The highly productive oilfields in North Dakota tap into a number of oil-rich sedimentary rock formations that underlie various parts of Williston Basin – a subsurface, oval-shaped structural-sedimentary depression that was formed by both tectonic events and the accumulation of large amounts of sediments. The Williston Basin has a major-axis length of about 760 km measured along a roughly north-south direction from western South Dakota to southern Saskatchewan and southwestern Manitoba, and a maximum minor-axis length of about 480 km along an east-west direction from central North Dakota to eastern Montana. The deepest part of the Williston Basin lies at the southwestern region of North Dakota. The Bakken, Three Forks, and Spearfish formations are among the most productive oil-and-gas bearing sedimentary rocks in the North Dakota area of the Williston Basin.
The Bakken Formation was formed at the beginning of the Mississippian Subperiod from about 417 to 350 million years ago. In the Williston Basin, the Bakken occurs as a three-layered sequence of rocks that includes a 50-foot-thick lower marine shale member, a middle marine clastic-carbonate (sandstone, siltstone, limestone and dolomite) member (85 feet thick), and an upper marine shale member (23 feet thick). The Bakken shales and mudstones are usually laminated and exhibit colors ranging from red to brown to black. The hydrocarbon-rich remains of ancient microscopic plants and animals were deposited and trapped within the pore spaces of these shales and mudrocks, and were then later transformed by high temperatures and pressures into “oil-prone” kerogen (and later on into oil and gas). Much of the oil and gas that were formed in the lower and upper shale layers remained in place because of the low permeability of the Bakken shales, but it seems that large amounts of oil and gas also found their way into portions of the middle, carbonate layer. Thus, oil and gas can be found in all three Bakken members, with the upper and lower shales functioning as both source and “continuous-type” reservoir rocks for the oil and gas. The middle clastic-carbonate member functions as a “continuous-type” reservoir, with the oil and gas being more or less evenly distributed within the rock body instead of being concentrated within a few structural or other (separate and distinct) rock traps. The middle clastic-carbonate member of the Bakken Formation is more porous and permeable than the lower and upper shale members. This middle layer is also the reservoir rock from which much of the oil extracted at the Elm Coulee Field in Montana originates. The current oil boom in North Dakota actually has its beginnings in 2006 when oil developers operating in Montana looked across the state border and traced the Bakken Formation eastwards from the Elm Coulee Field to other potentially promising plays (such as the Parshall Field) in North Dakota. Note that hydraulic fracturing of oil-producing layers through vertical wells had been first applied to the Bakken rocks in the 1960s.
The Three Forks Formation lies just below the Bakken and consists predominantly of limestone, clayey-limestone, dolomite, shale, and sandstone rock layers. It has a maximum thickness of 270 feet. The Three Forks Formation serves as as reservoir rock for the oil and gas originating from the lower shale member of the Bakken Formation. The upper shale member of the Bakken becomes the source rock for the Three Forks at the southern edge of the Williston Basin where the lower and middle members of the Bakken thin out. Note that before the current boom, oil had been extracted from the Three Forks beginning in the 1950s. An oil-rich sandstone unit designated as the Sanish lies between the Bakken and the Three Forks. Geologists are still divided on whether to include this unit with the lower Bakken shale member or to designate it as the topmost unit of the Three Forks formation.
The Spearfish Formation (sandstones, siltstones, shales, and evaporites) occupies part of the northeastern region of the Williston Basin. The sediments of the Spearfish were accumulated during the Triassic Period and the Spearfish is therefore much younger than the Bakken and the Three Forks. The Spearfish is so far the youngest known liquid-petroleum-bearing formation in North Dakota. The Spearfish formation is known primarily as the reservoir rock for oil that migrated upwards from the Madison Group of source rocks that lie just below the Spearfish, but other sources of the oil might still come to light.
References:
1) The Bakken Source System: Emphasis on the Three Forks Formation
2) Technology-Based Oil and Natural Gas Plays: Shale Shock! Could There Be Billions in the Bakken?
3) Bakken Mudrocks of the Williston Basin, World Class Source Rocks
4) Isopach of the Three Forks Formation - State of North Dakota
5) Bakken Tricks Work on the Three Forks
6) The Spearfish Formation -- Another Unconventional Target
7) Information on the Spearfish Formation
8) The Spearfish oil play in Manitoba and North Dakota/
9) North Dakota Geological Survey
10) Three Forks Formation to yield lots of oil in North Dakota
11) Geology -- Bakken
