ANALYSIS OF CARBONATE-EVAPORITE FACIES AND DEPOSITIONAL ENVIRONMENTS 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 “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 and accompanying gamma-ray and combined neutron/density logs from southwestern North Dakota in Hettinger, Williams, and Mountrail County were examined. The thickness of “B” interval in these cores varies from 47 to 55 feet; the top anhydrite member is from 5 to 11 feet thick. The lower part of the Red River “B” Interval consists of ~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 in the burrowed member is very low, approximately 1% and concentrated largely in molds. The overlying laminated member (15-25 feet thick) 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 breccias horizons. Also, acicular crystals of anhydrite are locally found within the uppermost part of the laminated member. The laminated member is non-fossilferous and highly porous with predominantly between crystal and separated vug porosity (5-15%). The overlying anhydrite member indicates supratidal setting. It is either laminated (with alternating dolomite laminae), or nodular with stringers of dolomite between nodules giving chicken-wire texture; coalesced nodules usually show contortions. The anhydrite member caps the third-order 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.