Geology
The rock formation consists of three members: lower shale, middle dolomite, and upper shale. The shales were deposited in relatively deep anoxic marine conditions, and the dolomite was deposited as a coastal carbonate bank during a time of shallower, well-oxygenated 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, in today's terms a light tight oil play. However, the presence of vertical to sub-vertical natural fractures makes the Bakken an excellent candidate for horizontal drilling techniques in which a well drills horizontally along bedding planes, rather than vertically through them. In this way, a borehole can contact many thousands of feet of oil reservoir rock in a unit with a maximum thickness of only about 140 feet (40 m). Production is also enhanced by artificially fracturing the rock, to allow oil to seep to the oil well.
Read more about this topic: Bakken Formation