ABSTRACT

Ideas on the origin and migration of oil in the highly productive Gulf Coast Tertiary are long on speculation and short on data. What is known with some certainty is that 1) crude oil is most often found in old salt-related structures, 2) it probably migrated vertically a considerable distance through fault associated fracture systems, and 3) it most likely originated in deeply buried high pressure marine slope shales. Oil compositions suggest multiple sources with mixed type II and type III kerogens. Although many migration modes have been suggested, most oil movement probably took place as a continuous oil phase or as a solute in a supercritical gas phase. Accumulation occurred most readily in structural closures within or just above the soft geopressure zone where pressure gradients are high and seals are effective.

Because of their location in the hard geopressure zone, oil source beds are rarely penetrated, and the lack of adequate samples has made practical application of geochemical data in the Gulf Coast difficult if not impossible. Nearly all published information defines only thermally immature, organic lean, gas-only source beds. Consequently, until appropriate samples can be collected and analyzed, it is important to develop conceptual models based on geologic history and seismic-stratigraphic methods to predict oil source bed distribution.

A model for oil source bed deposition in anoxic, salt controlled intraslope basins has been developed. Most modern, silled basin analogs however, are oxic and do not contain oil-generating kerogens. Reduced bottom circulation and resulting anoxia could have better existed during periods of global warmups and high sea level stands which occurred in Pliocene and Middle Miocene times. Definition of these oil source bed sites, either directly by geochemical analysis or indirectly with seismic investigations, can greatly enhance our ability to predict oil occurrences and to separate oil from gas prospects. It should also stimulate collaboration among Gulf Coast geologists, geophysicists, geochemists, biostratigraphers, and petroleum engineers, which will result in improved geological models and exploration successes, quite possibly including the discovery of oil in obscure and subtle traps.