GEOLOGIC MODEL FOR ASSESSMENT OF TECHNICALLY RECOVERABLE OIL IN THE DEVONIAN-MISSISSIPPIAN BAKKEN FORMATION, WILLISTON BASIN, U.S.A

The Upper Devonian to 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 (USGS) assessment estimates a mean volume of undiscovered technically recoverable oil for the Bakken Formation at about 3.65 billon barrels. The estimate was based on a geologic model and methodology that defines different assessment units by accumulation type (conventional or continuous), petrophysical properties, fracture occurrence and prediction, and level of thermal maturity and oil generation 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 (up to 35 percent) and are the petroleum source rocks, whereas the middle sandstone member varies in lithology and locally exhibits good matrix porosity (up to 14 percent) with permeabilities characteristic of tight reservoirs. Additional comingled production occurs locally from matrix porosity in the underlying informal “Sanish sand” unit of the Devonian Three Forks Formation. Combined, the Bakken Formation and Sanish sand define the “Bakken composite continuous reservoir”.

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 (TR) from 1D burial models; and 3) calibration of HI to total organic content.

The geologic model used to define continuous assessment units (AU) generally assumed levels of thermal maturity and generation capacity of the Bakken shale members from HI and TR, relation of HI and TR to potential fracturing and structural complexity of the Williston Basin, and lithofacies and petrophysical character of the middle sandstone member. Five continuous AUs were defined within the Bakken oil generation window. Using the established USGS methodology, assessment of each Bakken continuous AU was performed after estimation of effective well drainage areas, estimated ultimate recoveries (EUR) from productive wells, and success defined by a minimum EUR of 2,000 barrels.