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ABSTRACT: Consideration of the Parameters for the Calculation of Paleogeothermal Gradients

Stephen R. Daines, R. W. Nopper, T. T. Y. Ho

Changes of temperature with time and depth are critical factors in modeling hydrocarbon maturation. Vitrinite reflectance values predicted from the present-day apparent geothermal gradient are often at variance with observed maturation indicators. A solution lies in the correct determination of the interplay of heat flow, rock thermal properties, and surface temperatures in the geologic past, so that reasonable paleogeothermal conditions can be used to produce a better history match in modeling. The temporal behavior of heat flow in sedimentary basins is the principal factor that influences the thermal history, whereas thermal properties of sedimentary rocks and pore fluids control the temperature gradient. The effects of compaction and different pore fluid types on the t ermal environment are illustrated. Surface temperatures also control subsurface temperatures. Global climate changes during the Phanerozoic have resulted in a total surface temperature fluctuation of 30C° (54F°). Drift of an area from polar regions to tropical latitudes, in conjunction with global cooling and warming, may have resulted in a surface temperature change of more than 50C° (90F°). Whereas surface temperature changes during the last ice ages perturb continental heat flow to about 1500 m (4900 ft), long-term global climate change allows thermal equilibrium to be maintained within the depth limits of hydrocarbon habitats.

We show the effect of changing surface temperatures on the subsurface thermal regime by constructing a model containing the thermal properties of compacting sedimentary rocks, granite, and mantle material. Empirical mixing laws are used to define the variation of thermal properties with porosity in the sedimentary section. We find that by applying a published global paleotemperature history of the Phanerozoic, the maximum deviation from a stabilized heat flow is only 0.3 mW/m2 (0.007 HFU). Comparative maturation models illustrate this strong effect of surface temperature change on hydrocarbon generation intervals.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990