Origin and Geometry of Red River Dolomite Reservoirs, Western Williston Basin

ABSTRACT Remarkably uniform distribution of limestone, laminated dolomite, and anhydrite as determined from compensated neutron-density logs suggests that the entire Ordovician Red River Formation of the central Williston basin was deposited in subtidal “brining-upward” sequences. Study of cores and thin sections verifies this interpretation and reveals that the Red River “B” and “C” zones consist of, from bottom to top: (1) highly burrowed locally dolomitized fossiliferous wackestones and packstones, (2) laminated to evenly bedded unfossiliferous mudstones (dolomitized in many wells), and (3) bedded anhydrite. No evidence of subaerial exposure was observed in these rock units. Dolomitization in the Red River “C” zone is highly localized. An empirical study of dolomite distribution using data from well logs reveals the presence of dozens of pods of dolomite immediately beneath the “C” anhydrite. The pods are up to 200 ft (60 m) thick and 1 mi (1.6 km) in diameter and consist of concentered lenses of (1) tight (locally anhydritic) cryptocrystalline dolomite up to 40 ft (12.2 m) thick and 3,300 ft (1,000 m) in diameter, (2) fine to medium-grained porous dolomite that forms the reservoirs, and (3), still farther from the cryptocrystalline dolomite, relatively tight partly dolomitized limestones. These dolomite pods formed by the subtidal downward migration of magnesium-rich brines. The brines formed during or immediately after precipitation of the subtidal anhydrite (probably originally gypsum). Brine migration was localized by “holes” in the anhydrite and/or upwelling compaction water from the sediments beneath the anhydrite. The “holes” probably formed by fracturing, minor faulting, or expulsion of water during compaction. Where the magnesium-rich brines first entered the carbonate sediments, they formed cryptocrystalline dolomite but as they migrated away from the “holes” and lost their potency, they produced porous dolomite (the nature of which was partly controlled by the host sediment) and, still farther away, partly dolomitized limestones. Because certain layers of the “C” zone carbonates were quite permeable to the dolomitizing brines, highly irregular reservoirs characterized by interfingering limestones and dolomites formed. Several oil fields in the western Williston basin could have been found through application of this model and others could be more efficiently developed.