Comparison of Maturation Data and Fluid-inclusion Homogenization Temperatures to Simple Thermal Models: Implications for Thermal History and Fluid Flow in the Midcontinent

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 50οC 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 postDevonian 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. Past thermal regimes and regional fluid flows have recently been considered as processes affecting the distribution of petroleum, minerals, and diagenesis in the midcontinent. Studies in the last 10 years (Gregg, 1985; Leach and Rowan, 1986; Bethke and Marshak, 1990; Barker et al., 1992; Ge and Garven, 1992; Wojcik et al., 1992, 1994; Luczaj, 1995; Walton et al., 1995) have invoked general northward flow of heated fluids onto the cratonic shelf out of the deep Anadarko and Arkoma basins. High-temperature basinal fluids have been postulated to account for lateral changes in chemistry and mineralogy of lead-zinc districts in southern Missouri, Kansas, and Arkansas (Gregg, 1985; Leach and Rowan, 1986), and for the contiguous occurrence of petroleum from the deep Anadarko basin of Oklahoma to the relatively thin strata of central Kansas (Rich, 1933; Walters, 1958; Price, 1980). Inference of an ancient thermal regime that was hotter than the current one principally relies on maturation measurements, such as vitrinite reflectivity or fluidinclusion homogenization temperatures, that are greater than what can be accounted for by current thermal conditions. If greater-than-expected maturation is measured, then the next step is to determine if this relatively high maturation occurred during a hitherto unexpected period of deep burial or a period of time in which the stratigraphic column was temporarily affected by greater heat flow. If excessive heat flow is indicated, it is important to determine whether this heat was principally transferred by conduction or by a flux of heated fluid from elsewhere. The inadequacy of simple thermal models to fit available maturation and temperature data is either explained by thermal perturbation or poor assumptions of