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The AAPG/Datapages Combined Publications Database

Rocky Mountain Association of Geologists

Abstract


The Bakken-Three Forks Petroleum System in the Williston Basin, 2011
Pages 490-507

Chapter 19: Petroleum Source Rocks and Stratigraphy of the Bakken Formation in North Dakota

Rick L. Webster

Abstract

The Bakken Formation (Devonian–Mississippian) of North Dakota is a relatively thin unit consisting of upper and lower black organic-rich shales separated by a calcareous siltstone middle member. 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 geochemical analyses revealed the Bakken shales to be very organic rich (average 11.33 wt.% of organic carbon), and visual kerogen typing revealed this organic matter to be predominantly an amorphous type inferred to be sapropelic. The onset of hydrocarbon generation was determined to occur at an average depth of 9000 ft (2745 m) by interpretion of plots of geochemical parameters with depth. Hydrocarbon content and thermal kerogen breakdown increase greatly in the shales where they are buried greater than 9000 ft. The effective source area of the Bakken lies mostly in McKenzie, Williams, Dunn, and Billings counties. Oil generation was probably initiated in the Bakken about 75 m.y. ago (Late Cretaceous time) at a temperature of about 100° C, with initial expulsion of oil from the Bakken probably occurring 70 m.y. ago.

Vertical fracture systems located primarily along the Nesson anticline, Antelope oil field, and the Billings anticline seem to be the most reasonable way for migration of oil to have occurred from the Bakken into adjacent rock units. The amount of oil generated by the Bakken in North Dakota is 92.3 billion bbl. If only 10% of this 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.


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