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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 68 (1984)

Issue: 7. (July)

First Page: 953

Last Page: 953

Title: Petroleum Source Rocks and Stratigraphy of Bakken Formation in North Dakota: ABSTRACT

Author(s): Rick L. Webster


The Bakken Formation (Devonian and Mississippian) 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 145 ft (44.2 m)--with the lower shale attaining a maximum thickness of 50 ft (15.2 m), and the upper shale a maximum thickness of 23 ft (7 m). 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 a stratified water column resulting from restricted circulation. Organic matter in the black shales was derived mostly from planktonic algae.

Organic-carbon measurements revealed the Bakken shales to be very organic-rich (average of 11.33 wt. % of organic carbon), and visual kerogen typing revealed this organic matter to be predominantly an amorphous type that is inferred to be sapropelic. The onset of hydrocarbon generation was determined to occur at an average depth of 9,000 ft (2.74 km) by interpreting plots of geochemical parameters with depth (e.g., ratios of hydrocarbon to nonhydrocarbon, saturated hydrocarbon to organic carbon, pyrolytic hydrocarbon to organic carbon, and the pyrolysis production index). Hydrocarbon content and thermal kerogen breakdown increase greatly in the Bakken shales where they are buried at depths greater than 9,000 ft (2.74 km). The effective source area of the Bakken, as determined by maps f the above geochemical parameters, lies mostly in McKenzie, Williams, Dunn, and Billings Counties. Oil generation was probably initiated in the Bakken about 75 Ma (Late Cretaceous) at a temperature of about 100°C (212°F), with initial expulsion of oil from the Bakken probably occurring 70 Ma (Late Cretaceous). The amount of oil generated by the Bakken in North Dakota, as calculated from pyrolysis data, is 92.3 billion bbl. If only 10% of this oil was actually expelled from the shales, it could easily account for the 3 billion bbl of known type II oil reserves in the Williston basin. (This paper is a summary of work done for an M.S. thesis at the University of North Dakota).

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Copyright 1997 American Association of Petroleum Geologists