ABSTRACT

Detailed mapping and analysis of 23 Wilcox fields in the subject fields in the subject trend indicates that gas production is related to trap type. Of total cumulative production of 3.5 TCFG, approximately 67% is from upthrown fault blocks implying very effective fault seals due to down faulted shale juxtaposed against gas reservoirs, differential pressures, and probable clay smears.

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

NE Thompsonville is a 9-mile-long (14.5 km) 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 evacuation of a salt substrate; the basin flanks are defined by basinward-dipping listric faults that accommodated subsidence and merge beneath its floor.

Bob West Field lies along the edge of the Laramide fold belt. The 1-1/2 x 4 (2.4 x 6.4 km) 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 more than 23 stacked, shelf-bar producing sands, upwarped the fault plane and resulted in rollover growth of the Wilcox anticline. The fault shows no downward decrease in dip as is typical of listric growth faults. NE Thompsonville and Bob West fields both produce upthrown along crestal faults. This analysis indicates that "high side" closures, irrespective of diverse origins, have achieved head-of-the class stature as Wilcox gas producers.