Origin, Migration, and Accumulation of Petroleum in Gulf Coast Cenozoic

R. W. Jones

Explanations of the origin, migration, and accumulation of petroleum in the Gulf Coast upper Cenozoic must accommodate the following facts. (1) No specific source of the petroleum has ever been identified. (2) The most probable source section is 10,000-20,000 ft of low TOC (0.4-1.0 wt. %) shale that underlies the reservoirs. (3) Tremendous volumes of dry gas have been generated in the middle and basal part of the source section. (4) More gas than oil is in the reservoirs. (5) The distribution of oil and gas accumulations in the Cenozoic is not primarily controlled by the distribution of terrestrial gas-prone organic facies and marine oil-prone organic facies, but by the relative ease of migration of the two hydrocarbon phases. For example, gas preferentially accumulates i the simpler structures, oil in the intrusive salt domes. (6) High pressure and high porosity in the source rock indicates that neither water movement nor continuous phase oil movement out of the source rock are likely to be significant factors in primary migration. (7) The situation is very dynamic, with generation, migration, and accumulation occurring today. (8) Faults are very important as controls on migration and accumulation of the petroleum.

The interaction of these (and other) factors suggests that most oil reservoirs in the Gulf Coast upper Cenozoic sediments probably initially became mobile after being dissolved in gas in the source rock. The gas-oil mixture moved toward lower pressure areas adjacent to and in faults, and moved upward into reservoirs and traps along faults.

Some depocenters in the middle Tertiary-Pleistocene section have productivity reaching 800,000 bbl of oil/mi₃ of sediments, whereas the entire section averages slightly over 200,000 bbl/mi₃. These numbers are impressive, but in no way outstanding. Despite a large volume of mature source rock underlying the good quality reservoir rocks that exist in a proliferation of traps and trap types, the generally poor quality of the source rocks, migration inefficiencies, and trap leakages have resulted in most of the reservoirs in actual or apparent trap position being dry.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.