Paleozoic sedimentary rocks fill the Michigan Basin and provide the sources of the oils produced from this basin. The present geothermal gradient is not high in this shallow sequence, raising questions as to whether Devonian and younger strata are sufficiently thermally mature to have sourced petroleum. The thermal maturities of rock samples from Late Cambrian to Pennsylvanian strata have been determined using the kinetics of apparent transformations of biomarker molecules as an index, augmented by vitrinite reflectance measurements and Rock-Eval pyrolysis data. From the extents of sterane isomerization and the degrees of steroid aromatization in biomarkers extracted from these samples, the geothermal history of the Michigan Basin has been reconstructed. Organic matter in Devonian strata from the center of the basin is thermally mature; it is overmature in Ordovician rocks. The geological history of this basin has evidently been more complex than generally thought, suggesting higher temperatures or lower conduction rates in the past.
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