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

Abstract

Differences in the apparent level of thermal maturity indicated by sterane and hopane stereoisomers 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. The authors 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. Other indicators of thermal maturity including the methylphenanthrene index and the ratio of {alpha}{alpha}{alpha}/{alpha}{alpha}{alpha} + {alpha}{beta}{beta} steranes are covariant with the 20S/20S + 20R ratio of C{sub 29} sterane. The ratios of pristane/nC{sub 17}, phytane/nC{sub 18}, and trisnorhopane/trisnorneohopane (Ts/Tm) are not covariant with the 20S/20S + 20R ratio of C{sub 29} sterane. The ratios of pristane/nC{sub 17}, phytane/nC{sub 18}, and trisnorhopane/trisnorneohopane (Ts/Tm) are not covariant with the 20S/20S + 20R ratio of C{sub 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 ofmore » C{sub 31} hopane equilibrates more rapidly than the C{sub 29} 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.« less