Depositional History, Dolomitization, and Reservoir Facies Development, Red River C Zone, Sheridan County, Montana

Mark G. Kittridge

The Ordovician Red River Formation C zone is present throughout the Williston basin of North America and is both oil and gas productive. In Sheridan County, Montana, the Red River C zone consists of, from bottom to top, a burrowed, variably dolomitic, fossiliferous mudstone-wackestone; a laminated to bedded, generally nonfossiliferous dolomitic mudstone; and a nodular to enterolithic anhydrite. The burrowed member contains abundant skeletal remains of corals, bryozoans, brachiopods, and echinoderms. The laminated member contains intraclasts in several locations. Thin interbeds of dolomite are found in the anhydrite member.

The C burrowed member was deposited subtidally in warm, normal marine salinity waters. The C laminated member was formed in the intertidal to supratidal(?) zone and was deposited across the study area as sabkha conditions prograded during basin filling. The C anhydrite formed as part of a coastal sabkha environment and blanketed the area as the sabkha prograded basinward during basin filling. Dolomitization of the C laminated member occurred penecontemporaneously and secondarily in the C burrowed member. The C cycle sediment was dolomitized in a sabkha setting by evaporative pumping. The C burrowed member was preferentially dolomitized within the burrows because permeability was higher within the burrows than in the surrounding matrix.

Regional dip of the Red River C zone is to the southeast, and the unit thins slightly to the west in the direction of the basin edge. Porosity development in the C laminated and burrowed members is highly variable between closely spaced wells. Both structural and combination structural-stratigraphic traps exist within the study area.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.