ABSTRACT

The Bakken Formation is recognized as an important hydrocarbon source rock for the Williston Basin. Recent developments in horizontal drilling technology have made this unique geologic formation a principle exploration target as well. Necessary ingredients for a Bakken play involve overpressuring, thermal maturity, stratigraphic thinning, hydrocarbon content, and especially, the presence of fractures to free the oil. Tectonic breaking of the Three Forks, Bakken, and lower Lodgepole rocks is likely to occur near local and regional tectonic elements associated with Laramide, reactivated, and salt dissolution events. Other Bakken cracking happens during compaction and hydrocarbon expulsion and during erosional pressure unloading.

Bakken thickness varies on the Fort Peck Reservation from 0 to 25 m (0 to 75 ft). The Upper Shale Member is remarkably uniform, ranging from 3 to 4 m (8 to 12 ft) in most places. Stratigraphic thickness appears to be related to a northwest structural grain, especially in the northeast where fold axes are located parallel to the Opheim Syncline. This strike is coincident with the salt edge of the Devonian Prairie Evaporite. The Bakken is about 16 m (50 ft) thick along this dissolution boundary and may be fractured. Structural flexure near the Wolf Creek Nose, and especially off the northeast and eastern flanks of the Poplar Dome, where seismic profiles display steep normal faults, may have suitably fractured the Bakken, as well. Well logs in this area reveal good resistivity separation in the Middle Siltstone Member of the Bakken, and this feature may be used to detect fracturing in the low porosity reservoir rock. In addition, the lower Lodgepole has a similar electrical resistivity response on some well logs. The Three Forks is anomalously thick northeast of Poplar Dome.

Depth below ground of the Bakken varies from about 2100 to 3050 m (6500 to 9500 ft). Electrical resistivities and sonic travel times indicate that much of the Reservation's Bakken lies at depths sufficient to generate hydrocarbons. Resistivity and gamma ray logs confirm that organic carbon content is adequate, if not exceptionally high. Regional Laramide uplift, coupled with glacial erosion and rebound, probably account for the present physical state of the Bakken in this area. Overpressuring probably exists over at least half of the Reservation as determined by sonic log calculations and sparse drill stem test pressures.