BURTNER, R. L., Case Western Reserve University, Cleveland, OH, and A. NIGRINI, TerraSpec Associates, La Habra, CA

ABSTRACT: A New, Calibrated Thermal Model for the Idaho-Wyoming Thrust Belt

In the Idaho-Wyoming thrust belt, gravity-driven fluid flow was responsible for moving large amounts of heat from the depths of the Early Cretaceous foreland basin eastward toward the stable platform. In the process, isotherms were depressed in front of the Paris-Willard thrust 100-140 Ma and geothermal gradients were abnormally high along the eastern flank of the foredeep. The abnormally high heat flow along the eastern flank of the foredeep caused early and relatively shallow generation of hydrocarbons from the Permian Phosphoria Formation in that area.

Development of the Crawford and Meade thrusts 80-90 Ma disrupted the Early Cretaceous hydrodynamic system, moving the locus of meteoric recharge eastward and causing sudden cooling of strata near the former center of the Early Cretaceous foreland. Although this hydrodynamic system continued to shift eastward as thrusting moved progressively eastward, its thermal impact decreased as the remaining foreland basin became shallower and narrower.

The new thermal model, calibrated with apatite fission track and organic maturation data, has significant implications for hydrocarbon exploration in thrust belts. The model suggests that the effect of fluid flow on temperature should not be ignored when reconstructing the thermal history of these complex areas. Areas of above-normal surface heat flow, within relatively short periods of time, can become regions of abnormally low surface heat flow. Depending upon the position of source rocks in the hydrodynamic system, this variability in heat flow can result in either earlier or later generation of hydrocarbons than normally would be expected when other process models are applied.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.