ABSTRACT: Exploration Models for Bakken Shale Reservoirs, Williston Basin, North Dakota

Jay Thomas Sperr

Recent horizontal drilling for fractured Bakken shale in the Williston basin has focused attention on this unconventional reservoir. Present production from the 185 vertical completions is about 19 million bbl.

Production from the Bakken occurs in five main trapping types. The first type of trap is 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. Secondly, tensional fracturing can occur at drapes over basement structures, such as at Antelope field, or in drape over the solution edge of the Prairie Evaporite. Third, the shale may be fractured along regional lineaments. Fourth are local "hot spots" where the fracturing caused by overpressuring may provide reservoir quality sufficient for economic production. Finally, conventional sandstone reservoirs may occur in the middle member, productive from primary porosity, as at McGregor field.

Regional mapping of log characteristics may help define prospects for Bakken production. Meissner 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 may also define areas of overpressuring. Combined with mudlog and drillstem test shows, and the correct geological models, prospecting for Bakken oil fields can be extended beyond the present producing area.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990