The Bell Creek field, one of the most important oil discoveries in the United States in 1967, is in T8 and 9S, 54, and 55E, in Powder River and Carter Counties, southeastern Montana. Daily production, currently 50,000 bbl of oil, should rise to 65,000 bbl, outstripping that of any other Rocky Mountain oil field. Reserves are estimated at more than 200 million bbl.

The field is a Lower Cretaceous stratigraphic trap uncontrolled by structure, along the gently dipping east flank of the Powder River basin. Located near the intersection of northeast-trending littoral marine bars and a northwest-trending delta system (evident from earlier regional stratigraphic studies), the Muddy Sandstone trap was formed in a complex assemblage of local, shallow-water, nearshore environments during a regressive phase between two major advances of Early Cretaceous seas. Very porous and permeable sandstone bodies pinch out eastward, contain organic matter, and are underlain and overlain by organic-rich marine shale, providing an ideal trap for indigenous oil. The sandstone tongues were tilted upward on the east shortly after deposition as a result of the westward sub idence of the basin in which the overlying Mowry Shale was deposited. Later subsidence of the east limb of the Powder River basin further increased the size and efficiency of the trap. At least four individual reservoirs are delineated in the complex facies assemblage by oil-water and gas-oil contacts.

The Bell Creek field is an excellent example of major oil fields remaining to be found in stratigraphic traps in the sparsely drilled Rocky Mountain region. Drilling, rather than other exploration methods, is the most efficient and conclusive test of evolving economically oriented stratigraphic concepts.