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Characteristics of Wilcox Gas Reservoirs, Northeast Thompsonville Field, Jim Hogg and Webb Counties, Texas
Robert R. Berg (2), Frederick J. Tedford (3)
Gas was discovered in upper Wilcox sandstones in 1959 at Northeast Thompsonville field. Reservoirs are found on a faulted anticline at depths ranging from 9,400 to 13,700 ft. The Wilcox is dominantly shale, and stratigraphic units thicken off structure. At the crest of the structure is a growth fault that has about 1100 ft of throw, downthrown to the basin, and has a low dip of about 30 degrees, decreasing in the deep section near 12,000 ft.
Sandstones are thin-bedded, 1 to 2 ft thick, with interlaminated black shale. Bed sets display a sequence of graded, structureless sandstone overlain by laminated sandstone. This sequence appears to represent an ABE turbidite, probably of channel origin. Thinner sandstones are laminated and ripple-bedded, representing more complete turbidite sequences of the ABCDE type, but some thin sandstones are less complete BCE and CDE turbidites. The thinner sandstones are probably of overbank origin. Average grain size is 0. 13 mm (fine-grained), and bed sets are commonly graded from 0. 15 mm at the base to 0.06 mm at the top. Average detrital composition is 64 percent monocrystalline quartz, 6 percent feldspar, 2 percent rock fragments including polycrystalline quartz, 27 percent matrix, and 1 percent other grains. Common cement is silica as grain overgrowths and, more rarely, siderite. Total cement averages 17 percent of bulk composition. The best average porosities and permeabilities are on the order of 23 percent and 200 md and are found in thicker, channel turbidites. The morphology of reservoirs is illustrated by the uppermost Wilcox, Hinnant 1 sandstone that has a gross thickness of about 90 ft. Within the Hinnant are massive sandstone sections composed largely of AE and ABE turbidite sets. The massive units range from 30 to 75 ft in thickness and form narrow, dip-trending concentrations of channel turbidites that are the principal reservoir sandstones.
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