Thermal Maturity of the 1.05 Ga Nonesuch Formation, North American Midcontinent Rift: Biomarker Ratios and Hopane and Sterane Stereoisomers

HIESHIMA, G. B., Indiana University, Bloomington, IN, R. E. SUMMONS, Bureau of Mineral Resources, Canberra, Australia, and L. M. PRATT, Indiana University, Bloomington, IN

Differences in the apparent level of thermal maturity indicated by sterane and hopane steroisomers arise from the relationship between time and temperature in the maturation process. As part of a continuing study of the approximately 1.05 Ga Nonesuch Formation, North American Midcontinent Rift, we have evaluated the thermal maturity of organic matter based on biomarker distributions and ratios of stereoisomers (17 samples).

Steranes and hopanes, once thought to be absent from Nonesuch bitumen and petroleum, have been identified and quantified by multiple-reaction-monitoring gas chromatography mass spectrometry. The 20S/20S + 20R ratio of C(29) sterane, a robust indicator of maturity in Phanerozoic strata, ranges from 32 to 52% and averages around 40% indicating a moderate level of thermal maturity with respect to petroleum formation and preservation. High concentrations of thermally unstable moretanes, 17b(H) 21a(H) hopanes, relative to the more stable 17a(H) 21b(H) hopanes corroborates this interpretation. In marked contrast, the 22S/22S + 22R ratio of C(31) hopane has attained a near equilibrium value averaging approximately 56% suggesting that the organic matter is overmature with respect to petroleum generation. Other indicators of thermal maturity including the methylphenanthrene index and the ratio of aaa/aaa + abb steranes are covariant with the 20S/20S + 20R ratio of C(29) sterane. The ratios of pristane/nC(17), phytane/nC(18), and trisnorhopane/trisnorneohopane (Ts/Tm) are not covariant with the 20S/20S + 20R ratio of C(29) sterane reflecting control by the source of organic matter.

The long burial history of the Nonesuch has influenced the kinetic transformations of hopane and sterane stereoisomers to a greater extent than thermal influence. The 22S/22S + 22R ratio of C(31) hopane equilibrates more rapidly than the C 20S/20S + 20R ratio even at relatively low burial temperatures given sufficient geologic time. The Nonesuch Formation is moderately mature with respect to petroleum formation and preservation despite a billion year long burial history.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)