The Bakken shale and underlying Three Forks formation are part of the larger Williston Basin which encompasses Saskatchewan, Manitoba, North Dakota, Montana, and South Dakota. Bakken producing zones are mainly present in Western North Dakota, Southern Saskatchewan, and Eastern Montana.
The Bakken shale is one of the largest (possibly the largest) continuous oil accumulations in the world. It is an over pressured system which is in part responsible for high IP rates. The high pressure in the formation suggests that the oil is contained within the petroleum system.
This means that the oil remains in place and is tightly contained throughout the geologic structure. Ten years ago both reserves and productive capability were highly uncertain. But today Bakken shale oil is not only accessible but also profitable given advances in technological know-how and sustained high oil prices.
The Three Forks formation, generally found immediately under the Bakken formation, has also proven to contain productive reservoir rock. The Three Forks formation typically consists of interbedded dolomites and shale with local development of a discontinuous sandy member at the top, known as the Sanish sand. The Three Forks formation is an unconventional carbonate play. Similar to the Bakken formation, the Three Forks formation has recently been exploited utilizing the same horizontal drilling and advanced completion techniques as the Bakken development. Drilling in the Three Forks formation began in mid-2008 and a number of operators are currently drilling wells targeting this formation.
Source : Continental Resources Investor Presentation
The Bakken is a shale oil play. It is conventional, light-sweet crude oil, trapped 10,000 feet below the surface within shale rock. The Bakken shale play consists of three layers, an upper layer of shale rock, a middle layer of sandstone/dolomite, and a lower layer of shale rock. The middle sandstone layer is what is commonly drilled and fracked with the horizontal lateral today.
The Three-Forks formation, which is being drilled just below the Bakken shale, is thought to be contributing oil volumes separate from the Bakken shale, potentially adding to total recoverable reserves. While the most heavily produced zones in the Williston Basin have been the Bakken shale and underlying Three Forks, there are other promising zones in the basin that offer potential as drillers learn more about the geology and oil field technology advances.