This comprehensive, basin-scale hydrodynamic study of the Williston Basin covers eastern Montana, all of North Dakota, southern Saskatchewan, and southern Manitoba and investigates 21 zones from the Upper Cretaceous to the Cambrian. The study area extends from 46°N to 51°N and from 100°W to 107°W, covering the depocentre and productive margins of the Williston Basin. Utilizing data from nearly 40,000 wells, the study provides maps of reservoir structure, formation water salinity, hydraulic head, and water and oil migration patterns. Provided on the oil migration maps are hydrocarbon occurrences (producing fields and shows) and rigorously defined source rock maturity outlines for the charging source horizon. Geochemical work on petroleum systems (source rock: reservoir oil correlations) has been incorporated and a regional temperature gradient map compiled to determine source rock maturity limits is also included. The study reveals extensive regions within each reservoir formation with excellent undiscovered reserves potential.