ABSTRACT

Recent horizontal drilling in the fractured Bakken Shale has focused attention on this unconventional reservoir of the Williston Basin. Production from the 185 vertical completions as of early 1990 was about 19 million barrels.

Production from the Bakken occurs in five main types of reservoirs. The first type occurs along the depositional edge of the Bakken shales where the thinner section of shale is more likely to be fractured, as on the Billings Nose in North Dakota. Secondly, tensional fracturing can occur at drapes over basement structures, such as at Antelope field, or over the solution edge of the Devonian Prairie Evaporite. Third, the shale may be fractured along regional lineaments. Fourth, overpressuring causes local hot spots where the fracturing may provide reservoir quality sufficient for economic production. Finally, conventional sandstone reservoirs may occur in the middle member, as at McGregor field.

Regional mapping of log characteristics can be used to define prospects for Bakken production. Meissner (1978) has shown that resistivity logs might be used to define the area of mature Bakken shale, and it appears that they may also define areas of oil-filled fractures. Sonic and density logs define areas of overpressuring. A combination of mud logs, drillstem test shows, and the correct geological models, can be used to prospect for Bakken oil fields beyond the present producing area.