CARBONATE-EVAPORITE FACIES IN THE “B” INTERVAL OF THE UPPER ORDOVICIAN RED RIVER FORMATION, WILLISTON BASIN, NORTH DAKOTA

The Ordovician Red River Formation of the Williston Basin, North Dakota is a major producer of oil and gas. The upper part of the formation is traditionally subdivided into 4 intervals labeled, from oldest to youngest: “D”, “C”, “B”, and “A”. The “B” interval, as defined by common industry usage, consists of three informal members: lower burrowed member, middle laminated member, and upper anhydrite member. The “B” laminated member has the most economic importance through petroleum production in the southern Williston Basin. Five cores from southwestern North Dakota in Hettinger, Williams, and Mountrail County and accompanying gamma-ray and combined neutron/density logs were examined in the study of the “B” member. The thickness of the “B” interval in these cores varies from 47 to 55 feet; the top anhydrite member ranges in thickness from 5 to 11 feet. The lower member of the Red River “B” Interval consists of a ~35 feet thick sequence dominated by burrow-mottled skeletal mudstone to wacke-packstone with echinoderms, brachiopods, and gastropods. This shallow subtidal facies grades up into skeletal packstone and grainstone (shoal), rarely floatstone, forming 2 to 15 feet thick shallowing upward cycles. The porosity concentrated in molds of the burrowed member is very low, approximately 1%. The overlying laminated member ranges in thickness from 15 to 25 feet, and is dominated by a fine crystalline dolomite, rarely limestone characterized by crinkly and flat microbial lamination. Periodic exposure of this tidal flat facies is indicated by repeated horizons with desiccation cracks, or by rare thin breccia horizons. Also, acicular crystals of anhydrite are locally found within the uppermost parts of the laminated member. The laminated member is non-fossilferous and moderately to highly porous (5-15%) with predominantly between-crystal and separate-vug porosity. The overlying anhydrite member indicates a supratidal depositional environment. It is either laminated (with alternating dolomite laminae), or nodular with stringers of dolomite between anhydrite nodules forming a chicken-wire texture. The anhydrite member caps the depositional sequence that corresponds to the “B” interval of the Red River Formation, and is overlain by the thin basal microbial laminite of the “A” interval.