Study of Shelf-Slope Environments of Deposition, High Island A-474/A-499 Area

David H. Anspach, S. E. Tripp, R. E. Berlitz, J. A. Gilreath

The focus of this study is the High Island A-474/A-499 fields located on the outer continental shelf 80 mi southeast of Galveston, Texas. The primary geologic structure is an elongated northwest-southeast-trending dome associated with a deep-seated shale or salt diapir. The dome is bisected by two large northwest-striking down-to-the-northeast growth faults. The productive intervals range in age from late Pleistocene to late Pliocene.

Paleontologic studies indicate the upper and middle Pleistocene sandstones were deposited in an outer shelf environment. These productive layers originated from prograding deltas located to the north, northeast, and southwest, the latter being transported as sand plumes by northeasterly flowing currents. The large growth faults, together with associated secondary faulting, provided the primary trapping mechanism for these reservoirs. Greatest sand accumulation occurred at the intersection of the growth faults and the northeast-trending sand plumes.

In contrast, stratigraphic traps are the primary trapping mechanism for the lower Pleistocene and upper Pliocene strata. These laterally discontinuous, highly productive sandstones consist of submarine fan and slope facies indicative of an upper slope depositional environment. Some of these sandstones were transported into the area by turbidity flows from the north and northeast. Other sandstones, many of which were deposited on the downthrown side of down-to-the-northeast growth faults, were transported by deep-water currents flowing to the northeast. Sand transport directions are supported by log analysis, seismic data, and recent work on current circulation in the Gulf of Mexico.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.