| Three Forks Group | |
|---|---|
| Stratigraphic range:Famennian toTournaisian | |
| 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 |
TheThree Forks Group is astratigraphical unit ofFamennianage in theWilliston Basin.
It takes the name from the city ofThree Forks, Montana, and was first described in outcrop near the city by A.C. Peale in 1893 (for the Three Forks Shale).[2]
The Three Forks Group is composed ofdolomite,mudstone and bituminousshale.[1]
In the subsurface of the Williston Basin, the Three Forks is referred to as the Three ForksFormation, which lies between theBirdbear Formation below, and the Bakken Formation above.[3]
Oil produced from the Three Forks Formation in the Williston Basin ofNorth Dakota and south-easternSaskatchewan 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 ofnatural gas liquids using current technology.[4][5][6]
The Three Forks Group reaches a maximum thickness of 80 metres (260 ft), but can be as thin as 35 metres (110 ft).[1]
| Sub-unit | Age | Lithology | Max. Thickness | Reference |
|---|---|---|---|---|
| Bakken Formation | Kinderhookian | bituminousshale | 40 m (130 ft) | [7] |
| Lyleton Formation | Famennian | red dolomitic and siltyshale, argillaceousdolomite | 40 m (130 ft) | [8] |
| Big Valley Formation | Famennian | mudstone | 40 m (130 ft) | [9] |
| Torquay Formation | Famennian | browndolomite,shale | 65 m (210 ft) | [10] |
The Three Forks Groupconformably overlies theSaskatchewan Group and isdisconformably overlain by theMadison Group.[1]
It is equivalent to the sum of theWabamun Group andExshaw Formation inAlberta.
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.
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.