Correlation and Stratigraphic Analysis of the Bakken and Sappington Formations in Montana

The Upper Devonian-Lower Mississippian (Late Fammenian-Tournaisian) Bakken Formation in the Williston Basin is one of the largest continuous oil fields in the U.S. The upper and the lower shale members are organic rich source rocks that supplied oil to the middle member, which is reservoir rock. Although the oil-producing Bakken Formation has been intensely studied in the Williston Basin, the lateral relationship between the Bakken Formation and the coeval Sappington Formation in western Montana remains cryptic. This study correlates the Sappington Formation in western Montana with the Bakken Formation in the Williston Basin in northeastern Montana. It clarifies the lateral relationship between these two units, and extent of their members across Montana and, the causes of these thickness variations. This study utilized 675 well logs (mostly gamma ray, caliper, sonic, density, neutron, resistivity logs) to make multiple E-W and N-S cross sections and isopach maps. Also, seven outcrops of the Sappington Formation in southwestern Montana and five Bakken Formation cores in the Williston Basin were tied to the subsurface data.

Variations in the distribution of the Bakken/Sappington Formation were caused by eustatic changes and local epeirogenic uplifts. The Bakken/Sappington Formation is thickest in the depressions in southwestern and the northeastern Montana, the Central Montana Trough and the Williston Basin in Montana. The Bakken/Sappington Formation is thin coincident with major structural uplifts that were active during the Late Devonian, such as Yellowstone Park Uplift, Bearpaw Anticline, Scapegoat-Bannatyne Anticline and Nesson Anticline. Devonian strata are difficult to identify in the subsurface of south-central Montana making the Bakken/Sappington correlation problematic in this area. The Lower Bakken/Sappington Member thickness is 15 ft (4.6 m) in northeastern and southwestern Montana. The Lower Bakken/Sappington Member is more continuous in western Montana than the other Bakken/Sappington Members. The Middle Bakken/Sappington Member is thickest (~55 ft; 16.7 m) in the northeastern Williston Basin and in the Central Montana Trough (~50 ft; 15.2 m). The Middle Bakken/Sappington Member was less affected by the tectonics and it is present from northwestern to northeastern Montana, except in far northwestern and central Montana. The Upper Bakken Member (~5-15 ft; 1.5 m-4.6 m) is the most continuous unit in the Williston Basin, as the Bakken Members show onlapping relationship that makes the distribution of each younger member greater. However, the Upper Bakken/Sappington Member is absent west of the Central Montana Trough due to basin inversion and it is also absent in far northwestern and central Montana as a result of the erosion or nondeposition caused by the local uplifts. Transgressions were responsible for the deposition of the upper and the lower black shales in offshore marine environments, whereas the Middle Bakken/Sappington Member was deposited during regression and records multiple offshore marine to tidal environments.