INTEGRATION OF LITHOFACIES AND PETROPHYSICAL PROPERTIES FOR BETTER UNDERSTANDING OF THE RESERVOIR ARCHITECTURE, RED RIVER B INTERVAL, WILLISTON BASIN, NORTH DAKOTA

The Upper Ordovician Red River Formation is the second major conventional hydrocarbon reservoir in the Williston Basin, North Dakota. This study focuses on its B porosity zone that is composed of three members: burrowed skeletal limestone, dololaminite (reservoir), and anhydrite (seal). The study is based on the integration of core- and wireline-log data in order to better develop a reservoir model using a rock fabric approach coupled with porosity and permeability data from the North Dakota Industrial Commission database. Twelve continuous cores were logged and integrated with rock-fabric characteristics derived from 476 core plugs for the "B" interval of the Red River Fm. Eight facies types were identified, from deepest to the shallowest: burrowed skeletal mudstone-wackestone (“deeper” subtidal), skeletal wacke-packstone (moderately shallow subtidal), skeletal packstone-grainstone (shoal-water), thrombolite (intertidal to upper subtidal), microbial laminite (tidal flat), and anhydrite (supratidal, subaqueous). Porosity of the analyzed samples is 0.2 to 33% (average 4.8%), with intercrystalline, moldic, and separate vug being the most abundant pore types; permeability varies from 0.01 to 176 mD (average 2.93 mD). Fine to lower medium crystalline laminated dolomite displays predominantly class 3 and class 2 petrophysical relationships, with many of data plotting left of the class 1 where the burrowed skeletal mudstone-wackestone predominates, i.e., in the low porosity and low to high permeability range. Grain-dominated facies are scattered across the petrophysical class fields. The average reservoir quality index (RQI) is low (0.02) with a maximum value of 0.43 within the B burrowed member. Relationship between the facies type and RQI is positive for the laminated and thrombolytic facies. In all other facies it is much more complex, largely reflecting the dolomitization pattern.