Assessing Undiscovered Resources in the Devonian Three Forks Formation, Williston Basin, USA

Marra, Kristen; Gaswirth, Stephanie; Cook, Troy

The Devonian Three Forks Formation has become a significant target for oil exploration in the U.S. Williston Basin due to recent increased drilling and production activity in the overlying Bakken Formation. The U.S. Geological Survey (USGS) assessed the Williston Basin Province in 2008, which included the Bakken Formation and 50 feet of strata below the lower shale member of the Bakken. This additional 50 feet encompassed the informal Pronghorn member of the Bakken Formation (also known as the "Sanish sand") and the uppermost Three Forks Formation in areas where the Pronghorn is thin or absent. A complete assessment of the Three Forks Formation has not been previously conducted by the USGS. The Three Forks Formation consists of interbedded grayish-green dolomitic mudstones, pink-tan silty dolostones, and anhydrite, which were all deposited in supratidal to intertidal environments. It is divided stratigraphically into lower and upper units or by some operators into four "benches." The Three Forks Formation is sourced by oil generated in the upper and lower Bakken shale members and is considered part of the Bakken Petroleum System. It is a low porosity (~6%) and low permeability (0.04 md) tight oil reservoir that requires multi-scale fracturing. Recent oil discoveries in the upper Three Forks (or "benches" 1 and 2) and saturations of oil in the lower Three Forks (or "benches" 3 and 4) suggest that these units contain technically recoverable oil. Although it is stratigraphically part of the Bakken Formation, the Pronghorn member will be assessed as part of the Three Forks Formation due to both units being similarly sourced from the overlying Bakken shales and the lack of seal between the Pronghorn and upper Three Forks. The current assessment of the Three Forks Formation will incorporate thermal maturity limits based on hydrogen index mapping of the upper and lower Bakken shales, geographic extents of the Bakken and Three Forks intervals, and structural features to define geologically based assessment unit boundaries for this emerging oil play. The complete USGS assessment of technically recoverable, undiscovered resources of both the Bakken and the Three Forks Formations will be completed in 2013.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013