The U.S. Geological Survey (USGS) recently completed an assessment of the undiscovered oil and gas potential of the Bighorn Basin Province, which encompasses about 6.7 million acres in north-central Wyoming and southern Montana. The assessment is based on the geologic elements of each total petroleum system defined in the province, including petroleum source rocks (source-rock maturation, petroleum generation, and migration), reservoir rocks (sequence stratigraphy and petrophysical properties), and traps (trap formation and timing). Using this geologic framework, the USGS defined two total petroleum systems: (1) Phosphoria, and (2) Cretaceous-Tertiary Composite. Within these two systems, eight assessment units (AU) were defined, and undiscovered oil and gas resources were quantitatively estimated within each AU.