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The Upper Birdbear Formation (Nisku) of Western North Dakota: Another Emerging Williston Basin Horizontal Play

Jay T. Sperr, Bill Barrett Corporation, 1099 18th Street Ste 2300, Denver, CO, phone: 303-312-8534, fax: 303-291-0368, [email protected] and Randolph B. Burke, North Dakota Geological Survey, 600 E. Boulevard Ave, Bismarck, ND 58505-0840.

A two to three foot thick dolomite reservoir in the upper portion of the Devonian Birdbear Formation has become a target for horizontal development drilling in Golden Valley and Billings Counties, North Dakota. This reservoir may cover over three hundred square miles and may yield reserves in excess of 45 million barrels of oil. Oil is trapped by anhydrite occlusion of porosity updip along the subcrop. Downdip of the oil column, the reservoir is wet except on some closures. Reservoir depths vary from 10,400 to 10,800 feet.

The upper Birdbear Formation consists of three upward-shallowing carbonate-cycles; the middle cycle is the drilling target. The middle cycle starts with a one foot thick black shale which is overlain by a one to three foot thick very-porous dolomite that is capped by two to three feet of tight anhydrite and dolomite. The dolomite has porosities in excess of 25% and oil saturations greater than 75%. Log analysis is confounded by thin bed effects on tool resolution in the varied lithologies. The reservoir is relatively continuous, but varies in thickness and quality.

Horizontal wells have initial rates that exceed 500 barrels of oil per day and may ultimately produce in excess of 300 thousand barrels of oil per well. Vertical wells were discouraging because of low initial production rates and small ultimate reserves. Current practice is to drill two opposing laterals from a single vertical borehole, spaced upon 1280 acres, at a cost of about $2 million.