Postdevelopment Analysis of Producing Shelf-Slope Environments of Deposition, High Island Area, Texas

D. A. Anspach, S. E. Tripp, R. E. Berlitz, J. A. Gilreath

High Island A-474/A-499 was originally acquired as a structural prospect, although it was suspected that stratigraphic traps were also present. Subsequent drilling demonstrated stratigraphic traps were the major important factor in hydrocarbon accumulation.

The study area is located on the outer continental shelf, 80 mi southeast of Galveston, Texas. The primary geologic structure consists of an elongate 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.

Paleontologic studies indicate the productive intervals were deposited during the late Pliocene-Pleistocene. The large growth faults, combined with associated secondary faulting, provide the primary trapping mechanism for the C-17 through F-8 zones. Paleontologic evidence indicates these pay zones are sands deposited in an outer shelf environment of deposition. These sands originated from prograding deltas located to the southwest, north, and northeast. Sediments with a southwesterly source were transported as sand plumes by northeast-flowing currents. 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 in zones G-5 through G-25. These laterally discontinuous, highly productive sands consist of submarine fan and slope facies indicative of an upper to lower slope depositional environment. The sands were transported into the area by turbidity flows from the north and northeast and by deep-water currents flowing to the northeast from a southwest depocenter. Many of these current-transported sands were deposited on the downthrown side of down-to-the-northeast growth faults.

AAPG Search and Discovery Article #91042©1987 GCAGS and GC-SEPM Section Meeting, San Antonio, Texas, October 28-31, 1987.