The Tensleep Sandstone of Pennsylvanian age thins northward in the Bighorn Basin indicating uplift and post-Des Moines beveling and channeling. These irregularities were filled by sediments as the Phosphoria Sea transgressed in Permian time, and the Permian strata also thin regionally northward. Oil accumulation in the Tensleep has been mostly in structurally controlled traps beneath impervious beds of the overlying Phosphoria (Park City) Formation. Accumulation in several of these traps is believed to have been favored by intraformational change in permeability or lithofacies of pervious strata against impervious channel-fillings at the base of the Phosphoria.

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