Date of Award
Master of Science (MS)
The Duperow Formation of northwestern North Dakota is a cyclical carbonate evaporite unit deposited on a shallow marine shelf during the early part of the Late Devonian (Frasnian). The formation forms the lower part of the Jefferson Group and lithostratigraphic review permits three informal members to be recognized: lower, middle, and upper. Petrographic, lithologic, and well log examination of drill cores and thin sections allows the identification of seven lithofacies within the three members. Lithofacies include; bioclastic wackestone/packstone (LF1), stromatoporoid boundstone/floatstone (LF2), Amphipora boundstone/floatstone (LF3), peloidal wackestone (LF4), dolomitic mudstone (LF5), microbially laminated dolowackestone (LF6), and anhydrite (Lf7). Lithofacies are further grouped into five lithofacies associations (LFA I to LFA V) which correspond to five depositional settings during accumulation of Duperow strata. Lithofacies were deposited within normal marine subtidal, stromatoporoid bank or mound, intertidal slightly restricted, intertidal more restricted, and supratidal conditions. Rocks of the Duperow were typically deposited in a transition from subtidal to supratidal environments, preserving several shallowing upward successions. Nine short term (1-2 million years) third order cycles are recorded by the rocks of the Duperow which represent two transgressive phases and one regressive phase.
Diagenetic features within the Duperow enhance porosity and establish units with appreciable amounts of reservoir characteristics. The main diagenetic features observed are micrite enveloping and micritization, calcite cementation, dolomitization, compaction, dissolution, and anhydrite cementation. Only dolomitization and dissolution increase porosity in the rocks of the Duperow. Dolomitization is fabric selective, as only the stromatoporoid boundstone/floatstone and microbially laminated dolowackestone lithofacies contain large amounts of dolomite and well developed intercrystalline porosity. These lithofacies are traceable and correlated throughout the entire study area and extend away from the known producing regions along the Nesson and Billings anticlines. The high degrees of dolomitization observed are the result of active fracture and fault systems acting as conduits for dolomitizing fluids. Paleostructural highs and lows allow for the migration of fluids into the highly susceptible carbonates of the Duperow Formation.
Alcorn, Zachary P., "Lithostratigraphic investigation of a Late Devonian carbonate-evaporite sequence; the Duperow Formation, Williston Basin, North Dakota" (2014). Theses and Dissertations. 1.