Little Knife Field--U.S. Williston Basin

WITTSTROM, M. D., and R. F. LINDSAY, Chevron U.S.A., Inc., Midland, TX

Little Knife field is a combination structural and stratigraphic trap located near the structural center of the Williston basin, North Dakota. The field is approximately 12 mi (19.3 km) long and 2.5 to 5.5 mi (4 to 8.9 km) wide. Little Knife was discovered by Gulf Oil in 1976 as part of a regional exploration play involving a transition from impermeable to porous carbonate rocks. In 1987, ultimate recovery from the Mission Canyon (Mississippian) reservoir was estimated to be 97.5 MMBO. This included 57.5 MMBO primary, 27 MMBO secondary, and 13 MMBO tertiary (CO) oil. At present the field is still under primary recovery, since unitization efforts have not been successful.

Approximately one-third of Little Knife's 130 ft (39.6 m) oil column is trapped by structural closure beneath a regional anhydrite seal in a north-south-trending anticline. The remaining two-thirds of the oil column is trapped where the reservoir beds change facies from porous dolostones and dolomitic limestones to nonporous limestones. Structural entrapment accounts for approximately 50% (127 MMBO) of the OOIP, but covers only 30% of the producing area.

Production is from the upper portions of the Mission Canyon Formation, a regressive, shoaling-upward carbonate-anhydrite sequence deposited in a slowly shrinking epeiric sea. The Mission Canyon in the Little Knife area is divided into six zones that record predominantly cyclic, subtidal deposition. These are overlain by prograding lagoonal, tidal flat, and sabkha beds. The source of Mission Canyon oil is thought to be the Bakken Formation, an organic-rich shale at the base of the Mississippian.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)