Analysis of petroleum source rocks of the Bakken Formation (Devonian and Mississippian) in North Dakota
Date of Award
Master of Science (MS)
F.D. Holland, Jr
The Bakken Formation of North Dakota consists of upper and lower, black, organic-rich shales separated by a calcareous siltstone middle member. The formation is a relatively thin unit (maximum thickness of 145 feet) with the lower shale .attaining a maximum thickness of 50 feet and the upper shale a maximum thickness of 23 feet. The shales are hard, siliceous, pyritic, fissile, and noncalcareous. They contain abundant conodonts and tasmanites and have planar laminations accented by pyrite. The upper and lower shales were apparently deposited in an offshore, marine, anoxic environment where anoxic conditions may have been caused by silling of the basin or the establishment of upwelling currents. Organic matter depositied in the black shales was derived mostly from planktonic algae.
Organic geochemical analyses used to evaluate the Bakken shales as petroleum source rocks include organic carbon measurements, chromatographic analysis of organic extracts, pyrolysis, vitrinite reflectance, and visual kerogen typing. Organic carbon measurements revealed the Bakken shales to be very organic-rich (average of 11.33 weight percent of organic carbon) and visual kerogen typing revealed this organic matter to be predominantly an amorphous type which is inferred to be sapropelic. The onset of hydrocarbon generation was determined to occur at an average depth of 9,000 feet by interpreting plots of geo chemical parameters with depth (e.g. ratios of hydrocarbon to non hydrocarbon, saturated hydrocarbon to organic carbon, pyrolytic hydro carbon to organic carbon, and the pyrolysis production index). Hydro carbon content and thermal kerogen breakdown increase greatly in the Bakken shales where they are buried greater than 9,000 feet. The effective source area of the Bakken, as determined by maps of the above geochemical parameters, lies mostly in McKenzie, Williams, Dunn, and Billings Counties. Oil generation was probably initiated in the Bakken about 80 million years ago (Late Cretaceous) at a temperature 0 of about 100 C, with initial expulsion of oil from the Bakken probably occurring 70-80 million years ago (Late Cretaceous).
Vertical fracture systems, located primarily along the Nessen anticline, Antelope Oil Field, and the Billings nose, seem to be the most reasonable way for migration of oil to occur from the Bakken into adjacent rock units. Lateral migration of Bakken oil took place in porosity zones of the Madison Group predominantly to the north and northeast of the effective Bakken source area. The migration of oil from the Bakken has apparently taken place as continuous-phase (or bulk) oil migration. Primary oil migration may have affected the hydrocarbon data used in this study by increasing or deleting the indigenous hydrocarbon content of the Bakken shales.
Webster, Rick L., "Analysis of petroleum source rocks of the Bakken Formation (Devonian and Mississippian) in North Dakota" (1982). Theses and Dissertations. 317.