The Bell Creek field, an important recent discovery on the east flank of the Powder River basin, currently produces 50,000 bbls of oil per day and has estimated reserves of more than 200 million bbl. At least four reservoirs are delineated in the field by oil-water and gas-oil contacts. Production is from stratigraphic traps in the Muddy Sandstone of Early Cretaceous age. The producing sandstone units are very porous and permeable, pinch out eastward, and are underlain and overlain by organic-rich marine shale. The reservoir rocks were deposited in shallow-water nearshore environments near the intersection of northeast-trending littoral marine bars and a northwest-trending delta system. The traps were formed during a regression between two major advances of the Early Cretaceous sea.