The Plains CO2 Reduction (PCOR) Partnership region has significant potential for carbon sequestration. While the methods employed in this study look specifically at the potential sequestration capacity of oil fields within the Williston Basin, they can be used to determine reconnaissance-level sequestration capacity in any oil-producing region. Two methods have been utilized to estimate capacity, one based on oil pools undergoing enhanced oil recovery (EOR) and the other assuming that the reservoir pore space can be filled to capacity with carbon dioxide (CO2). The focus of the work described in this paper is to broadly characterize the potential CO2 sequestration capacity of Williston Basin oil fields.