Integrated sequence stratigraphy, depositional environments, diagenesis, and reservoir characterization of the Cotton Valley Sandstones (Jurassic), east Texas Basin, United States

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 Jurassic-Lower 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 strike-oriented 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.