Abstract: Trap Types vs. Productivity of Significant Wilcox Gas Fields in the South Texas, Listric Growth Fault Trend, and the Divergent Origin of Its Two Largest Producers

Fred L. Stricklin Jr.

Detailed mapping and analysis of 23 Wilcox fields in the subject trend indicates that gas production is related to trap type. Of total cumulative production of 3.4 TCFG, 65% is from upthrown fault blocks implying very effective fault seals due to differential pressure and/or shale smears.

NE Thompsonville and Bob West fields have produced 650 and 200 BCFG, respectively, with 400 BCFG remaining reserves in the latter. The field structures are not attributed to listric growth faulting, as is suggested by their trend location.

NE Thompsonville is a 9-mile-long turtle structure that originated through depositional loading of an upper slope basin, followed by tilting, and then eventual collapse of a sediment squeeze-up mound due to gravitational instability. These events provide an excellent example of basin evolution through sediment loading accompanied by withdrawal of a salt-shale substrate; the basin flanks are defined by basin-dipping listric faulting that accommodated subsidence and merge beneath its floor.

Bob West Field lies along the edge of the Laramide fold belt. The 1-1/2 × 4 mile field anticline adjoins a deep-seated fault that slices over and across a buried structural ridge of probable Cretaceous age. Uplift of the latter, immediately following deposition of 20+ stacked, shelf-bar producing sands, upwarped the fault and resulted in rollover growth of the Wilcox anticline. The fault shows no downward decrease in dip typical of listric faults.

NE Thompsonville and Bob West fields both produce upthrown along crestal faults. This analysis indicates that "highside" closures, irrespective of diverse origins, have achieved head-of-the-class stature as Wilcox gas producers.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas