Abstract

Mudlogs and drillstem tests from the sparse drilling in the structurally deeper areas of the Bighorn Basin of Wyoming and Montana suggest the presence of an overpressured basin-centered gas accumulation of limited size. The areas of overpressuring cover about 135 square miles at the stratigraphic level of the top of the Upper Cretaceous Mesaverde Formation, 290 square miles at the level of the base of the Mesaverde, and 920 square miles at the level of the Upper Cretaceous Frontier Formation. These areas are considerably smaller than the areas of overpressuring in the Wind River Basin to the south, a basin of similar size to the Bighorn Basin, where overpressuring at the base of the Mesaverde Formation, for example, occurs over an area of about 1,600 square miles. The limited number of available drillstem tests and mudlogs suggest an extensive area of underpressured gas-bearing rocks surrounding this overpressured core in the Bighorn Basin. The combined area of underpressured and overpressured gas-bearing rocks may cover as much as 1,980 square miles at the Mesaverde level, suggesting that the majority of the basin-centered accumulation in the basin may be underpressured. This contrasts with the Wind River Basin to the south where the majority of the basin-centered gas accumulation appears to be overpressured. Present-day temperatures and levels of thermal maturity appear to have been somewhat higher at the onset of overpressuring in the Bighorn Basin than in the Wind River Basin.