SEDIMENTARY FACIES ARCHITECTURE OF THE UPPER ORDOVICIAN RED RIVER FORMATION, NORTH DAKOTA

The Red River Formation is a prominent producer of hydrocarbons in the Williston Basin, and is the second most important hydrocarbon producing formation in North Dakota. The upper, oil-rich part of the formation is usually subdivided into four units, annotated as “A,” “B,” “C,” and “D,” from the shallowest to deepest, respectively. The formation is a mixed carbonate-evaporite, with tight limestones and dolomites being the reservoirs; these lithologies are relatively easy to recognize on combined porosity-neutron density wireline logs.

In this study we used the wireline log data available from the North Dakota Geological Survey to construct structural and isopach maps of the Red River Formation and its confining facies/horizons in North Dakota. For each well, the following data were recorded in Microsoft Excell®: county, NDIC file number, elevation, latitude, longitude, top Red River, top “A” anhydrite, bottom “A” anhydrite, top “B” anhydrite, top “B” laminite, top “B” burrowed, top “C” anhydrite, top “C” laminite, top “C” burrowed, and bottom Red River. The tops and bottoms of individual horizons were picked based on combined neutron porosity-bulk density logs and gamma-ray logs; additional control was obtained by comparison with 25 logged cores. Isopach and structure maps were constructed in Surfer®, and include Red River Formation, “C” interval, “C” interval burrowed member, “C” interval laminated member, “C” interval anhydrite, “B” interval, “B” interval burrowed member, “B” interval laminated member, “B” interval anhydrite, “A” interval, and “A” interval anhydrite. Isopach maps indicate a number of minor structures; they also clearly show eastwards thinning of the formation and its confining members. The formation reaches its maximum thickness of slightly over 214 meters in Dunn County; the maximum thickness of anhydrite horizons with laminated dolomite interbeds increases from 4 meters (“A” anhydrite) to 9 meters (“B” anhydrite) and 16 meters (“C” anhydrite). The occurrence of evaporites and their argillaceous shale caps (visible on gamma-ray logs only) is used to evaluate local paleoclimate, and examine basin-marginal versus basin-central origin of evaporites.