Episodic, variable, large scale horizontal and vertical translation of source and reservoir rocks is a characteristic of thrust belts which complicates petroleum exploration. Understanding recent oil and gas finds in the Wyoming-Northern Idaho thrust belt requires a comprehensive analysis which includes seismic, surface, aero-magnetic, gravity, petrographic and well data. These are integrated with the aid of empirical structural models developed earlier, primarily by workers in the Appalachian and Albertan thrust belts. Knowledge of times of trap formation relative to times of oil generation and migration is critical. This required palynological dating of elastic wedges and unconformities, as well as source rock identification, oil analyses, thermal alteration studies, and restored structural cross-sections and maps.

The Wyoming-N. Utah-E. Idaho thrust belt may be divided into four major thrust fault systems of different ages. They are, from west to east: Paris Willard (uppermost Jurassic), Meade-Crawford (Coniacian), Absaroka (Santonian-Maastrichtian), and Darby (Paleocene-L. Eocene). During the past three years sizeable new reserves have been found in folds in the hanging wall of the Absaroka Thrust at Pineview, Ryckman Creek, Painter Reservoir and Yellow Creek. Studies which consider time as well as temperature, show that the oil and gas in these fields probably originated in Cretaceous source beds in the footwall of the Absaroka thrust sheet. The oil migrated laterally and upwards into concurrently forming hanging wall fold traps in porous Triassic Nugget sandstone, and was preserved by bedded anhydrite and halite caprock. Studies of other parts of the thrust belt suggest that some structural traps are barren because they were formed after oil generation in nearby source rock. Ideally, trap formation should coincide with flush oil generation in laterally equivalent or thrust juxtaposed source beds. These concepts and techniques are applicable to exploration in all thrust belts.