The Triassic Spearfish Formation of North Dakota is equivalent to the Lower Watrous in Saskatchewan and the Lower Amaranth in Manitoba. In the study area, the Spearfish Formation onlaps the unconformity surface of the Mississippian Madison Group. The Spearfish oil play is both geologically and hydrogeologically complex. The play extends from a stratigraphic and structural trap at Newburg / Westhope South in North Dakota updip to Pierson and Waskada in Manitoba, where a combination of trapping mechanisms may be at work. The controls on the limit of this play appear to be Mississippian paleotopography, facies distribution within the Spearfish, and hydrodynamic influence. Within the study area, many of the depositional trends were controlled by paleotopographic features. A second-order structural residual map on the top of the Mississippian unconformity gives a good indication of the paleotopography at the time of Spearfish deposition. On the basis of this map, several depositional settings are recognizable. Three sub-basins are observed in the southwest, north-central, and eastern portions of the study area. A series of structural highs form ridges separating these sub-basins, resulting in separate hydraulic systems. Interpretation of the hydrodynamics identified eight water systems and five oil systems over the study area. Petrophysical log analyses show the Spearfish top can range from a primarily clastic facies (shale) to more of a carbonate and evaporite facies (limestone, dolomite, and anhydrite) with very little quartz content. Petrophysical evidence also suggests that, at some well locations, carbonates rather than clastics occur in greater quantities and, thus, represent the dominant lithology.