The U.S. Geological Survey completed an assessment of the undiscovered oil and gas potential of the Williston Basin Province in 2008. The assessment applied the total petroleum system concept, which includes mapping the distribution of potential source rocks and known petroleum accumulations, and determining the timing of petroleum generation and migration. Geologically based, it focuses on source rock and reservoir rock stratigraphy, timing of tectonic events and the configuration of resulting structures, formation of traps and seals, and burial history modeling. The total petroleum system is subdivided into assessment units based on similar geologic characteristics and accumulation and petroleum type. For the Williston Basin Province, 10 total petroleum systems, 13 conventional assessment units, and 6 continuous (or unconventional) assessment units were defined, and the undiscovered oil and gas resources within each assessment unit were quantitatively estimated.