Chapter 8: Stratigraphic Relationships and Reservoir Quality at the Three Forks–Bakken Unconformity, Williston Basin, North Dakota

Abstract The Three Forks and Bakken formations are reservoirs in a large basin-centered oil accumulation in the North Dakota and Montana parts of the Williston Basin. Oil was discovered in reservoirs immediately below the lower Bakken shale in 1953 in Antelope Field, McKenzie County, North Dakota. Oil and gas production was established from the upper Three Forks, and the sandstone between the upper Three Forks and the lower Bakken shale was informally named the Sanish sandstone. Due to the poor definition of this sandstone and later discovery of other strata that straddle the Bakken-Three Forks contact, the Sanish has been misidentified and miscorrelated, resulting in the recent recommendation to abandon its use and instead include these strata in the Pronghorn Member of the Bakken Formation. Recent advances in horizontal drilling and multi-stage fracture stimulation techniques have increased oil production from Three Forks and Pronghorn reservoirs to more than 38,000 BOPD in late 2010. The main target of this drilling activity is the upper Three Forks. The upper Three Forks consists of interbedded pinkish-tan dolostones and green dolomitic mudstones. Parallel to subparallel laminations, uni- and bi-directional ripple cross laminations, mudcracks, and soft-sediment deformation structures are common and indicate deposition in a shallowing-upward succession of tide-dominated nearshore facies ranging from subtidal sand flats to intertidal mudflats. The dominant grain size in the upper Three Forks is silt, and reservoirs have low porosity and low permeability but are enhanced with fractures on several scales. The overlying Pronghorn Member of the Bakken Formation contains a complex mix of lithologies including burrowed dolomitic siltstones, bioturbated dolomitic sandstones, and interbedded medium grayish-green to medium to dark-gray shales. Where siltstones and sandstones are present in the Pronghorn, they have low porosity and permeability and are in communication with reservoirs of the underlying Three Forks. The Pronghorn was deposited in an overall deepening-upward sequence in a shallow marine environment and is the initial deposit of the Bakken transgression. Regional and local stratigraphic correlations lead to the recognition of an unconformity between the upper Three Forks and the overlying Pronghorn Member of the Bakken Formation. Based on these stratigraphic relationships, an erosional surface observed in numerous cores throughout the basin, and a change from progradation to transgression, the upper Three Forks-Pronghorn contact is interpreted as a sequence boundary.