Organic-carbon values are calculated from formation-density logs using the equation: TOC = (154.497/rho ) - 57.261, where TOC is organic-carbon content (wt. %) and rho is formation density (g/cm 3 ). 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. The mass of organic carbon in the Bakken Formation is approximately evenly divided between the upper and lower members, and it totals about 126 X 10 12 kg in the study area, of which 102 X 10 12 kg are in the thermally mature region. Tentative estimate that hydrocarbons equivalent to 132 billion bbl of 43 degrees (API gravity) oil have been expelled.--Modified journal abstract.