ABSTRACT: Environment of Deposition of Downdip Lower Wilcox Sandstones, Provident City Field, Lavaca County, Texas
Steven W. Vest
The Lower Wilcox section at Provident City field produces dry gas from thin-bedded, silty sandstones, at depths of 12,500 to 14,100 ft (3810 to 4298 m). Cores show that sandstone cosets range 0.1 to 2.7 ft (0.03 to 0.82 m) and average 0.58 ft (0.18 m) in thickness. Sedimentary structures within the cosets range upward from a massive unit (A) to a planar-laminated unit (B) to a ripple-laminated unit (C). The cosets have an average composition of lithic arkose and show textural grading indicative of deposition from turbidity flows.
The sandstones lie within the Wilcox fault zone, downdip of the Colorado and Guadalupe deltas of the Rockdale Delta System. Regional stratigraphy and structural trends indicate that the sandstones were deposited in a deep marine environment. A growth fault, having approximately 1000 ft (3048 m) of throw at a depth of 12,300 ft (3750 m), bounds the field to the northwest and largely controls the distribution of lithofacies. Stacked, AB-type, turbidite cosets indicate channel facies. The "M" Sandstone was deposited as a constructional channel, with abrupt lateral grading to overbank facies, where turbidites of the BC- and C-type are dominant. The "S" Sandstone was deposited as a series of thin, constructional channels, mostly with turbidites of the AB- and ABC-type that are generally st cked, causing superimposed, dip-trending lobes on an otherwise strike-trending sandstone.
AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990