Abstract: Characteristics of Wilcox Gas Reservoirs, Northeast Thompsonville Field, Jim Hogg and Webb Counties, Texas

Robert R. Berg, Frederick J. Tedford

Gas was discovered in upper Wilcox sandstones in 1959 at Northeast Thompsonville field. Reservoirs are found on a faulted anticline at depths ranging from 9,500 to 14,000 ft (2,900 to 4,300 m). The Wilcox is a dominantly shale section, and shale units thicken off structure. At the crest of the structure is a growth fault that has 1,100 ft (335 m) of throw, high dip in the shallow section, but low dip of about 20° in the deep section near 10,000 ft (3,000 m).

Sandstones are thin bedded, 1 to 2 ft thick (0.3 to 0.6 m), with interlaminated black shale. Bed sets display a sequence of graded, structureless sandstone overlain by laminated sandstone and thin black shale. This sequence appears to represent an ABE turbidite (Bouma cycle) 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 commonly are graded from 0.15 mm at the base to 0.06 mm at the top. Average detrital composition is 64% monocrystalline quartz, 6% feldspar, 2% rock fragments including polycrystalline quartz 27% matrix, and 1% other grains. Common cements are silica overgrowths and siderite that comprise an average of 17% of bulk composition. The best average porosities and permeabilities are on the order of 23% and 200 md. The morphology of reservoirs is illustrated by the uppermost Wilcox, first Hinnant sandstone, that has a gross thickness of about 90 ft (27 m). Within the Hinnant are "massive" sandstone sections composed largely of AE and ABE turbidite sets. The "massive" units range from 30 to 75 ft (9 to 23 m) in thickness and form narrow, dip-trending concentrations of channel turbidites that are the principal reservoir sandstones.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas