Environment of Deposition of Upper Wilcox Sandstones, Katy Gas Field, Waller County, Texas: ABSTRACT

Abstract

ABSTRACT At Katy gas field, sandstones of the upper Group produce gas at depths of 10021 to 11000 ft (3054 to 3353m) in reservoirs controlled by stratigraphic and structural characteristics. Producing zones are from 6 to 42 ft (1.8 to 12.8m) in the upper first lower massive sandstones, and D, C, B, A and second Wilcox interbedded sandstones and shales. The reservoir sandstones are dip-trending with production being localized on the top of the anticline. The upper sequence has been interpreted as delta-front grading upward to bay-marsh transitional deposits (Williams, et al., 1974) and, alternatively as deep-water turbidite deposits (Berg and Findley, 1973). The field is located downdip from the fault zone, downdip from known delta-destructional deposits in the upper Wilcox, and is as much as 45 miles (75 km) downdip from the postulated late Sabinian shoreline. Full-diameter cores from the upper sequence show the sandstones are submarine, constructional-channel turbidites, giving way vertically to thinner turbidite sandstones in a predominantly shale section. The sandstones are representative of submarine fan deposits, having bedset associations characteristic of channel deposits (A, AB and ABD) becoming middle fan associations (AE, BE, ABCE, and BCE) and then outer fan associations (ABE, BDE, CDE and DE) upward in the section. The thicker channel sandstones show limited lateral extent along strike, grading to thin, overbank sandstones. Sandstones are sparsely bioturbated, and shales are bioturbated only when they immediately overlie sandstones. The burrows are characteristic of a wide range of water depths from middle neritic to bathyal. Benthonic forams found in the cores are abraded by transport and represent a range of water depths from middle to outer neritic. Therefore, water depths during deposition were probably bathyal, as indicated by deeper-water trace fossils. The deposition of the upper Group is associated with transgression during late Sabinian and incipient uplift of a deep-seated, diapiric mass under the field. Electric log correlations and sandstone isopach maps suggest that the sands were deposited as parts of a submarine fan that shifted northwestward through time.