McDADE, ELIZABETH CHNN, Texaco Inc., New Orleans, LA; ROGER SASSEN, Geochemical and Environmental Research Group, Texas A&M University, College Station, TX; LLOYD WENGER, Exxon Production Research, Houston TX; and GARY A. COLE, Saudi Aramco Laboratories Department, Dhahran, Saudi Arabia

ABSTRACT: Identification of Organic-Rich Lower Tertiary Shales as Petroleum Source Rocks, Southern Louisiana

Comprehensive organic geochemical evidence of organic-rich, marine shales in the lower part of the middle Eocene Claiborne Group and the lower Eocene-Paleocene Wilcox Group of southern Louisiana is now available. This evidence influences models for Gulf Coast petroleum origin. The shales are the only post-Cretaceous sediments in the northern Gulf of Mexico that meet recognized criteria for oil source rocks. Many of organic-rich Paleogene shales contain terrestrially derived, amorphous kerogen altered by microbial activity, and display pyrolysis results consistent with type II/III kerogen. Shelf-edge depositional environments favored preservation of hydrogen-rich kerogen. Seismic and sedimentologic interpretations suggest that marine character and thickness increase on the Paleogene co tinental slope to the south. The shales at burial depths in the 3050-4600 m depth range, at present, are thermally immature to late mature with respect to oil generation. Detailed geochemical analyses of extractable organic matter and kerogen isolates suggest an oil-source correlation with Tertiary-reservoired oils in southern Louisiana and offshore in the adjacent Gulf of Mexico. Biomarkers of selected samples display high concentrations of C28-bisnorhopane and 18a-oleanane that are also found in Tertiary-reservoired oils. The key 18a-oleanane biomarker is absent or not reported in Gulf crude oils from Cretaceous and Jurassic source rocks. Burial and thermal history models suggest the timing of oil migration from Paleogene source rocks is consistent with emplacement of oils in Tertiary eservoirs. The lower Tertiary source rocks described here could offer new insight to understanding the origin of oil in other Tertiary deltas.

AAPG Search and Discovery Article #90989©1993 GCAGS and Gulf Coast SEPM 43rd Annual Meeting, Shreveport, Louisiana, October 20-22, 1993.