ABSTRACT

The High Island A-474/A-499 prospect was originally acquired as a structural play with some suspicion that stratigraphic traps were present. Subsequent drilling demonstrated stratigraphic traps played the most important role in hydrocarbon accumulation.

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

Paleontologic studies indicate the productive intervals were depositecl during late Pliocene and Pleistocene time. The large growth faults, combined with associated secondary faulting, provide the primary trapping mechanism for the Upper and Middle Pleistocene C-17 through F-8 horizons. Paleontologic evidence indicates these pay horizons were deposited in an outer shelf environment of deposition (Ecozone 3). These sands originated from prograding deltas located to the southwest, north, and northeast. Sediments with a southwesterly source were transported as sand plumes by northeasterly flowing currents. Greatest sand accumulation occurred at the intersection of the growth faults and the northeasterly trending sand plumes.

In contrast, stratigraphic traps are the primary trapping mechanism in the Lower Pleistocene and Upper Pliocene G-5 through G-25 horizons. These laterally discontinuous, highly productive sands consist of submarine fan and slope facies indicative of an upper to lower slope depositional environment (Ecozones 4 and 5). The sands were transported into the area by turbidity flows from the north and northeast and by deepwater 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.