THERMAL HISTORY OF THE 1.1-GA NONESUCH FORMATION, NORTH-AMERICAN MIDCONTINENT RIFT, WHITE-PINE, MICHIGAN

The Nonesuch Formation, a black siltstone, was deposited in the Lake S uperior portion of the midcontinent rift system during middle Proteroz oic rifting, Despite its age of nearly 1,1 Ga, the Nonesuch Formation contains liquid petroleum and solid pyrobitumen. Numerical modeling te chniques were used in this study to constrain the thermal history of t he Nonesuch Formation at White Pine, Michigan, Thermal modeling addres ses two issues: (1) the utility of illite-smectite expandability as a limiting parameter for describing the thermal history of ancient (Prot erozoic) sedimentary rocks, and (2) the potential for hydrocarbon matu ration within the Nonesuch Formation at White Pine, A range of potenti al burial histories for the Nonesuch Formation was constructed based o n geological evidence. Time-temperature histories were calculated for each burial history model assuming one-dimensional, transient, conduct ive heat flow. Results of numerical time-temperature models were then evaluated on the basis of organic and inorganic thermal maturity indic ators including Rock-Eval(R) pyrolysis data and biomarker indices (cor related to equivalent vitrinite reflectance values), in addition to il lite-smectite expandability, Illite-smectite expandability values appe ar generally consistent with measured organic thermal maturity values, The preferred set of cases also produces calculated illite-smectite e xpandability and vitrinite reflectance values consistent with thermal maturity indicators (that do not include vitrinite) collected in the f ield. Modeling results demonstrate that a combination of elevated basa l heat flow and rapid burial during the Proterozoic is required to pro duce the observed thermal maturities at White Pine, Michigan, More spe cifically, modeling indicates that maximum temperatures for the Nonesu ch Formation, 110 to 125 degrees C, were reached at about 1075 Ma, coi ncident with a maximum burial depth of about 6 km, although 4 km repre sents a more plausible value. The results support a thermal history co nsistent with in-situ oil generation at White Pine; however, thermal m odeling alone cannot rule out the possibility that oil was generated a nd expelled elsewhere and migrated into the area with fluids; expelled during an episode of postrift compression.