The upper and lower members of the Mississippian and Devonian Bakken Formation in the United States portion of the Williston basin are black shales that are extremely rich in organic matter and are the source of much of the oil found in the basin.
Organic-carbon values are calculated from formation-density logs using the equation: TOC = (154.497/ρ) – 57.261, where TOC is organic-carbon content (wt.%) and ρ is formation density (g/cm3). Test calculations comparing this equation to laboratory organic-carbon analyses from 39 wells in North Dakota show an average absolute difference of 1.1% in organic-carbon content.
Organic-carbon content, calculated at 159 locations in North Dakota and 107 in Montana, averages 12.1% for the upper member of the Bakken Formation and 11.5% for the lower member. There is a regional depletion of organic carbon, paralleling present-day isotherms, that reflects the conversion of organic matter to oil and subsequent expulsion of the oil from the formation.
The mass of organic carbon in the Bakken Formation is approximately evenly divided between the upper and lower members, and it totals about 126 × 1012 kg in the study area, of which 102 × 1012 kg are in the thermally mature region. The assumption that 167 mg HC/g TOC have migrated out of the mature Bakken shales leads to a tentative estimate that hydrocarbons equivalent to 132 billion bbl of 43° (API gravity) oil have been expelled from the United States portion of the upper and lower members of the Bakken Formation.