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The AAPG/Datapages Combined Publications Database

Montana Geological Society

Abstract

MTGS-AAPG

Bakken Formation Short Course: Study Notes No. 1
March 28-29, 1990

Pages 16 - 16

ABSTRACT: The Bakken Formation of the Williston Basin: Deposition, Previous HitMaturationNext Hit, and Fracturing

Joe Carlisle, Geologist, American Hunter, Denver

The Bakken Formation of the Williston Basin: Deposition, Previous HitMaturationNext Hit, and Fracturing

The Bakken Formation is a Devonian-Mississippian age clastic sequence in the Williston Basin of North Dakota, Montana, and adjacent Canada. The Bakken has a tripartite division of two organic rich shales separated by a silty-sandy middle member. It is one of the richest source rocks in the world and is thermally mature over an area of approximately 15,000 square miles.

The Bakken is a marine sequence. The shales are of deep marine deposition, deposited below wave base and under anoxic conditions. The middle member consists of shallow marine, higher energy deposits. The source of the clastic material in all members is from the Precambrian Shield. The significantly different depositional environments indicate a rapidly fluctuating sea level. In fact, several rapid transgressive and regressive pulses are indicated in the Bakken sequence. Several previously unrecognized unconformities in the sequence are evidence of the rapidly changing sea level.

Hydrocarbon generation in the Bakken began in early Cretaceous time. Peak Previous HitmaturationNext Hit coincided with maximum burial that probably occurred a few million years ago. The Previous HitmaturationTop process caused overpressuring and a bulk volume change in the shales. Hydrocarbon expulsion fracturing on a micro and macro scale resulted from the overpressuring. The Laramide Orogeny caused a compressional stress regime that resulted in an anisotropic system of regional fractures, and also resulted in tectonic fracturing associated with local folding.