Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 240, part 2
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Introduction

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 fluid-inclusion 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 the simple thermal model. If the assumption of simple burial heating can be shown to have been unlikely, then additional heating events, possibly ephemeral or short-lived, may have to be invoked. Understanding the thermal history is important, for it is intimately linked with the geologic evolution of the setting, its petroleum occurrence, and mineralization. In particular, orogenic belts, with their adjacent forelands and cratonic basins, have linked geologic histories. If significant transfer of heat from an orogenic belt into an adjacent shallow cratonic basin can occur by processes such as fluid movement, then thermal and geologic models that require significant burial and subsequent exhumation of this basin may be in error. Inasmuch as petroleum-exploration strategies and models of resource emplacement largely depend on concepts of heat transfer and burial history, it is important to evaluate various maturation and temperature data to determine which of the models or geologic processes are correct.

In this paper, organic maturation measurements in Paleozoic strata and fluid-inclusion temperatures in the Middle and Upper Ordovician rocks in central Kansas will be compared to theoretical maturation calculations produced by a simple model of thermal maturation. This model, a time-temperature index (TTI) calculation, utilizes a geothermal gradient at a given locality and mathematically combines it with a geologic history of subsidence and uplift. It will be demonstrated that the directly measured maturation parameters are higher than can be accounted for by most variations of any geological parameters used to compute the theoretical maturation, and a thermal event is necessary to account for this maturation. Next, spatial and stratigraphic characteristics of the maturation measurements will be examined to determine whether this anomalous maturation is due to fluid flow or a temporary flux of conductive heat from basement rocks. Evidence favors the former process.

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