Area 6 of the northern Rocky Mountain oil province includes the prolifically oil-productive Big Horn basin and Wind River basin-Casper arch in Wyoming and the less productive Crazy Mountain and Bull Mountains basins in Montana. Most of the study area was a part of the Wyoming shelf east of the Cordilleran miogeosyncline during the Paleozoic and Mesozoic Eras, and the present structural features were formed during the Laramide orogeny. The district has produced 2.4 billion bbl (end of 1968) of the 10.9 billion bbl of oil originally in place, and 1.4 trillion cu ft (end of 1967) of the 2.6 trillion cu ft of natural gas in place. More than 80 percent of the oil is contained in Paleozoic rocks. With present technology, only 10 percent of the remaining known oil will be recove ed, but secondary recovery may yield more. Structural traps have been drilled extensively; stratigraphic traps, which have had little exploitation, can be expected to add significantly to reserves, especially in the Big Horn basin and Wind River basin-Casper arch structures.