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The temperatures within a sedimentary basin during its evolution are controlled more by the variation in thermal properties of the contained rocks with space and time and with hydrodynamic effects such as dewatering and regional ground-water flow than with transient heat flow variations at the base of a thick sedimentary pile. Although the effect of basal heat flow has been extensively discussed, the effects of thermal property variations and the evolution of fluid flow systems in basins have rarely been addressed. The thermal properties of many sedimentary rocks are not well known and the depth and time variable changes associated with compaction, diagenesis, etc are difficult to evaluate. In one of the dominant rock components, clays, anisotropy, and sampling difficulti s make laboratory measurements difficult if not impossible. Hydrodynamic systems associated with basins are just beginning to be understood and much remains to be learned. Heat flow and temperature studies are techniques for investigating these effects.
Accurate temperature logs associated with measurements, where possible and suitable, of the thermal conductivities of the rock result in two quantities which can be used to unravel some of the unknown temperature controls in a sedimentary basin. These accurate data can be sued for correlation of geothermal gradient and thermal conductivity with well log properties such as seismic velocity (travel time), density, and gamma ray activity. These resulting correlations can then be used to infer the spacial variations in heat flow within the sedimentary basin and to accurately evaluate the effects of present fluid motions in the basin. The data can also be used to develop a catalog of thermal property variations as a function of the many variable parameters.
Examples are presented from the Mid-Continent showing the correlation between geothermal gradient, natural gamma ray activity, and seismic travel time for the suite of rocks occurring there. A major result of this study is that the thermal properties of shale have been misestimated in the literature and that the thermal properties of Paleozoic shales appear to be 50 to 100% lower than those assumed in most thermal modeling, leading to a consequent error of 50 to 100% in temperature calculations of basin thermal history.
Temperature and heat flow data are used to evaluate regional fluid circulation, with possible associated petroleum migration, in units such as the Madison Limestone and the Dakota Group. In addition, heat flow studies may outline areas where conditions are locally favorable for maturation. An example of large-scale basement heat flow variation in Nebraska is used to illustrate an area of unusually radioactive basement rocks producing local areas of higher temperature.
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