Bakken Formation - Wikipedia, the free encyclopedia
The rock formation consists of three members: lower shale, middle dolomite, and upper shale. The shales were deposited in relatively deep marine conditions, and the dolomite was deposited as a coastal carbonate bank during a time of shallower water. The middle dolomite member is the principal oil reservoir, roughly two miles (3.2 km) below the surface. Both the upper and lower shale members are organic-rich marine shale.
Porosities in the Bakken average about 5%, and permeabilities are very low, averaging 0.04 millidarcies—much lower than typical oil reservoirs.[4] However, the presence of horizontal fractures makes the Bakken an excellent candidate for horizontal drilling techniques in which a well drills along the extent of the rock layer, rather than punching a hole vertically through it. In this way, many thousands of feet of oil reservoir rock can be penetrated in a unit that reaches a maximum thickness of only about 140 feet (40 m).[5] Production is also enhanced by artificially fracturing the rock,[6] to allow oil to seep through for centralized collection.
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