Sequence Stratigraphy of the Bakken and Three Forks Formations, Williston Basin, USA
Authors: Sonnenberg Stephen A. 
Published: November 2019

The Williston Basin Bakken petroleum system is a giant continuous hydrocarbon accumulation. The petroleum system consists of source beds in the upper and lower Bakken shales and reservoirs in the middle and upper Three Forks, the Pronghorn member of the Bakken, and the middle Bakken. The petroleum system is characterized generally by low-porosity and permeability reservoirs, organic-rich source rocks, and regional hydrocarbon charge. The USGS (2013) mean technologically recoverable resource estimates for the Bakken Petroleum System is 7.375 billion barrels oil, 6.7 TCF gas, and 527 million barrels of natural gas liquids (Gaswirth et al., 2013).

In the western US, relative sea level changes may be a combination of glaciation in the southern hemisphere, regional flexural tectonics related to the Antler orogeny, epeirogenic uplift, and/or localized structural movement (Cole et al., 2015). The controls are not fully or clearly differentiated in the rock record.

The Three Forks is a silty dolostone throughout much of its stratigraphic interval. The Three Forks ranges in thickness from less than 25 ft to over 250 ft in the mapped area. Thickness patterns are controlled by paleostructural features such as the Poplar dome and the Nesson, Antelope, and Cedar Creek anticlines. Thinning and/or truncation occurs over the crest of the highs and thickening of strata occurs on the flanks of the highs.

The Three Forks unconformably (?) overlies the Birdbear in the Williston Basin and in turn is unconformably overlain by the Bakken Formation. The Three Forks consists of one overall deepening upward third-order sequence consisting of continental sabkha dolostones and anhydrites at the base changing to supratidal dolostones in the middle part to intertidal dolostones and mudstones in the upper part. The unit is subdivided into six shallowing upward parasequences by various authors. For mapping purposes, a three subdivision scheme has been adopted for this paper (i.e., upper, middle, lower). Most of the development activity in the Three Forks targets the upper Three Forks.

The Bakken Formation in the Williston Basin consists of four members: a basal member (dolostone, limestone, and siltstone) recently named the Pronghorn; a lower organic-rich black shale; a middle member (silty dolostone or limestone to sandstone lithology); and an upper organic-rich shale member. The Bakken Formation ranges in thickness from a wedge edge to over 140 ft with the thickest area in the Bakken located in northwest North Dakota, east of the Nesson anticline.

The Bakken appears to be composed of one complete third order sequence and part of a second third order sequence. The basal Pronghorn to middle part of the Middle Bakken represents one complete sequence (lowstand to transgressive to highstand system tracts). The Pronghorn to Lower Bakken Shale represent lowstand to transgressive system tracts. The lower Middle Bakken (facies A-C) represents a highstand system tract (falling stage system tract in upper part). The upper Middle Bakken (facies D-F) through the Upper Bakken Shale represents part of another third order sequence (lowstand to transgressive system tract). The Middle Bakken has an oolitic, bioclastic, sandy middle facies (facies D) which represents a lowstand deposit. This is overlain by the upper Middle Bakken (facies E-F) and the Upper Bakken Shale which is a transgressive system tract. Part of the overlying Lodgepole represents the highstand part of the second sequence.

Sharp downlap surfaces are noted at the base of the Middle Bakken and the base of the Lodgepole. The downlap surfaces represent the transition from transgressive system tracts to highstand system tracts. Maximum flooding surfaces are found in the middle and upper portions of the upper and lower Bakken shales.