The Three Forks Formation is an important hydrocarbon reservoir in the Williston Basin, North Dakota. High production rates associated with new drilling and completion techniques has created interest in this formation. Core data have increased an understanding of depositional environments, facies distribution, and other parameters that impact reservoir quality.Six cores were described in detail and additional 23 wells were evaluated to propose a depositional model for the Three Forks Formation.The upper Devonian Three Forks Formation was deposited on a shallow and extensive epeiric platform. During the late Devonian, an arid climate and sea level changes controlled deposition across a storm and tidal-dominated, evaporitic platform. Stacked cyclic patterns are evident in cores as well as logs, especially the gamma ray.The Three Forks Formation is a mud-dominated system composed of dolomitic siltstones, claystone, and mudstone. Dolomitic siltstones are tan-brown, while claystones and mudstones exhibit colors that range from gray-green to red. The reddish color is a product of erosion, weathering, and oxidation.Six facies were identified and described for the Three Forks Formation. Facies F and Porosity-permeability and irreducible oil-irreducible water saturation plots were also constructed using available core data. These showed the best reservoir quality in the Upper Three Forks.Digenesis was also examined using thin section analyses from a well that cored the entire Three Forks interval (EOG Liberty 2-11H). The Lower Three Forks has significantly more calcite and anhydrite cementation, oxidation, and anhydrite. Collectively, these components have a negative effect on reservoir quality. In the Middle and the Upper part of Three Forks, dolomitization affects reservoir quality in a positive way, enhancing both porosity and permeability.Mineralogy also affects reservoir quality. An increase in calcite decreases reservoir quality while an increase in dolomite is associated with better reservoir quality and higher production in the Upper Three Forks.Three Forks production is influenced by thermal maturity of the Bakken Formation. The up-dip thermal boundary of the petroleum system ties directly to thermal maturity of the Lower Bakken Shale.In summary, the reservoir quality in Three Forks Formation depends on the interplay of factors such as depositional environment, mineralogical composition, diagenetic processes, and factors affecting petroleum generation and entrapment. These have a profound impact on porosity, permeability, and productivity.

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