Geologic Occurrence of Oil and Gas in Montana
The Great Plains of Montana contain many structural features apparently favorable for oil and gas accumulation, but tests of most of them have disclosed only a small number of important fields. The region is characterized by several isolated mountainous groups of diversified origin, each being essentially a complete structural unit. Oil and gas are produced from formations ranging in age from early Mississippian to Upper Cretaceous. The producing structural features are various types of anticlines, noses, and terraces. The major tectonic movements responsible for the development of most of these structural features took place sometime in the Eocene subsequent to the deposition of the Fort Union formation. The major factors responsible for oil and gas accumulation are anticlines and domes, lenticular sandstones, porosity, faults and circulating ground water.