ABSTRACT

In the Idaho-Wyoming portion of the Idaho-Wyoming-Utah 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 the vicinity of the Paris-Willard thrust 100 to 140 m.y.a., and geothermal gradients became 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. Movement on the Crawford and Meade thrusts 80 to 100 m.y.a. disrupted this major hydrodynamic system, moving the locus of meteoric recharge eastward and causing sudden cooling of strata along the western margin of the 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. It suggests that the effect of fluid flow on temperature should not be ignored when reconstructing the thermal histories of these complex areas. Areas of above-normal surface heat flow can, within relatively short periods of time, become regions of abnormally low surface heat flow. Depending on the positions of source rocks in the hydrodynamic system, this variability in heat flow can result in either earlier or later generation of hydrocarbons than would normally be expected when other process models are applied.

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