ABSTRACT

The several provinces are described and explanations offered concerning the reasons for the differing types of producing reservoirs.

The petroleum provinces in Montana are the Big Snowy Complex, the Bighorn Basin, the Powder River Basin, the Williston Basin, the Sweetgrass Arch, and post-Triassic Montana, the last one being a geologic time bracket covering the entire state. The Big Snowy Complex is unique to Montana. The other provinces thin stratigraphically towards the Big Snowy Complex beneath the post-Triassic blanket.

The prominent structures in the central Montana plains are not the result of horizontal compression but are rather due to upwelling of Tertiary magmas and emplacement of igneous intrusive rocks which post-date thrust faulting in the western part of the state. These structures include Porcupine Dome, Ingomar Dome, Womans Pocket Anticline, Big and Little Elk Domes, to name a few. The late development of closed structures explains why they did not trap significant amounts of hydrocarbons-having been formed after generation, migration, and emplacement of most of the oil and gas.