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Breaking into Bakken Potential on the Fort Peck Reservation, Northeastern Montana

MONSON, LAWRENCE M., and DUANE F. LUND,* Minerals Resource, Fort Peck Tribes, Polar, MT

Necessary ingredients for a Bakken play involve overpressuring, thermal maturity, stratigraphic thinning, hydrocarbon content, and the presence of fractures to free the oil.

Bakken thickness varies on the reservation from 0 to 25 m. The Upper Shale Member is uniformly 3-4 m. Thickness is 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 general salt solution edge of the Devonian Prairie Evaporites. The Bakken is about 16 m thick along this dissolution boundary and may contain the necessary fracturing. Structural flexure near the Wolf

Creek Nose, and especially off the northeast and eastern flanks of the Poplar dome, may have suitably fractured the Bakken as well. Well logs in this area have good resistivity separation in the Middle Siltstone Member of the Bakken, which may be used to detect fracturing in this low-porosity reservoir.

Present depth of the Bakken varies from about 2100 to 3050 m. Electrical resistivities indicate, however, that much of the reservation's Bakken was subjected to sufficient depths to generate hydrocarbons. Other physical properties, based on porosity and gamma-ray logs, confirm that organic carbon content is adequate, if not exceptionally high. Regional and Laramide uplift, coupled with glacial erosion and rebound, probably explain the present elevation of the Bakken in this area. Significant overpressuring exists in the Bakken over at least half of the reservation as determined by sonic-log calculations and sparse drill-stem test pressures.

 

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)