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South Hallettsville field, discovered in 1976, produces gas and condensate from a lower Wilcox clastic wedge. The discovery well, General Crude Oil 1 Anderson, was drilled on an Edwards seismic closure, encountered high pressure gas sands in the lower Wilcox, blew out, and was abandoned. The replacement well had potential of 23,926 MCFGD flowing and 38.7 bbl condensate per MMCF of gas through perforations from 9,909 to 10,011 ft (2,973 to 3,003 m). Within 2 years this well had produced 2.1 Bcf of gas and 74,543 bbl of condensate.
The wedge has a sandstone shale ratio of 20%. Reservoir sands are medium to very fine grained, individual or stacked, and several inches to 25 ft (7.5 m) thick. The sand is 40 to 70% quartz with 10 to 35% feldspar and lithic fragments. The matrix is kaolinite, chlorite, illite, and illite/smectite with minor carbonate cement. Reservoir sands have porosities of 18 to 24.7% and permeabilities
of 0.5 to 83 md. Primary sedimentary structures, textures, mineralogy, and stratigraphic sequences from core analysis, log character, and paleontology suggest a delta to prodelta environment. Convolute structures within reservoirs and chaotic dipmeter patterns suggest complex subaqueous mass movement, similar to that within the modern Mississippi delta complex.
General Crude Oil has drilled 17 wells in the field with 15 containing hydrocarbon-bearing sands. The productive sandstones rarely correlate between wells, thus masking reservoir geometry and indicating a restricted aerial extent for most reservoir sandstones. Production in most wells has declined rapidly, and results of stimulation by fracturing, acidizing, and clay stabilization appear to substantiate the interpretation of multiple, small, discontinuous reservoirs.
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