The Niobrara Formation (Upper Cretaceous) has recently gained attention as a shallow, low-permeability reservoir for natural gas. Understanding its distribution and the conditions under which it was deposited will contribute to its evaluation as a source of hydrocarbons in this region.
On the basis of outcrop sections and cores in northeastern North Dakota, the Niobrara Formation is approximately 64 m thick and can be divided into two subequal units. The lower 31-m unit is medium dark gray and medium olive-gray, laminated calcareous shale with “white specks” (fecal pellets), comminuted fish remains, Lingula, and thin fine-grained sand stringers near the base. The upper 33-m unit is light-gray to light olive-gray, shally chalk containing abundant “white specks”, with a thin (5 m) very light gray, bioturbated chalk at its base. Sediments are bioturbated at the top of the lower unit and the base of the upper unit.
The main controls of sediment character are rates of calcareous plankton productivity and aerobic versus anaerobic bottom conditions. The Niobrara represents, from bottom to top, the following sequence of environments: (1) low productivity anaerobic conditions; (2) low productivity aerobic conditions; (3) high productivity aerobic conditions; and (4) high productivity anaerobic conditions.
Over the eastern half of North Dakota, the Niobrara ranges in thickness from less than 17 m to greater than 75 m. Alternating thinning and thickening bands trend northwest-southeast and suggest structural control of deposition.