Source of Oils in Gulf Coast Cenozoic Reservoirs

Doris M. Curtis

Many Gulf Coast geologists have assumed that shales interbedded with or adjacent to the reservoir sandstones are source rocks for oils in Cenozoic reservoirs, but few source-rock quality shales have been identified in Cenozoic strata. Reservoirs and their associated shales are in thermally immature and organic-poor intervals. Based on geothermal gradient, age, and depth, it can be shown that thermally mature source rocks should be present in older slope shales beneath each producing trend. Assumptions regarding the source rock potential of the interbedded thermally immature shales derive from the fact that hydrocarbons migrated into traps soon after burial of the reservoir (early migration). Early migration from the source rock was therefore also assumed (shallow burial, arly migration model). Review of the geochemical requirements for a source rock shows that geochemical constraints demand late migration from the source rock after many thousands of feet of burial (deep burial, late migration model). Geological and geochemical concepts are compatible, however, if migration out of the source rock was late (long after deposition and deep burial of the source rock) but migration into the reservoir was early (soon after shallow burial of the reservoir and trap system).

A model that reconciles the "shallow burial" and the "deep burial" concepts (i.e., the geological and geochemical constraints) provides for (1) deposition of source sediment, (2) maturation of source rock by deep burial under a younger depocenter that contains reservoir sands, (3) generation of hydrocarbons and expulsion from the source rock, and (4) onset of migration into traps formed within a few thousand feet of burial of the reservoir in the young depocenter that overlies the source rock. Thus, there is late expulsion and migration out of the deeply buried source rock and early migration into the young, shallow-buried trap. Primary migration may take place by diffusion through shale pore systems, or by microfractures in the geopressured shales, but once hydrocarbons have been exp lled from the source rocks and have migrated through the geopressured shales, they can migrate through fault systems into younger reservoir rocks. Geochemically immature reservoir sections contain mature oils derived from older, more deeply-buried source rocks. Source rocks of Cretaceous and Tertiary age have been identified, and have been inferred from the chemistry of reservoired oils. The model provides for multiple sources for the oil in Gulf Coast Cenozoic reservoirs and appears to be consistent with analytical findings.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.