ABSTRACT

Hydrocarbon source-rock bodies comprising the upper and lower Bakken shale units are mature in the deeper portions of the Williston Basin where they have been exposed to present-day maximum burial-related temperatures of about 165° F or more. Hydrocarbon generation within the zone of maturity has caused the creation of a zone of abnormally high fluid pressure within the Bakken and closely adjacent beds. The abnormally high fluid pressures have created vertical fractures in the adjacent confining beds comprising the Lodgepole and Three Forks Formations.

Fracturing appears to be preferentially upward towards the Mission Canyon interval on the eastern flank of the Basin. The Lodgepole on this flank of the basin must have a lower "fracture gradient" than the Three Forks. On the western flank of the basin, where the Lodgepole becomes siliceous and thicker, fracturing evidently occurs downward through the Three Forks to the Nisku. Fracturing in the Bakken shale and siltstone members also takes place in areas where Bakken is highly stressed, as in the zone of strong "bending" or "curvature" at Antelope Field.