USGS – American Oil Reserves

https://www.usgs.gov/centers/central-energy-resources-science-center/science/united-states-assessments-undiscovered-oil

https://www.usgs.gov/centers/central-energy-resources-science-center/science/williston-basin-oil-and-gas-assessments

Using a geology-based assessment methodology, the U.S. Geological Survey estimated undiscovered, technically recoverable mean resources of 4.3 billion barrels of oil and 4.9 trillion cubic feet of gas (associated) in the Bakken and Three Forks Formations of the Williston Basin Province, North Dakota and Montana.