Depositional History, Facies Analysis, and Production Characteristics of Hydrocarbon-Bearing Sediments, Offshore Texas

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 barrier-strandplain facies that prograded over thick prodelta, shelf, and slope muds initiated contemporaneous faulting, resulting in displacements of as much as 10,000 ft ( 3 km) .The steep fault surfaces extend to great depths at delta depocenters along former shelf margins. 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 normally slightly sinuous and continuous parallel to depositional strike. In contrast, fault traces are highly irregular and discontinuous where expansion faults and salt diapirs formed contemporaneously. Fault surfaces flatten at relatively shallow depths and become broad bedding-plane faults along mud-rich shelf-margin embayments. Gas and some liquid hydrocarbons are produced from a broad spectrum of reservoir sandstones deposited as fluvial channel till, barrier strandplains, distributary-mouth bars, distal delta fronts, and submarine channel fill and fans. Delta-slope and distal delta-front facies yield the most hydrocarbons. Barrier-strandplain and submarine channel-fill facies are also significant exploration targets for primary hydrocarbon 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 Island and Mustang Island Areas and by a smaller delta located in the Galveston Area. The depth and basinward limits of reservoirs in these areas have not been delineated by current drilling. Miocene strata, which compose the largest volume of sedimentary fill in the western Gulf Coast Basin, account for two-thirds of the total Texas offshore production. The Miocene producing trend extends southwestward from Louisiana across the Texas continental shelf. Oil and gas are produced from Miocene strata that are successively younger basinward. Earliest Miocene depocenters (base of the Miocene to Marginulina ascensionensis) were shelf-margin deltas that reached their greatest basinward extent in the High Island, 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. After the middle Miocene transgression (Amphistegina Bto Textularia strapperi) shelf-margin deltas prograded within the Galveston, Brazos, Brazos South Addition, and Mustang Island East Addition Areas, whereas distal deltaic sandstones and shelf muds accumulated the North Padre Island East addition and South Padre Island Areas. Meanwhile, strandplain deposits continued to aggrade in the High Island Area. Even though the principal late Miocene depocenter was in South Louisiana, uppermost Miocene sediments (Textularia strapperi to Bigenerina A) are locally sandy and thick in the South Padre Island, Galveston, and High Island Areas. Pliocene deposits beneath rnost of the Texas shelf are sand-poor and probably have negligible petroleum potential because equivalent fluvial and deltaic systems having Texas sediment sources appear to be minor. Recent speculation has focused on the essentially untested High Island and High Island East Addition 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. These prolific deposits lie within a western extension of the offshore Louisiana deltaic depocenter. The geology of the Plio-Pleistocene depocenter is difficult to interpret because of complex structural deformation and rapidly changing facies. Major growth faults, salt dornes, and Quaternary sea-level fluctuations have combined to produce erosional unconformities and depositional environments ranging from fluvial channels to submarine canyons and fans. Cumulative oil production from the Texas continental shelf between 1955 and 1982 was about 20 million barrels from State waters and about 72 million barrels from Federal waters for a total of 92 million barrels. Cumulative natural gas production during this period was 6.8 trillion cubic feet (Tcf); 2.3 Tcf was produced from State leases and 4.5 Tcf from Federal leases. Large areas of offshore Texas basinward of the present shelf margin also contain sedimentary facies and structural traps favorable for the generation and accumulation of hydrocarbons. The substantial water depths and remoteness of drilling sites in this region may put additional economic limitations on future exploration.
Robert A. Morton
Lee A. Jirik
Ricahrd Q. Foote

Morton, R. A., Jirik, L. A., and Foote, R. Q., 1985, Depositional History, Facies Analysis, and Production Characteristics of Hydrocarbon-Bearing Sediments, Offshore Texas: The University of Texas at Austin, Bureau of Economic Geology, Geological Circular 85-2, 31 p.

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The University of Texas at Austin, Bureau of Economic Geology
Geological Circular