Depositional environments and diagenesis of the lower Duperow Formation (Devonian), Billings Anticline, North Dakota

The Duperow Formation is an Upper Devonian (Frasnian) subsurface unit in the Williston Basin in the north-central United States and south-central Canada. Despite its economic importance for hydrocarbons, no detailed accounts have been published on the Duperow Formation that integrate depositional environments, lithology, diagenesis, and pore studies on either a regional or local scale. It was the purpose of this study to construct a model of the depositional environments and diagenesis of the lower Duperow Formation in the Billings Anticline of west-central North Dakota, based on detailed analysis of 17 cores (225 m, 739 ft) and associated wireline logs and well files. The primary study area comprised nine townships (approximately 840 sq km or 324 sq mi) and included 11 oil fields and a portion of another. Analytical methods included macroscopic examination of cores and examination of thin sections using a petrographic microscope and a scanning electron microscope (SEM). Insoluble residues were analyzed using SEM, microprobe, and X-ray diffractometry systems. Eight lithotypes were defined in the Duperow Formation on the basis of mineralogy, sedimentary structures, types of constituents, and textural relations: 1) lithoclastic, intraclastic floatstone; 2) burrowed brachiopod wackestone; 3) stromatoporoi d, coral floatstone; 4) ostracode, peloidal grainstone; 5) poorly foss i liferous peloidal wackestone to packstone; 6) mudstone; 7) laminated to disturbed-bedded anhydrite; and 8) argillaceous, siliciclastic dolomudstone. These lithotypes represent deposits formed in three environments: 1) marine sublittoral; 2) metahaline to hypersaline perilittoral; and 3) evaporite-basin supralittoral. Cementation and replacement were the major di agenetic processes that affected lower Duperow Formation rocks. Calcite, primarily in the form of micrite and microspar, is the primary cement, with lesser amounts of anhydrite, dolomite, and quartz. Thorough dolomitization produced large volumes of intercrystalline porosity. The dolomitized lithotypes form reservoirs that trap economic volumes of hydrocarbons. Circumstantial evidence suggests that calcite cement formed at its present depth of approximately 11,000 ft (3,353 m), and that the replacement dolomitization that forms reservoirs for hydrocarbons occurred during, or before, Mesozoic time