Chapter 5 Geologic Assessment of Technically Recoverable Oil in the Devonian and Mississippian Bakken Formation
Pollastro, Richard M.
Roberts, Laura N.R.
Cook, Troy A.
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The Upper Devonian and Lower Mississippian Bakken Formation in the U.S. portion of the Williston Basin is a giant continuous (unconventional) oil resource. A recent U.S. Geological Survey assessment estimated a mean volume of undiscovered technically recoverable oil for the Bakken Formation at about 3.65 billion barrels 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. The lower and upper shale members are rich in marine organic matter (as much as 35 percent 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 percent) but with low permeability characteristic of tight reservoirs. Additional commingled production occurs locally from matrix porosity in the immediately underlying Sanish sand unit (informal name) 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 also source 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 U.S. portion of the Bakken Formation is defined as the area in which the organic-rich shale members are thermally mature with respect to oil generation. The area of the oil-generation window for the Bakken 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 from one-dimensional burial history models, and (3) calibration of HI to total organic content. The area of the oil-generation window was divided into five continuous assessment units: (1) Elm Coulee-Billings Nose, (2) Central Basin-Poplar Dome, (3) Nesson-Little Knife Structural, (4) Eastern Expulsion Threshold, and (5) Northwest Expulsion Threshold. One hypothetical conventional assessment unit, the Middle Sandstone Member, was defined external to the area of oil generation.