Facies Analysis of Hydrocarbon-Bearing Sediments, Offshore Texas

Abstract

Large areas of offshore Texas remain unexplored even though sedimentary facies and structural traps favor the generation and accumulation of hydrocarbons. Rapid deposition of sand-rich deltaic and Strandline facies that prograded over thick prodelta, shelf, and slope muds caused contemporaneous faulting with displacements up to 10,000 feet. Sandstones thicken substantially on downthrown sides of faults, but shale out a few miles basinward of the ancient shelf margins. These regional expansion faults are slightly sinuous and laterally extensive parallel to depositional strike. Fault traces are highly irregular and discontinuous where expansion faults and salt diapirs formed simultaneously. Fault surfaces flatten at relatively shallow depths and become broad bedding-plane faults across mud-rich shelf-margin embayments; in contrast, steep fault surfaces extend to greater depths at delta depocenters along former shelf margins. Gas and some liquid hydrocarbons are produced from a broad spectrum of reservoir sandstones deposited as fluvial channels, barrier-strandplains, distributary-mouth bars, distal delta fronts, and submarine channels and fans. Delta-slope and distal delta-front facies volumetrically account for the most prolific production. Barrier-strandplain and submarine channel-fill facies also serve as significant exploration targets for primary accumulations, whereas fluvial and coastal plain facies mainly trapped hydrocarbons during secondary migration. The oldest producing sandstones (Oligocene Frio Formation) were deposited by a large shelf-margin delta system located in the North Padre and Mustang Island areas, while a smaller delta was located in the Galveston area. The depth and basinward limits of these reservoirs have not been delineated by current drilling. Miocene strata, which compose the largest volume of sedimentary fill in the western Gulf Basin, account for two thirds of the total Texas offshore production. Earliest Miocene depocenters (Anahuac to Marginulina ascensionensis) were shelf-margin deltas that reached their greatest basinward extent in the High Island and in the Brazos, Matagorda Island, Mustang Island, and North Padre Island areas. Slightly younger sediments (Marginulina ascensionensis to Amphistegina B) accumulated mainly by aggradation in the High Island area and by minor progradation in most other areas. Following the middle Miocene transgression (Amphistegina B to Textularia stapperi) the Galveston, Brazos, Brazos South, and Mustang Island East areas were sites of shelf-margin delta progradation, whereas distal-deltaic sandstones and shelf muds accumulated in the North Padre East and South Padre Island areas and strandline deposits continued to aggrade in the High Island area. Even though the principal late Miocene depocenter was located in South Louisiana, upper Miocene sediments (Textularia stapperi to Bigenerina A) are locally sandy and thick in the South Padre Island, Galveston, and High Island areas. Upper Miocene production generally follows the middle Miocene producing trends. Pliocene deposits beneath most of the Texas shelf are shaly and probably have negligible petroleum potential because equivalent fluvial and deltaic systems with Texas sediment sources appear to be minor. Recent speculation has focused on the essentially untested High Island and High Island East areas, where large shale-ridge structures may trap hydrocarbons in overlying Pliocene sediments. One third of the Texas offshore production comes from Plio-Pleistocene deposits in the High Island South Addition and South Extension and Galveston South Addition areas that lie within a western extension of the offshore Louisiana deltaic depocenter. Major growth faults, deep-water sediments, Quaternary sea-level fluctuations, and salt diapirs make the geology of this area extremely complex. The substantial water depths and remoteness of drilling sites in these areas put economic limitations on future offshore exploration.