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North Dakota Geological Survey, Bismarck, ND

Abstract: Mississippian Sherwood Lagoonal Reservoir, Lucky Mound Field, McLean Co., North Dakota

Lucky Mound Field has produced over 1.4 million barrels of oil from finely dolomitized mudstones and peloidal packstones of the Sherwood interval of the Mission Canyon Formation. Dolomitization of the fine-grain lagoonal deposits creates homogeneous porosity and permeability in the upper Sherwood. These reservoir rocks differ from the majority of Sherwood reservoirs which produce from coarse pisolitic grainstones or dolomitized open marine wackestones.

Lucky Mound reservoir appears to be compartmentalized on more than one scale: 1) on the field scale, the upper Sherwood is separated from the lower Sherwood; and 2) on the localized scale, within the grainstone shoal facies of the lower Sherwood. The Upper Sherwood is separated from the lower Sherwood by subaerial exposure surfaces that are tightly cemented where they developed on the tops of islands built on shoals. Off the island crests the exposure surfaces are more variably cemented. Production data suggests that this exposure surface is sufficiently tight to essentially segregate the field into two reservoirs one in the upper Sherwood and one in the lower. Subaerial exposure of islands locally produced karst features including solution cavities and collapse breccias. Horizontal solution channels in the lower Sherwood larger than the 4 inch core could complicate enhanced recovery efforts by water flooding by channelizing the flow and reducing sweep efficiency. No karst features were seen in the upper Sherwood carbonates which grade upward into anhydrite. The anhydrite serves as a seal providing an updip trap.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana