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ABSTRACT: Novel Isotopic Compositions of Total Organic Carbon and Bitumen in the Nonesuch Formation, North American Mid-Continent Rift

G. B. Hieshima, L. M. Pratt, J. M. Hayes

The middle Proterozoic Nonesuch Formation is an important potential source rock in the North American Mid-Continent rift. As part of a continuing study of the petroleum potential of the Nonesuch Formation, stable isotopic compositions of total organic carbon and extracted bitumen were measured from core and mine samples.

Total organic carbon contents of finely laminated samples range from 0.25 to 2.8 wt. % and average 0.6%. The carbon isotope composition of total organic carbon ranges from -33.4^pmil (vs. PDB) to -33.9^pmil and averages -33.6^pmil. Nonesuch bitumens are dominated by the saturated hydrocarbon fraction. Gas chromatographs show homologous series of n-alkanes and isoprenoids with low concentrations of biomarkers including hopanes, steranes, and monomethyl-substituted cyclohexylalkanes. The carbon isotope composition of total bitumen ranges from -29.6^pmil to -32.5^pmil and is significantly heavier than the isotopic composition of total organic carbon. The isotopic values of total organic carbon and bitumen are the opposite of the relationship reported for most Phanerozoic petroleums and b tumens. Contamination and migration of bitumen from other sources are rejected as explanations because (1) lithologies with low total organic carbon do not contain bitumen, (2) alternative source rocks are not recognized, (3) gas chromatographs of bitumens from solvent and pyrolytic extractions are similar, and (4) bitumens contain a suite of biomarkers associated with Precambrian source rocks. Our petrographic, geochemical, and isotopic data suggest multiple sources of organic detritus in the Nonesuch, with bitumen generated by an isotopically heavy, subordinate fraction of the organic matter.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990