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Mineralogy and Petrology Controls on Hydrocarbon Saturation in the Three Forks Reservoir, North Dakota


The Three Forks reservoir forms the lower part of the “Bakken pool” in the North Dakota portion of the Williston basin. Oil production occurs from sandy to silty dolomitic lithologies in the upper portion of the Three Forks Formation. In most oil-productive areas of western North Dakota, three reservoir rock types can be defined in the upper Three Forks based on mineralogy, capillary pressure characteristics and water saturation distributions. The best Three Forks hydrocarbon saturations occur in brown to tan, sandy to silty dolostone. Within the productive oil column, this end-member lithology typically has 2-7% porosity (4.3% average) and 5-25% water saturation. The average mineral content is 63% dolomite, 31% quartz-feldspar and 3% illite (values less than 1% not listed). A second end-member rock type is green, silty, dolomitic mudstone that typically has 5-11% porosity (8.9% average) and 40-90% water saturation. The average mineral content is 35% dolomite, 31% quartz-feldspar, 30% clay minerals (23% illite, 4% chlorite, 3% illite-smectite), and 2% pyrite-marcasite. The third rock type consists of mixed brown and green, sandy to silty dolostone, with intermediate reservoir rock properties. It includes laminated and brecciated lithologies. Due to small pore-throat sizes, oil column heights greater than 3,000 feet would have been needed to achieve observed hydrocarbon saturations in a water-wet system. Under these conditions, the oil column is too thin to be explained by simple buoyancy-driven oil emplacement. Based on an analogy with very low permeability, continuous gas reservoirs, it is inferred that either: 1) hydrocarbon emplacement occurred at a shallower depth where permeability was higher, or 2) overpressure (current or ancient) that developed during maturation of overlying Bakken shales was required to emplace oil in rocks with existing very low permeability.