Richard M. Pollastro

Geologic Model for the Assessment of Technically Recoverable Oil in the Devonian–Mississippian Bakken Formation, Williston Basin

The Upper Devonian and Lower Mississippian Bakken Formation in the United States part of the Williston Basin is a giant continuous (unconventional) oil resource. A recent U.S. Geological Survey (USGS) assessment estimated a mean volume of undiscovered technically recoverable oil for the Bakken Formation of about 3.65 billion bbl of oil. The estimate is based on a geologic model and a methodology that defines different assessment units by accumulation type (conventional or continuous), structural control, fracture occurrence and prediction, lithology and petrophysical properties, formation thickness, underlying salt movement or dissolution, and level of thermal maturity and oil-generation capacity of Bakken source rocks. The Bakken Formation consists of three informal members: (1) lower shale member; (2) middle sandstone member; and (3) upper shale member. Shale members are rich in marine organic matter (as much as 35% by weight) and are the petroleum source rocks, whereas the middle sandstone member varies in depositional facies and lithology and locally exhibits good matrix porosity (as much as 14%) but with low permeability, a characteristic of tight reservoirs. Additional commingled production occurs locally from matrix porosity in the immediately underlying, informally named, Sanish sand unit of the Upper Devonian Three Forks Formation. Combined, the Bakken Formation and Sanish sand define the Bakken composite continuous reservoir. On a larger scale, thermally mature organic-rich Bakken shale members are also the source for oils produced from locally occurring Waulsortian mounds or porous strata immediately above the upper shale member in the overlying Lower Mississippian Lodgepole Limestone. As a whole, elements of petroleum source, reservoir, seal, migration, and trap define the stratigraphic and geographic character of a Bakken-Lodgepole Total Petroleum System. The geographic extent of the continuous oil accumulation within the United States part of the Bakken Formation is defined as the area in which organic-rich shale members of the Bakken Formation are thermally mature with respect to oil generation. The area of the oil-generation window for the Bakken Formation continuous reservoir was determined using a combination of the following: (1) contour mapping of both the hydrogen index (HI) and log-resistivity well data of the upper shale member, (2) calibration of HI to the transformation ratio (TR) from one-dimensional burial history models, and (3) calibration of HI to total organic content. The geologic model used to further define continuous assessment units (AUs) within the Bakken Formation continuous oil accumulation was, in general, based on assumed levels of thermal maturity and generation capacity of the Bakken shale members as determined from HI and TR, relation of HI and TR to potential fracturing and structural complexity of the Williston Basin, and lithofacies distribution and petrophysical character of the middle sandstone member. The area of the oil-generation window was divided into five continuous AUs: (1) Elm Coulee-Billings Nose AU, (2) Central Basin-Poplar Dome AU, (3) Nesson-Little Knife Structural AU, (4) Eastern Expulsion Threshold AU, and (5) Northwest Expulsion Threshold AU. One hypothetical conventional AU, a Middle Sandstone Member AU, was defined external to the area of oil generation. Using the established U.S. Geological Survey methodology, assessment of each Bakken continuous AU was performed after estimation of effective well drainage areas, estimated ultimate recovery (EUR) from productive wells, and production success defined by a minimum EUR of 2000 bbl of oil. The AUs with the greatest resource potential are the Eastern Expulsion Threshold AU (mean volume, 0.973 billion bbl of oil), which is best represented by the Parshall and Sanish fields of Mountrail County, North Dakota, and the Nesson-Little Knife Structural AU (mean volume, 0.908 billion bbl of oil), where structural reservoir development exists, the middle sandstone member is thick and porous, the underlying Sanish sand reservoir unit is commonly present, and shale members have high oil-generation potential and the probability of abundant natural fracturing.