Organic-Rich Lower Tertiary Shales, South Louisiana: Implications for Petroleum Source Rock Deposition.

This study presents comprehensive organic geochemical evidence of organic-rich marine shales in the middle Eocene lower part of the Claiborne Group and the lower Eocene-Paleocene Wilcox Group of south Louisiana. The shales are the only postCretaceous age sediments in the northwest Gulf of Mexico basin that fit the conventional criteria for liquid hydrocarbon source rocks. Visual and geochemical evidence suggests that the Paleogene shales contain terrestrially derived, amorphous kerogen with high hydrogen indices whose oil potential has been upgraded by bacterial degradation. The Paleogene section is at immature to late mature levels with respect to petroleum generation, at presentday burial depths of 10000 to 15000 ft (3048 4572 m). Three shelf-edge depositional environments favored accumulation and preservation of hydrogen-rich organic matter 1) marine transgressive; 2 ) hemipelagic, deposited during progradation, and; 3) interdistributary bays. Increased marine character and thickening of source beds on the Paleogene continental slope to the south is expected according to seismic and sedimentologic interpretation. Paleogene source units of excellent quality, but high thermal maturity, are presently buried beneath upper Tertiary reservoirs in the sourh Louisiana salt basin. Detailed geochemical analyses of extractable organic matter (EOM) and kerogen isolated suggest an oil-source correlation with Tertiary-reservoired oils in south Louisiana and the Louisian shelf. EOM samples have immature characterisitics, but compare favorably with south Louisian crude oils. Significant quantities of the biomarker species C 28,30bisnorhopane and 18a-oleanane are present in the EOM, and in south Louisiana Tertiary crude oil, but are not reported from any known Mesozoic-sourced oils in the northwest Gulf of Mexico. Stable carbon isotope analysis of kerogen, EOM, and crude oils further suggest

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