ABSTRACT

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 south Louisiana is now available that influences models for Gulf Coast petroleum origin. The shales are the only post-Cretaceous age sediments in the northern Gulf of Mexico that meet recognized criteria for oil source rocks. Many 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 favor preservation of hydrogen-rich kerogen. Seismic and sedimentologic interpretations suggest that marine character and thickness increase on the Paleogene continental slope to the south. The shales at burial depths in the 3050-4600 m depth range are at present thermally immature to late mature with respect to oil generation. Detailed geochemical analysis of extractable organic matter and kerogen isolates suggest an oil-source correlation with Tertiary-reservoired oils in south Louisiana and offshore in the adjacent Gulf of Mexico. Biomarkers of selected samples display high concentrations of C₂₈-bisnorhopane and 18-oleanane that are also found in Tertiary-reservoired oils. The key 18-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 reservoirs. The lower Tertiary source rocks described here could offer new insight to understanding the origin of oil in other Tertiary deltas.