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

BURTNER, R. L., A. NIGRINI, and P. F. GREEN, Chevron Oil Field Research Company, La Habra, CA

In the Idaho-Wyoming thrust belt, gravity-driven fluid flow was responsible for moving large amounts of heat from the depths of the Lower Cretaceous foreland basin eastward toward the Moxa Arch. 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 generation of hydrocarbons from the Permian Phosphoria Formation and may even have caused some generation from Lower Cretaceous source rocks in an area east of the deepest part of the foredeep. Development of the Crawford and Meade thrusts 80-90 Ma 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 became shallower and narrower.

The new thermal model, calibrated with apatite fission track and maturation data, has significant implications for hydrocarbon exploration in thrust belts. It suggests that the effect of fluid flow on temperature cannot be ignored when reconstructing the thermal history of these complex areas. Areas of above-normal heat flow can within short periods of time become regions of abnormally low heat flow. Depending upon the position of organic-rich 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.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)