grains and sea water. 
Porosity 
Porosity in the Caballos Formation observed in thin sections ranges from 0 to 19 percent. Primary intergranular porosity is very low (2%) in all facies, while secondary porosity is about 12 percent on average. This secondary porosity forms inhomogenous packing, floating grains, and channelized pores as a result of dissolutional diagenetic processes (Figure 43). Higher porosity values are associated with lower tidal channels ( 19 % ) and fluvial channel facies ( 15 % ), while low porosity is observed in tidal mouth bars and tidal flat deposits (0-5% ). 
Much original porosity was reduced by mechanical and chemical compaction expressed in intergranular pressure solution and stylolitization, processes that generated the main source of silica, involved in the subsequent quartz cementation (Figure 44). Core porosity in these rocks correlates with petrographic porosity and 
sonic porosity. Final stages of diagenesis such as oil migration and subsequent generation of dead oil have contributed to reduction of secondary porosity. 
Figure 42. Siderite cement composed of small rhombic crystals, filling primary and secondary pores. This cement is present only at top of the Caballos Formation in tidal mouth bar facies. Plane parallel light. Long dimension of photo 1.09 mm Figure 43. Thin-section photograph showing secondary porosity formed by floating grains and channelized pores (blue areas). The rock is a quartzarenite deposited under fluvial conditions. Note quartz cementation. Cross-polarized light. Long dimension of photo 4.2 mm. 
Figure 44. Deformed stylolite in a highly cemented quartzarenite. Stylolites were probably formed as consequence of the high pressure generated during the Andean orogeny, and represent one of the last diagenetic stages in Orito field. Cross-polarized light. Long dimension of photo 4.2 mm. 
IMPLICATION FOR EXTENDED FIELD DEVELOPMENT 
The depositional and petrological model discussed here demostrates the existence of four different north-west oriented stratigraphic units, whose reservoir properties have influenced the production history in Orito field. 
Historical production trends of the Caballos Formation correlate with the major depositional axes defined in this study, while low production trends are located on the west and east areas of the field. These alignments are due in part to the presence of the C02-rich gas cap to the east, and the water-leg to the west. These relationships are indicated in Figure 45, which presents cummulative well production in the form of a bubble map. Larger cummulative productions are associated with the sedimentary units, such as fluvial sandstones and lower tidal channel deposits, that present better sand and porosity development, 
Because production in the Caballos reservou m Orito field is not unit­selective, and horizons with different petrophysical properties are produced simultaneously, early entry of water through higher permeability zones occurs and leads to decreased recovery efficiency. Furthermore, porosity and permeability profiles (Figure 14), present decreasing upward patterns, as in a channel sand, indicating the need of close-spaced wells to improve the sweep efficiency (Parker, 1986). 
78 
N 
~ 
+ 39 
"6. • 
. •· 15 
566000 N 
2 
I~ 

EXPLANATION 
I: : : : : : :1 Unit 1 (Fluvial channels) 
D Unit 38 (Lower tidal ch .) 
E:::::J Unit 3A (Lower tidal ch.) 
Cumulative oil production. The sphere diameter is proportional to the cumulative well production. 
• 
23 
Oilwell 
23 
Gas well 
Dry well 
*~ 
566000 N 
562000 N 
km
0 Q f
'l1 
0 mi 1.2
§ 
m 
Figure 45. Cumulative oil production from the Caballos reservoir in Orito field compared with distribution ofunits 1, 3A and 3B which present thickest sand and highest porosity development. Oil volume is showed with spheres whose size is proportional to the cumulative production, but it does not reflect drainage area. 
The Caballes Formation currently contains an estimated volume of 250 million barrels of unrecovered mobile oil that remains as a resource target of additional perforation and secondary recovery techniques. Part of this resource target can be translated to reserves by means of targeted infill drilling in the major depositional axes that are structurally high (Fisher, 1994). 
Potential areas for this purpose are pointed out in Figure 46 which combines the structural map at top of the Caballos with major depositional axes and cumulative productions. In these areas the identified stratigraphic units must be produced separately until economic limit is reached, in order to control the water influx and maximize the sweep efficiency. 
Stimulation techniques to improve reservoir properties must be appropriate to the mineral composition of framework grains and cements. To avoid the precipitation of insoluble hydroxides and iron-rich carbonate minerals, rocks with high content of siderite and glauconite, such as tidal mouth bar facies, must not be treated with hydrochloric acid. By contrast, in tidal channels and tidal flat deposits, acidization and fracturing practices to improve permeability are strongly recommended. 
High glauconite content in the tidal sand bars located at top of Caballos Formation causes low resistivity response and therefore inaccurate log calculation. A few wells have tested these sandstones individually with negative results, but the possibility of production remains open to strategically located wells in the cores of the sand bars, out side of the area of gas cap influence. 
1 N 


EXPLANATION e Oil Well 
* 
Gas Well 
-4-DryWell 
.---SOOO~ Structural ccntour 
~Thrust 
_.-----/' Oil-Water contact 
----_,,, ... Oil-Gas contact 
-5052G 1 Original oil-gas contact 
-5250G2 Current oil-gas contact 
-6010W1 Original oil-water contact 
-5700W2 Current oil-water contact 
A Curnmulative oil
WI' production and well number 

• 
a 
"'v7, 

High potential 
0 
areas for infill 
drililg 

km 
Figure 46. Structural map at the top of the Caballos Formation in Orito field showing distribution of units 1 and 3B as well as cumulative production and high potential areas for infill drilling. Key for the pattern scheme as in Figure 45. 
Vertical heterogeneity in these units is low to moderate considering the limited shale development and the vertical stacking of sandy bodies in channel fills and tidal sand bars (Tyler and Finley; 1991 ). On the other hand, facies boundaries in the same unit create multiple compartments that increase the lateral heterogeneity and lead to partially drained or untapped reservoirs at the current well spacing. Dip elongate channel-fill pods and lobate-splay deposits in the meandering system constitute a clear example of these reservoir compartments (Ambrose et al; 1991). Therefore, targeted infill drilling and/or horizontal drilling in areas with biggest potential may be considered to improve mobile oil recovery (Tyler and Finley, 1991). 
CONCLUSIONS 
The detailed analysis of the Caballos Formation in Orito field suggests that this Cretaceous interval represents a retro~adational sequence composed of four stratigraphic units made up fluvial deposits that grade upward into estuarine deposits and are in turn overlain by tidal mouth bars. The sedimentary succession was deposited during coastal onlap of a tide-dominated estuary. 
Diagenetic processes influenced each unit differently, modifying the petrophysical properties of the facies that compose the Caballos reservoir in Orito field. Net sand distribution and facies maps show northwest oriented sand bodies that are composed of fluvial facies (unit 1), tidal flat and marsh facies (unit 2), lower tidal channel facies (unit 3), and tidal mouth bar facies (unit 4). 
Petrographic description indicates that the framework mineralogy of the sandstones is dominated by quartz grains, highly compacted and cemented by quartz overgrowths and kaolinite resulting in decreased porosity and predominance of secondary porosity in all reservoirs. 
An estimated volume of 250 MMBLS of unrecovered mobile oil offers a tremendous resource target for advanced field development technologies such as infill and horizontal drilling. This type of additional development should be focused to target the fluvial and lower tidal channel deposits. 
83 
APPENDIX 
PETROGRAPIDC DESCRIPTIONS 
In the following section, petrographic descriptions, diagenetic history, and porosity-permeability relationships are presented according to facies and discussed separately. Textural and diagenetic features for each facies are illustrated with representative photomicrographs at the end of each facies description. 
Composition of the rocks is presented in terms of volumetric fractions of framework (F), matrix (M), cement (C), and total porosity (P). Mineral composition considers the three basic elements, quartz (Q), feldspar (F) and rock fragments (R), established by Folk (1974) to obtain a clan classification. Volume percent compositions are based on counts of 200 points for each thin section. 
Mean grain size, shape, and sorting were visually estimated by comparing each thin section to standardized charts as viewed under the petrographic microscope. Identification and classification of framework grains, authigenic minerals and pore types was performed according to standard petrographic methods (Folk, 1974; Pettijohn et al., 1987). 
84 

FLUVIAL CHANNEL FACIES 
Samples 6442,6466,6468,6487 ,6495,6498.5,6502,6503,6516 Bedding : Minor orientation of elongate grains forming planar, parallel interbedding of very fine-grained sandstone and fine-grained sandstone. Stylolites are parallel to this lamination, and there is also an alignment of kaolinite cement. Fabric : Highly compacted rock with linear, concave-convex and sutured contacts between grains. Texture : Bimodal texture with two types of grain size (very fine and fine), locally coarse grains. Grain size increases toward bottom of channels (6498, 6502, 6503). In general sorting is fair to poor. Composition: F62 Mt C24 P13 Q99 Fo Rt Framework : Quartz grains represent almost the total framework in the rocks of this facies. Grains are monocrystaline usually with straight extinction, most of them with clean surfaces, others with some inclusions. Rock fragments are polycrystalline, strong undulose extinction, compose by elongate quartz crystals. Feldspars are represented by traces of untwinned plagioclase, partially or totally sericitized (Figure 47). Matrix: Mixture of detrital clay minerals (sample 6487). Miscellaneous : zircon, muscovite, pyrite. Authigenic Minerals : Cements: Quartz cement (14%) of the total rock volume, forms overgrowths in optical continuity with grains. Kaolinite (5%) forms well-developed crystals with vermicular texture and is hosted between framework grains, associated regularly with dead oil. Dead oil is also an important cement (2% ), adhered to pore space walls, decreasing petrophysical properties of the rock. Pyrite cement (2%) is present as scattered patches, cementing several framework grains. Hematite and calcite cements form locally but their volume is very low (less than 1 %). Porosity : A high percent (90) of the porosity is secondary, represented by floating grains, oversized pores and some channelized pores. Better porosity is related to larger grain sizes and lack of kaolinite cement and dead oil. Diagenesis: Dissolution of feldspars and rock fragments constitute the most important diagenetic process to increase the reservoir properties in this facies. Inhomogenous packing, floating grains, and channelized pores are the result of dissolution processes. These processes have improved porosity and permeability in rocks with several stages of precipitation of authigenic minerals. A possible sequence for the occurrence of these processes is: 
1. 
Compaction 

2. 
Precipitation of quartz overgrowth cement 

3. 
Dissolution of feldspars 

4. 
Precipitation of kaolinite 

5. 
Dissolution of rock fragments 

6. 
Precipitation of pyrite 

7. 
Precipitation of calcite 

8. 
Precipitation of hematite 

9. 
Deposition of dead oil 


Figure 47. Photomicrograph showing the framework composition of fluvial channel facies. The rock is a quartzarenite cemented by quartz and presents dead oil as the last diagenetic stage decreasing the porosity. Cross-polarized light. Long dimension of photo 4.2 mm 
OVERBANK SUBFACIES 
Samples 6522, 6525 
Bedding : Continuous wavy lamination, formed by interbedding of very fine 
sandstone and mudstone. 
Fabric : Sandstone formed by framework grains in an abundant clay matrix, 
generating a rock with extremely low porosity, locally mud-supported framework. 
Texture : Fair to poor sorting of angular grains, cemented by quartz, pyrite and 
hematite. 
Composition : F54 M36 C9 P 1 Q90 F3 R7 
Framework : Quartz monocrystalline, angular shape, non undulose extinction, highly 
fractured grains. Feldspars represented mainly by plagioclase, usually altered to 
sericite. Rock fragments mainly of volcanic origin, strongly altered (Figure 48). 
Matrix : Formed by organic-rich clay, amorphous. occurring as small irregular blebs 
disseminated throughout the samples, and as wavy continuous laminations in the 
muds tones. 
Miscellaneous : Muscovite, zircon, and pyrite. 
Authigenic Minerals : 
Cements : The sandy fraction is cemented by quartz (8% of the total rock volume), 
forming overgrowths around quartz grains, hematite (1 % ), disseminated and located 
mainly in the finer fractions, and pyrite (0.5%) in a patches forming local aggregates 
of grains. 

Porosity : Primary porosity was occluded by quartz cementation. Secondary porosity is very low and is the result of local dissolution of framework grains, particularly feldspars and volcanic rock fragments. 
Diagenesis: 
1. 
Compaction. 

2. 
Precipitation of quartz overgrowth cement 

3. 
Dissolution of feldspars 


5. 
Dissolution of rock fragments 

6. 
Pyrite cement. 

7. 
Formation of hematite. 


Figure 48. Thin-section photograph of overbank deposits. The brown matrix is organic-rich clay. The rock is a very fine-grained sandstone locally coarse composed of quartz grains. Cross-polarized light Long dimension of photo 1.09 mm 
TIDAL FLATS AND MARSH FACIES 
Samples: 6395, 6398, 6400, 6402.5, 6422, 6424, 6427, 6429, 6430, 6435. Bedding : Minor lamination shown by orientation of elongate grains, stylolites, dead oil and secondary pores. Fabric : Extremely highly compacted rock. The fabric is also affected by abundant quartz cement. Texture : Very fine-grained, fair to good sorting. Composition : Fss M4 C3s Po Q9s Fo R2 Framework : Quartz monocrystalline, straight and non-undulose extinction, generally with clean surfaces, sometimes with inclusions. Rock fragments are mainly polycrystalline, with internal elongate microcrystals. Traces of partially sericitized volcanic fragments (Figure 49). Matrix : Detrital clay is composed of kaolinite and a mixture of illite-smectite occurs between grains as consequence of compaction. Miscellaneous : muscovite, zircon, tourmaline, glauconite Authigenic Minerals Cements: Quartz overgrowths represent the main authigenic mineral in this facies, 55 percent of the total cement. The other 47 percent of cement is kaolinite (18%), which is present as vermicular crystals among framework grains. Calcite cement (15% of the total cement) is also present, forming scattered patches of cement, coating framework grains with local poikilotopic texture. Hematite ( 12% ) present as aggregate of crystals, it is probably a result of recent oxidation. Porosity : Porosity in this facies has been destroyed by precipitation of authigenic minerals. However some samples (6398, 6400), show an important amount of secondary porosity (10%), represented by oversized and channelized pores. Diagenesis : Diagenetic history in this facies is very complex, as a result of several events of dissolution and precipitation of framework components and authigenic minerals, respectively. A probable diagenetic sequence may be: 
1. 
Compaction. 

2. 
Precipitation of quartz overgrowths cement in the sand layers, helped by introduction of silica from water expelled from shaly layers during compaction (diffusion), and by direct precipitation of quartz as consequence of supersaturation of silica. 

3. 
Dissolution of feldspars 

4. 
Precipitation of kaolinite cement in an acid environment rich in silica, and helped by bacterial oxidation of organic fragments present in this tidal flat deposits. 

5. 
Fracturing of the rock by orogenic events forming intra and intergranular fractures generating secondary porosity. 

6. 
Influx of solutions rich in C02 through fractures, and precipitation of carbonate cement. 


Figure 49. Thin-section photograph of tidal flat facies. The rock contains quartz overgrowths cementation (q) and authigenic kaolinite (k) occluding the porosity. Cross-polarized light. Long dimension of photo 1.09 mm 
LOWER TIDAL CHANNEL FACIES Samples: 6349, 6351, 6354, 6360, 6392, 6407 Bedding : Structureless Fabric : No orientation, but there is a minor alignment of elongate grains; no evidence of bioturbation or slumping. Highly compacted rocks. Linear and concave­convex contacts between grains. Texture : Fair to good sorting, fine to medium grained. Composition : Fs1 M2 C22 Pis Qgs Fo R2 Framework : Formed by quartz, mainly monocrystalline, straight and non undulose extinction, all the grains have quartz overgrowths. Polycrystalline grains, with internal orientation represent the lithic fraction in the rock, probably high metamorphic grade rocks (Figure 50). Matrix : Amorphous clay matrix is hosted among grains. Miscellaneous : Muscovite, zircon, tourmaline. Authigenic Minerals : Cements: Quartz represents 85 percent of the total cement in this facies. It forms overgrowths on all grains usually completely surround each quartz grain, except for the points of contact with adjacent framework grains. Kaolinite cement (10%) forms vermicular stacks of crystals, and it is associated with dead oil, hosted in the pores reducing porosity and permeability of the rock. Although dead oil is not a mineral, is considered here as cement, and it has been counted to define the volumetric fractions. 
Hematite is only 5 percent of the total cement, and it is especially located where the framework grains are relatively finer. Porosity : Framework grain dissolution is the most important porosity enhancing diagenetic modification. This secondary porosity is preserved as oversize pores and larger pore throats connecting adjacent pores. At bottom of active channels, where the grain size is larger, porosity increases dramatically, reaching values of 25 percent (Samples 6351, 6392). 
Diagenesis: 
1. 
Compaction 

2. 
Silica dissolution by pressure solution 

3. 
Precipitation of quartz overgrowths 

4. 
Dissolution of feldspars 

5. 
Precipitation of kaolinite 


4. 
Formation of grain and rock fractures 

5. 
Precipitation of hematite cement. 


Linear and concave-convex contacts between grains indicate that quartz overgrowth cementation occurred in an intermediate stage in the diagenetic history of the rock. Textural relationships indicate that quartz overgrowths preceded hematite 
cementation. 
Figure 50 .. Framework grain structure of the lower tidal channel facies. Quartz grains (Q) cemented by quartz cement and kaolinite (K). Note the chanelized pore (P) typical in this facies and responsable of the localy high permeability. Cross-polarized light. Long dimension of photo 4.2 mm 
TIDAL MOUTH BAR FACIES 
Sam.pies: 6326,6328,6330,6334,6337 Bedding : Structureless Fabric : Highly compacted rock, no evidence of bioturbation or slumping. Contacts between grains are linear, concave-convex and sometimes sutured. Texture : Fair to poor sorting, very fine to fine-grained. Composition : F66 Mo C2s P6 Qs3 Fo G11 Fram.ework : Formed by quartz grains, glauconite and rare feldspars. Quartz is monocrystalline, straight and non undulose extinction, most of grains with authigenic overgrowths showing right edges, and sometimes thin layers of inclusions on their surface. A few polycrystalline grains are present. Detrital pale green glauconite, present as individual grains deformed by compaction, with some alteration on their surfaces, probably oxidation. Feldspars: Less than 1 percent of untwinned plagioclase partially to totally seritized (Figure 51 ). Miscellaneous : Kaolinite, muscovite, and polycrystalline grains of quartz. Authigenic Minerals : Cements : Siderite represents about 66 percent of the total cement, forming small flattened rhombic crystals, reddish and brownish color. Locally altered to hematite. Calcite: represents 34 percent of the total cement, forming scattered patches showing local poikilotopic textures. Blue stain indicates iron content. 
Porosity: 90 percent of the total porosity is secondary as result of dissolution of glauconite and feldspars. This porosity is represented by oversize pores. An increment of primary porosity is noted toward bottom of this unit, where porosity reaches 10 percent of the total volume of the rock. Diagenesis : Diagenetic events listed in order of occurrence are: 
1. 
Compaction 

2. 
Precipitation of carbonate cement from pore water, through increase temperature in descending pore fluids. 

3. 
Siderite cementation in a highly reducing environment replacing carbonate cement. 

4. 
Partial dissolution of feldspars and glauconite. 


Figure 51. Thin-section photograph of a sample deposited in a tidal mouth bar. The rock is composed of quartz (Q) and glauconite (G), and is cemented by siderite (S). Cross-polarized light Long dimension of photo 1.09 mm 
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The vita has been removed from the digital copy.