Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 240, part 2
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Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 240, part 2
Next Page--Introduction
K. David Newell, Kansas Geological Survey, The University of Kansas, Lawrence, KS 66047, USA
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Time-temperature index (TTI) modeling is used to establish a simple theoretical thermal maturity for Paleozoic strata in central Kansas. These thermal maturation calculations are based on estimates of likely geothermal gradients and best knowledge of the tectonic history of the region, as derived from stratigraphic thicknesses and estimates of erosion at unconformities. Major uncertainties in the data for the TTI modeling are burial during Cretaceous time and geothermal gradient, thus several models were calculated in which ranges of these two variables were considered. Results of the thermal modeling are then compared to available data on the thermal maturation. These data are principally derived from subsurface samples, on which vitrinite-reflectance, pyrolysis, and fluid-inclusion analyses have been performed.
Vitrinite-reflectance and Rock-Eval maturation measurements indicate that Middle and Upper Ordovician strata (i.e., Simpson, Viola, and Maquoketa formations) in the study area are in initial phases of oil generation. Maturation modeling can match the results of the organic analyses, but geothermal gradients and burial during the Cretaceous have to be maximized.
Although the TTI modeling utilizing very high geothermal gradients and near-excessive thicknesses of Cretaceous strata can match the observed maturation, the modeled results are probably not correct because fluid-inclusion data from saddle dolomites from the Upper Ordovician Viola Limestone indicate this unit reached temperatures 50° C higher than the maximum modeled temperature. A thermal event is inferred to account for the excess maturation and elevated fluid-inclusion homogenization temperatures. This thermal event may be manifested in the erratic increase of vitrinite-reflectance with depth for post-Devonian strata, as well as for pyrolysis measurements in wells for which maturation profiles are available. Flow of heated water onto the cratonic shelf out of the Anadarko basin during the late Paleozoic Ouachita orogeny may be responsible for the maturation anomalies.
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