ABSTRACT

Overpressured Yegua gas-condensate 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) lie downdip of a pronounced flexure, are associated with middle to outer neritic fauna, and represent a complex of channeled turbidite deposits on the outer shelf or upper slope. 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 ("Cook Mountain") reservoir interval in 'mid-dip' fields. The '40' sandstones at the base of the upper Yegua Formation form a small but thick gas-productive birdsfoot delta in the Phase Four field area fed by channels which expand across 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' sand stones (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 the Phase Four and Shanghai fault trends, downthrown closures against sealing faults, upthrown closures, and stratigraphic traps. Most reservoirs are small in area (200 to 400 acres) but prolific (1100 to 2100 MCF/ac-ft), although traps of several thousand acres are present in the El Campo complex. Stratigraphic complexities which form reservoir boundaries 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 of gas-bearing Yegua sandstones 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 structurally complex areas.

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, perhaps in more laminated zones which do not have a characteristic seismic expression; from more sophisticated use of seismic signature analysis for direct hydrocarbon detection; and from improved data quality in known trends.