2014 Rocky Mountain Section AAPG Annual Meeting

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Mosser Dome Field, Yellowstone County, Montana: A Giant Stratigraphic Oil Accumulation


Surface and subsurface mapping of a valley fill depositional system in the Fall River Fm. (L. Cretaceous) in southern Montana reveals the presence of a giant oil accumulation that may have originally contained more than two billion barrels of oil. This major stratigraphic trap on the northeast flank of the Bighorn Basin is one of the largest in the Rocky Mountain region of the U.S. Except for a small area at Mosser Dome oil field, the oil contained in this trap is viscous heavy oil (<15° API). Extending a distance of twenty-two miles with a maximum width of four miles, the forty-eight square mile accumulation is found in porous sandstone that reaches a thickness of over ninety feet. The sandstone reservoir, known as the Greybull sandstone, was deposited in a westward flowing valley fill system. In the area of the Mosser Dome accumulation, two separate fault zones deflected the fluvial system to the northeast for a distance of sixteen miles before it resumed its westward flow direction. The large deflections of the valley fill system combined with southward regional dip to form a giant stratigraphic oil trap. Geochemical analysis of the oil indicates a source in the Permian age Phosphoria Fm. Subsequent introduction of bacteria-bearing fresh water caused the oil to be degraded, leaving relatively immobile asphaltic oil in the reservoir. The Mosser Dome stratigraphic trap is an excellent analog for non-degraded oil and gas accumulations in similar settings in the Fall River Fm. in the deeper portions of the Bighorn Basin.