SEDIMENTOLOGY OF A BLACK BOX – DEFINING SHALE FACIES BELTS IN THE UPPER DEVONIAN-LOWER MISSISSIPPIAN BAKKEN FORMATION, WILLISTON BASIN OF NORTH DAKOTA, USA

The Upper Devonian-Lower Mississippian Bakken Formation is one of the most prolific oil reservoirs in the continental United States. A mostly dolomitic unit, the middle Bakken member, is sandwiched between two high-total organic carbon-bearing shales, the lower and upper Bakken shale members, thought by others to have been deposited in an anoxic environment. Sedimentological investigations to date have concentrated on the middle Bakken member, the producing interval, and have largely ignored the two shales. This study concentrates on the upper Bakken member using continuous ultra-thin thin sections from North Dakota core samples for detailed sedimentological analysis.

The upper Bakken shale member exhibits three distinct facies irregularly stacked on a millimeter- to centi­meter-scale throughout the cored interval investigated. On a depositional transect from relatively shallow to deep water, these facies form belts as follows. Facies belt 1 is heavily bioturbated with well-defined horizontal burrows that forms massive mudstone with no observable sedimentary features (e.g. laminations). The intense bioturbation of facies belt 1 is attributed to well-oxygenated conditions within the sediments during and after deposition. Facies belt 2 is minimally bioturbated with some vertical burrows, some ripples, and laminations that are coarsening-upward from clay particles at the base to silt-sized grains at the top. Less intensive bioturbation in facies belt 2 points to the likelihood of dysoxic conditions even at a few to several centimeters below the sediment-water interface. Facies belt 3 is a non-bioturbated, massive radiolarian facies. Only facies belt 3 remains as a candidate for an anoxic environment during sediment deposition.

According to our model, traction deposition, evidenced by ripples, episodically occurred in facies belt 2 and is inferred in the shallower facies belt 1, and only the radiolarian-prone facies 3 is envisioned to have been deposited entirely by suspension settling. The small-scale stacking patterns observed argue for frequent changes in the oxygen content of the water column and immediately below the sediment-water interface, supporting a model for dysoxic/oxic, not anoxic conditions for much of Bakken shale deposition.