Integrated sequence stratigraphy, depositional environments, diagenesis, and reservoir characterization of the Cotton Valley Sandstones (Jurassic), East Texas Basin, USA
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The cumulative production of the Cotton Valley Sandstones in East Texas Basin as of 1993 was 2,666 BCFG with estimated reserves of 24.2TCFG. The Upper JurassicLower Cretaceous Cotton Valley Sandstone is a thick siliciclastic unit in East Texas Basin. It is characterized by low porosity (<6%) and low permeability (<0.5 md) and requires hydraulic fracturing to yield its gas. This study concerns the sequence stratigraphy, depositional environments and diagenesis of the Cotton Valley Sandstones in order to gain an insight into the processes that influenced their reservoir quality. The research involved the use of 4000 feet of conventional core, 350 well logs, and petrographic and geochemical data on sandstones, shales and organic matter. Various analytical techniques were used, including optical petrography, SEM/EDS, CL, EMP, XRD, stable and radiogenic isotope geochemistry and fluid inclusions. Six major thirdorder depositional sequences were identified by correlating SP and resistivity logs on 60 wells. Each sequence has identifiable highstand, transgressive, and/or lowstand system tracts. Sandstones and shales of the transgressive system tract were deposited in strikeoriented barrier island complexes and associated shelf and slope environments. Deposits of the highstand system tracts were most probably deposited in wave-dominated deltaic settings. No cores are available in the lowstand system tracts. Twenty five facies (F1- F25) have been grouped into four main facies associations that represent fully marine (FA1), marginal marine (FA2 and FA3) and back-barrier coastal plain (FA4) environments of deposition. Cotton Valley sandstones are very fine-to fine-grained, moderately-to well-sorted quartzarenite and subarkose. The Ouachita Mountains and Arbuckle Mountains located north and northwest of the study area are most probably the main sources of sediments for the Cotton Valley clastics. Sandstones experienced extensive diagenesis that altered the original detrital composition of the sandstone and modified initial porosity and permeability. The paragenetic sequence of the Cotton Valley sandstones include: 1) clay coatings around the detrital grains (chlorite and illite); 2) early pore-filling pyrite; 3) calcite 1 precipitation; 4) quartz cement; 5) dissolution of feldspars; 6) potash-feldspar overgrowths; 7) kaolinite and chlorite cement; 8) oil emplacement; 9) illite; 10) calcite 2; 11) dolomite and anhydrite; and 12) late diagenetic pyrite. There are two principal internal sources of silica in the Cotton Valley Sandstones, intergranular pressure solution and dissolution along stylolites. The average quartz cement in the Cotton Valley is 11% and the calculated amount of silica derived internally from both pressure solution and stylolites constitute up to 48% of the total quartz cement. The remaining 52% come from sources that could not be quantified. Other significant local sources of silica include dissolution of quartz grains at clay laminae and at contacts between sandstone and interbedded shales, and dissolution of micron-scale detrital quartz within interbedded shales and clay laminae. High content of clay laminae are observed in cores and shale interbeds constitute up to 35% of the total CV rock volume. These sources could not be quantified. A possible external source, a silica-rich fluid expelled from the Bossier shales. Authigenic carbonates occur as pore-filling cement and replacive phase. Calcite cement is the dominant carbonate type in the Cotton Valley Sandstones. The source of Ca for authigenic calcite is the local dissolution and reprecipitation of the carbonate shell fragments in the sandstones and the oyster-rich lagoonal limestone beds interbedded with the sandstone. The limestone beds, which range in thickness from 35 ft (10.5 m) to 50 ft (15.5 m), served as a good sealing rock as well as a potential source for calcite cement. The Cotton Valley has an average of 20%-35% shales interbedded with the sandstones; these shales could have been a significant source for iron as well as carbonates and quartz cements to the sandstones. Original porosity loss by compaction was much more than that lost by cementation in the bioturbated Ophiomorpha-dominated sandstones and cryptobioturbated sandstones, whereas porosity loss by cementation was more significant in the cleaner sandstones of the laminated, tidally influenced shoreface and sand flat. The reservoir quality of the studied sandstones is controlled mainly by depositional environment and subsequent diagenesis. Depositional environment controlled the distribution of clay matrix and oyster fragments. Clay coatings in the Cotton Valley Sandstones had a direct influence on the precipitation of quartz cement. Clay coatings retarded quartz cementation and resulted in a better reservoir quality. It was also observed that shell fragments were the primary source of carbonate cements. So, depositional environment indirectly controlled the distribution of quartz cement and carbonates in various facies. Three main parameters influenced reservoir quality: (1) the amount of clay matrix introduced by bioturbation, (2) the amount of quartz cement, and (3) the percentage of clay coating. Cotton Valley Sandstones are divided into three main reservoir types: (1) poor reservoir quality: quartz-cemented sandstones, (2) poor reservoir quality: bioturbated sandstones with more than 15% clay matrix; and (3) good reservoir quality: clean bioturbated sandstones with less than 10% clay matrix.