The Powder River basin in northeastern Wyoming and southeastern Montana is an asymmetric structural depression, deepest along the west flank. All but 6 percent of its 42,000-sq mi (108,780 sq km) area has oil potential. Pre-Pennsylvanian strata offer possibilities for production, especially in the Ordovician Red River or Bighorn, which have been only partly tested. Good recoveries from the upper Minnelusa (Lower Permian) probably can be extended beyond the areas of present production. Lower Mesozoic sedimentary rocks have limited potential because good source beds are absent and trapping conditions are poor. The Cretaceous System, which has yielded two thirds of the oil already produced in the Powder River basin, offers the best prospects for the future. All formational u its except the Lance have been productive, but 90 percent of the oil has come from the upper Inyan Kara (Fall River) and Muddy-Newcastle zones. Although only a small fraction of the volume of Cretaceous sedimentary rocks appears to offer favorable reservoir potential, it is estimated that total oil in place is 6 billion bbl; 10 billion bbl is the speculative figure. Cumulative production to the date of this report is a scant 349 million bbl.