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Geology and Oil Accumulation Central Gulf Coastal Plain Abstract
Thirty to forty thosand feet of continental, marginal and marine Mesozoic and Cenozoic sediments, constitute the Central Gulf Coastal Plain above Palezoic and Precambrian basement rocks. Sparse data permits extension of the Ouachita and Appalachian trends of folding into the subsurface beneath the Central Gulf Region to within approximately 50 miles of each other. Northwest of the Appalachian trend of folding and northeast of the Ouachita trend, less deformed, primarily argillaceous and calcareous Paleozoic sediments exist in the Warrior Basin of Alabama and Mississippi.
Mesozoic and Cenozoic deposits of the Central Gulf Coastal Plain constitute a great, roughly lenticular mass of inter-tonguing, primarily arenaceous, deltaic deposits and, predominantly argillaceous--calcareous, marine sediments. Areas or sites of maximum deposition (depocenters) occur along the axes of thickness at approximately the centers of deltaic deposition during each sedimentary epoch. These individual deltaic masses coalesce to form elongate complexes resembling flattened link sausages.
Lithologically, the Jurassic is a clastic-evaporite sequence with locally developed reef limestones. Comanchean sediments vary from predominantly argillaceous - calcareous marine beds in the west to deltaic "red-bed" clastics in the east. Gulfian facies are mainly calcareous and argillaceous but appreciable quantities of arenaceous materials occur in the eastern portion of the region. Tertiary strata consist of cyclic calcareous-argillaceous marine beds and
arenaceous marginal strata. All exhibit greater structural deformation with increasing age.
Commercial production of oil or gas has been obtained from Ordovician, Mississippian, Pennsylvanian, Jurassic, Comanchean, Gulfian and Tertiary strata. Reserves approximate 4 billion barrels of oil and 35 trillion cubic feet of gas. Major accumulations appear to be controlled both structurally and stratigraphically. Common structural types effecting accumulation are (1) domes and anticlines, faulted and unfaulted; (2) normal faults downthrown on the coastal side or on the landward side; (3) structural nosings; and (4) structural terraces. Most structural anomalies increase in size with depth. Stratigraphic mastery of accumulation occurs as a result of porosity variations and wedgeouts, especially in updip or downdip directions and in association with structural anomalies. Major accumulations are most common in the area of 30-50% arenaceous material.
Approximately 7 billion barrels of oil and 20 trillion cubic feet of gas have been produced from a known minimum sedimentary volume of 375,000 cubic miles. Reserves in these same rocks are about 8 billion barrels of oil and 50 trillion cubic feet of gas, making a total of approximately 15 billion barrels of discovered oil and 70 trillion cubic feet of discovered gas. The amount of discovered oil per cubic mile (cuben) is 40,000 barrels. This is approximately four-fifths of Weeks' estimated potential total oil discovery averaging 50,000 barrels per cubic mile.
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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