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
Prev Page--Direct Maturation Indicators ||
Next Page--References
TTI modeling and organic maturation measurements generally indicate the Viola Limestone would be in initial stages of oil generation if it were a petroleum source rock. Theoretical maturation from TTI modeling can match maturation indicated by analyses of organic material, but constraints have to be placed on the interplay of the geothermal gradient and thickness of Cretaceous strata. Even when either of these two parameters are maximized though, the maximum expected burial temperature of the Viola Limestone falls well short of the temperatures indicated by Th measurements, and patterns of Th distribution and thermal maturation are inconsistent with assumptions of normal burial heating.
The geologic history of subsidence and uplift in the two localities modeled are fairly well constrained. If geologically unreasonable assumptions such as extreme geothermal gradients or excessive subsidence or uplift have to be invoked to achieve agreement with laboratory results, then chances are that the tectonic or geothermal inputs to the model are incorrect, or the modeling method itself may be too simple to model the actual situation presented by the maturation data. TTI models presented in this study implicitly assume the simple case of a constant geothermal gradient and vertical transfer of heat by conduction, and do not account for uneven heating of the stratigraphic column by heated brines. The poor fit of the maturation model to measured maturation indicates a different method of heat transfer may be likely.
A heat pulse, probably of short duration, appears necessary to account for the relatively high Th measurements. A hint as to the nature of this heat pulse may lie in the depth profiles of the Rock-Eval and Ro measurements, and with the erratic distribution of Th measurements with respect to their geographic distribution and depth. Organic-maturation parameters and maturation profiles in individual wells reveal well-to-well variations in maturation, erratic increases of maturation parameters with depth in individual wells, and even decreases of maturation with depth (figs. 8, 10). These anomalies indicate nonuniform heating of the rock column and temperatures that vary over short distances. Thermal anomalies may also be indicated by the homogenization temperatures of the saddle dolomites in that there is not a correspondence of greater Th with depth. Although a thermal event is indicated by the fluid inclusions, even then the fluid inclusions do not necessarily have to record the maximum temperature experienced over the geologic history of these rocks, nor do the saddle dolomites necessarily have to be contemporaneous, so perhaps a good correlation of temperature with depth would be unusual.
Localized heating by vertical and lateral movement of formation waters may best account for these thermal anomalies and overall thermal maturity of the Viola Limestone. The most likely time for this fluid movement was probably during the Ouachita orogeny in late Paleozoic time inasmuch as this tectonic event had substantial structural effects inboard on the craton. Advective flow from the Ouachita orogen was shown by Wojcik et al. (1992, 1994) to affect Pennsylvanian strata in southeastern Kansas. Detailed work by Barker et al. (1992) and Walton et al. (1995) in the Cherokee basin of southeastern Kansas has also revealed marked spatial variations in maturation that indicate local "warm spots." These warm spots may have been formed by upward flow of warm waters through fractures into the Pennsylvanian section (Barker et al., 1992).
Heat transfer by movement of water onto the craton from peripheral orogens has been explained by a variety of processes. The efficacy of compaction (Cathles and Smith, 1983; Bethke, 1985; Hermanrud, 1986), tectonic compression (Oliver, 1986; Ge and Garven, 1989); topographic differences (Smith and Chapman, 1983; Garven and Freeze, 1984; Bethke, 1985; Bethke and Marshak, 1990; Deming et al., 1990; Deming and Nunn, 1991; Garven et al., 1993; Yao and Demicco, 1995) for long-distance movement of waters onto the craton have been quantitatively investigated, but more modeling will be needed. At present, compaction is possibly only of local importance (Cathles and Smith, 1983; Bethke, 1985; Hermanrud, 1986; Bethke et al., 1991). Thus the timing of significant fluid flow onto the craton and heating of the studied section may correspond to events during the Ouachita orogeny, possibly Pennsylvanian to Permian time (Oliver, 1986).
Further analyses of organic materials and fluid inclusions in saddle dolomites and related diagenetic minerals are needed to understand the spatial and stratigraphic pattern of the anomalous thermal event(s) in the midcontinent. Mapping and measuring these properties could help in understanding the process of fluid movement, as well as aid in ascertaining how orogenic events at the edge of the continental plates affect economic mineralization and petroleum migration on the craton inboard of the orogenic belt.
This work stems from dissertation research completed at The University of Kansas in Lawrence, Kansas. I thank Paul Enos and Robert Goldstein for their guidance and suggestions to improve the research and this resulting manuscript. I am also grateful to Charles Barker and Joseph Hatch for their suggestions to improve the manuscript.
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