Stratigraphic Framework, Structural Styles, and Seismic Signatures of Downdip Yegua Gas Condensate Fields, Central Wharton County, Texas Gulf Coast

EWING, THOMAS E., Frontera Exploration Consultants, San Antonio, TX, and W. GRANT FERGESON, Venus Oil Company, San Antonio, TX

Overpressured Yegua gas reservoirs in central Wharton County occur within four cycles of sand deposition which are found far downdip of the major Yegua sand depocenters. Gas condensate fields are found where these sandstones were deposited within at least two distinct Yegua expansion-fault systems, possibly near or beyond the most seaward Wilcox shelf edge.

Four sand-bearing intervals are present within sequences designated as 10 through 70. The "60" sandstones of the lower Yegua Formation (Anomalina umbonatus zone) were deposited in a complex of channels, are associated with a middle to outer neritic fauna, and represent channeled turbidite deposits on the outer shelf. The "60" interval was deposited before movement on most Yegua faults. The sandstones are productive at Phase Four and Gresham fields. An upper lower Yegua sand-bearing interval ("50") has not produced in the downdip fault trend but is the main reservoir interval in "middip" channel sandstones. The "40" sandstones at the base of the upper Yegua Formation form a small but thick and gas-productive

birdsfoot delta in the Phase Four field area fed by channels which expand over several faults. The "30" sandstones (Eponides yeguaensis zone) occur primarily as thick strike-elongate sand bodies, interpreted to represent a barrier-bar complex. This sandstone sequence thickens markedly, from 200 ft to over 800 ft, over a fault of the Phase Four system at Gresham field; the sandstones produce at Menefee, Black Owl, and Gresham fields. The "20" sandstones (Discorbis yeguaensis zone) occur downthrown to the Shanghai fault within the Shanghai domino-style glide-fault system in central Wharton County. They represent proximal to distal delta-front deposits of the Shanghai delta system, inferred to be fed by a channel. The sandstones are productive at Shanghai, Shanghai East, and the El Campo field complex. The upper Yegua intervals carry beach to middle neritic faunas and represent mostly shallow-water high-energy deposition. The "20," "30," and "40" sandstone intervals may represent drops in relative sea level in the area.
Gas has been trapped in rollover anticlines downthrown to expansion faults of both Phase Four and Shanghai fault trends, downthrown closures against sealing faults, upthrown closures, and stratigraphic traps. Most reservoirs are small in area (200-400 ac) but productive (1100-2100 mcf/ac-ft), but traps of several thousand acres are present in the El Campo complex. Stratigraphic complexities which limit production include channel boundaries and unconformities related to sandstone deposition. Structural complexities include antithetic faults, closely spaced expansion faults, and horsetail faults which die out upward into sand sequences.

Seismic signatures are of three classes: (1) strong fluid-contact reflectors with AVO anomalies from thick sandstones, as at Black Owl and Shanghai fields; (2) strong bedding-parallel reflectors with AVO anomalies, derived from thinner, gas-filled sandstones, as at Phase Four; and (3) little reflection character from gas-filled, highly laminated sequences, as at El Campo. AVO anomalies can be severely affected by out-of-plane energy, by skips in data acquisition, and by masking in the structurally complex area.

The Downdip Yegua fields have been found to date by recognizing anomalous seismic signatures and delineating small closures. Future discoveries will probably result from exploration for specific reservoir intervals, particularly for more laminated zones which do not have a characteristic seismic expression, and from improved data quality in known trends.

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)