Three Forks Group

The Three Forks Group is a stratigraphical unit of Famennian age in the Williston Basin.

Three Forks Group Stratigraphic range: Famennian to Tournaisian PreꞒ Ꞓ O S D C P T J K Pg N
Type	Geologic group
Sub-units	Bakken Formation, Lyleton Formation, Big Valley Formation, Torquay Formation
Underlies	Madison Group
Overlies	Saskatchewan Group
Thickness	35 metres (110 ft) to 80 metres (260 ft)[1]
Lithology
Primary	Dolomite, mudstone, shale
Location
Region	Western Canadian Sedimentary Basin, Williston Basin
Country	Canada  United States
Type section
Named for	Three Forks, Montana
Named by	A.C. Peale, 1893

It takes the name from the city of Three Forks, Montana, and was first described in outcrop near the city by A.C. Peale in 1893 (for the Three Forks Shale).[2]

Lithology

The Three Forks Group is composed of Dolomite, mudstone and bituminous shale. [1]

Hydrocarbon production

In the subsurface of the Williston Basin, the Three Forks is referred to as the Three Forks Formation, which lies between the Birdbear Formation below, and the Bakken Formation above.[3]

Oil produced from the Three Forks Formation in the Williston Basin of North Dakota and south-eastern Saskatchewan is often included in production statistics with the overlying Bakken Formation. For instance, the Three Forks and Bakken were combined in estimates of potential production released by the United States Geological Survey on April 30, 2013. The estimate by the USGS projects that 7.4 billion barrels of oil can be recovered from the Bakken and Three Forks formations and 6.7 trillion cubic feet of natural gas and 530 million barrels of natural gas liquids using current technology.[4][5][6]

Distribution

The Three Forks Group reaches a maximum thickness of 80 metres (260 ft), but can be as thin as 35 metres (110 ft).[1]

Subdivisions

Sub-unit	Age	Lithology	Max. Thickness	Reference
Bakken Formation	Kinderhookian	bituminous shale	40 m (130 ft)	[7]
Lyleton Formation	Famennian	red dolomitic and silty shale, argillaceous dolomite	40 m (130 ft)	[8]
Big Valley Formation	Famennian	mudstone	40 m (130 ft)	[9]
Torquay Formation	Famennian	brown dolomite, shale	65 m (210 ft)	[10]

Relationship to other units

The Three Forks Group conformably overlies the Saskatchewan Group and is disconformably overlain by the Madison Group.[1]

It is equivalent to the sum of the Wabamun Group and Exshaw Formation in Alberta.

References

1. Lexicon of Canadian Geologic Units. "Three Forks Group". Archived from the original on 2009-04-16. Retrieved 2010-01-01.
2. Peale, A.C., 1893. The Paleozoic section in the vicinity of Three Forks, Montana. U.S G.S., Bull. 110, p. 9-56.
3. US Geological Survey, Three Forks, accessed 6 Jan. 2013.
4. "Archived copy". Archived from the original on 2013-10-29. Retrieved 2013-05-01.CS1 maint: archived copy as title (link)
5. Lenny Bernstein (April 30, 2013). "Northern plains site has twice as much oil as previously thought, Interior says". The Washington Post. Retrieved May 1, 2013. The doubling of the estimate resulted largely because of the first look at the Three Forks Formation in North Dakota, which the Geological Survey said contains 3.73 billion barrels of oil. Its reassessment of the Bakken Formation, which lies above Three Forks, showed 3.65 billion barrels, only a little more than a 2008 USGS estimate. The agency’s conclusion that the formations contain 6.7 trillion cubic feet of natural gas and 530 million barrels of natural gas liquids represented a tripling of previous estimates.
6. Stephanie B. Gaswirth; Kristen R. Marra; Troy A. Cook; Ronald R. Charpentier; Donald L. Gautier; Debra K. Higley; Timothy R. Klett; Michael D. Lewan; Paul G. Lillis; Christopher J. Schenk; Marilyn E. Tennyson; Katherine J. Whidden. "National Assessment of Oil and Gas Fact Sheet Assessment of Undiscovered Oil Resources in the Bakken and Three Forks Formations, Williston Basin Province, Montana, North Dakota, and South Dakota, 2013" (PDF). Fact Sheet 2013–3013. United States Geological Survey. Retrieved May 1, 2013. Using a geology-based assessment methodology, the U.S. Geological Survey estimated mean undiscovered volumes of 7.4 billion barrels of oil, 6.7 trillion cubic feet of associated/dissolved natural gas, and 0.53 billion barrels of natural gas liquids in the Bakken and Three Forks Formations in the Williston Basin Province of Montana, North Dakota, and South Dakota.
7. Lexicon of Canadian Geologic Units. "Bakken Formation". Retrieved 2010-01-03.
8. Lexicon of Canadian Geologic Units. "Lyleton Formation". Retrieved 2010-01-03.
9. Lexicon of Canadian Geologic Units. "Big Valley Formation". Retrieved 2010-01-03.
10. Lexicon of Canadian Geologic Units. "Torquay Formation". Retrieved 2010-01-03.