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Geology and Oil Accumulations of the Gulf Coastal Plain
The Central Gulf Coastal Plain includes an area between Dallas and Houston on the west; Macon, Georgia, and Tallahassee on the east; the Appalachian and Ouachita Mountains on the north and the Gulf of Mexico on the south. The basement rocks are Paleozoic and Pre-Cambrian in age. Forty to forty-five thousand feet of Mesozoic and Cenozoic continental, marginal and marine sediments rest upon the truncated, south dipping surface of the basement rocks.
The Ouachita and Appalachian lines of folding extend under the Coastal Plain and may intersect or cross. Subsurface data extends them to within fifty miles of each other.
The Coastal Plain is an area of vertical movements only, with no compressive forces having acted. The only known thrusting is associated with salt domes and is rare. The vertical movements are due to isostatic adjustments caused by density differences in sediments, intrusion of large igneous bodies or of very large bodies of salt. An almost continuous zone of staggered or offset, strike faulting, both up and down to the coast, is peripheral to the Coastal Geosyncline. This zone, which includes the Balcones, Mexia-Luling, Arkansas and Pickens-Quitman-Gilbertown fault zones, from west to east, roughly parallels the Marathon-Ouachita folding. The present geosynclinal axis trends eastward and is near the present coast line.
Salt domes are common in the area and were emplaced by forces due to density differences in surrounding rocks. The source of the salt is thought to be in the Jurassic. It has been computed that at least a 2,000 foot bed of salt must be available at depth for a salt dome of any vertical magnitude to form.
The Mesozoic and Cenozoic deposits constitute a great, roughly lenticular mass of inter-tonguing, primarily arenaceous, deltaic deposits and, predominantly argillaceous-calcareous marine sediments. These sediments were laid down in irregular arcs or areas of maximum deposition (depocenters) along the north shore of the ancestral Gulf during each sedimentary epoch. The centers of deposition shifted from north to south in the lower and middle Cretaceous; shifted north again during the upper Cretaceous and gradually moved south during the Tertiary.
The Jurassic deposits are clastic in the northeast and grade south and west into evaporites, limestone, and dark shales. The lower and middle Cretaceous exhibit the same lithic trend, with deltaic "red-bed" clastics to the northeast and shale and lime facies to the southwest. The upper Cretaceous is mainly shale and chalk although some sandy facies occur in the eastern part of the area. The Upper Cretaceous is one of the most important oil producing series of the area. Limestone reefs occur in this series on old highs and are mainly reservoirs for gas.
The Tertiary strata consist primarily of cyclic interfingerings of sandy deltaic deposits from the north with marine shale and limestone beds from the south and reach a total thickness of more than 25,000 feet. Of the series, only the Oligocene (Vicksburgian) and Upper Eocene (Jacksonian) are predominantly marine throughout the Central Gulf.
Commercial production of oil or gas has been obtained from Ordovician, Mississippian, Pennsylvanian, Jurassic, Cretaceous, Teritary and Quaternary (?) strata. Reserves are about four billion barrels of oil and 35 trillion cubic feet of gas.
Major accumulations are controlled by both structure and stratigraphy and are most common in areas containing 30-50 per cent sand. Common structural types affecting accumulation are domes and anticlines, faulted or unfaulted; normal faults, downthrown on either the coastal or landward side; structural salients of various kinds. Most structural anomalies increase in size with depth. Stratigraphy affects accumulation as a result of updip or downdip directions and in association with structural anomalies. Variations in permeability and porosity often control the local occurrence of production in closed structures and in some instances account for oil accumulation in the absence of structure.
Approximately fifteen billion barrels of oil and 70 trillion cubic feet of gas have been discovered in the Central Gulf Coastal Plain. The known minimum sedimentary volume of the area is 375,000 cubic miles. Therefore, about 40,000 barrels of oil have been discovered per cubic mile. This figure is four-fifths of L. G. Weeks' estimated average potential total oil discovery figure of 50,000 barrels per cubic mile, and may indicate that appreciably more oil will be discovered in the area.
If hydrocarbons are present in similar quantities in the possible 350,000-600,000 cubic miles of offshore deposits in the Central Gulf Region, estimated potential reserves in the area could be 14-25 billion barrels of oil and 70-125 trillion cubic feet of gas. Not all of this can presently be considered exploitable.
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