Natural fractures orientation and density in the Bakken formation, local and regional stresses
Both Induced and natural fracturing occur within the middle Bakken from where most Bakken wells are drilled. The level of thermal maturity, source rock thickness, proximity to source facies and level of hydrocarbon generation can be pointed out as the reason behind the distribution and frequency of horizontal, open fractures in the middle member of the Bakken. The Bakken formation comprises three distinct members, upper and lower Bakken, and middle Bakken member, which is the primary reservoir rock, although the other members also have reservoir potential.
Horizontal fracture development in the middle member increases towards the boundaries of the lower and upper shales, in areas where source rocks exist at the hydrocarbon generation onset. The broadest and well-developed fracture network takes place in oil-saturated reservoir rocks, next to mature or over-matures source rocks, which generate hydrocarbons actively.
Horizontal fracturing in the middle members is said to have resulted from bitumen expulsion from lower and upper shales into interbedded sandstones with low permeability and siltstones through early-generated Carbon dioxide (CO2). Abnormal pressure development and the fracturing extent, which occurs at the same time with oil migration was because of the organic richness, thickness, and maturity of lower and upper shale.
In the middle Bakken interval, low frequency fracturing exists, because fractures commonly occupy the upper middle Bakken, which is a high CaCO3 Para-sequence. There is also inadequate vertical fracture connectivity for the bed bound fractures that are constrained by the height of thin-beds. Fracture spacing varies, and fractures relatively do not exist in the lower Middle Bakken dolomitic rocks. Laterals in this zone encounter natural fractures and drill faster. In the consortium wells, there is a bi-modal fracture orientation with NW-SE striking fractures being dominant, while NE-SW with extension fractures being subordinate. Fracture planes dip in different directions at mostly temperatures exceeding 80o. Fractures are not related to mud gas, whose increase is generally observable at the laterals toe. Lastly, the fractures are filled partially with carbonate cement with a fracture porosity ranging from 0.003% to 0.005%, and the fracture aperture being at the nannodarcy range.
The Bakken formation comprises a sequence that begins with tectonically enhanced unconformity at the top of the three forks formation. The top of low density values mid-way through the shale is overlain by high stand systems tract including upper portion of lower Bakken and facies of Middle Bakken member. The shift to lower density is likely a shift to higher organic content within the shale, meaning there is higher preservation of organic matter as the basin deepens during transgression. Maximum flooding surface occurs again within the upper Bakken shale at another shift to lower density and a high gamma ray value.
Elm Coulee is a large field located at the transition from mature to immature level of maturation. Volume increases during maturation providing necessary overpressure to allow highly productive wells. Regional fractures and faults form an orthogonal set with NE-SW and NS-SE orientations in the Bakken formation in Elm Coulee. Various horizontal wells are drilled perpendicular to sigma one direction to intersect these fractures. Local structures formed by salt dissolution or basement tectonics form both hinge parallel and hinge oblique fractures that may dominate and overprint the local fracture signature. Horizontal fractures resulting from oil expulsion in shale and maybe along bedding plane lamination of Middle Bakken C and E facies enhance production by providing permeability pathways. Large accumulations such as Elm Coulee have significant trap and seal components.
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