Date of Award
Master of Science (MS)
The Middle Devonian Dawson Bay Formation carbonate unit is present in the subsurface of North Dakota except where truncated by post-depositional erosion. The carbonate unit thickens from the erosional limit to a maximum thickness of 47.5 m (156 ft.) in Renville County and reaches a maximum depth of 3798 m (12, 432 ft.) below the surface in McKenzie County. In North Dakota, a submarine hardground separates the carbonate unit from the underlying “Second Red Bed member” of the Dawson Bay Formation. The upper contact with the Souris River Formation is conformable except in those areas where the Dawson Bay Formation was exposed to subaerial erosion prior to deposition of the Souris River sediments.
The Dawson Bay carbonate unit is predominantly dolomitic and fossiliferous limestone or fossiliferous dolostone. The carbonate unit can be subdivided into five lithofacies on the basis of characteristic fossil fauna, flora, and other lithologic features. In ascending order, the carbonate lithofacies are: lithofacies B (brachiopod-echinoderm wackestones and mudstones), lithofacies C (stromatoporoid boundstones, wackestones, and mudstones), lithofacies D (gastropod-ostracod-brachiopod-echinoderm wackestones and mudstones), and lithofacies F (cryptalgal boundstone and mudstones).
Lithofacies analysis of the Dawson Bay carbonates suggests a shallowing-upward succession of depositional environments and associated energy zones as follows: shallow epieric sea (very low energy), stromatoporoid biostrome/bioherm (low energy), very shallow epeiric sea (very low energy), restricted shallow epeiric sea (extremely low energy), and shallow epeiric sea shoreline (variable energy).
Eogenetic diagenesis includes: color-mottling, dolomitization of micrite to microcrystalline dolomite with penecontemporaneous anhydrite replacement of cryptalgal mudstones and boundstones, cementation by sparry calcite, and vuggy porosity development. Mesogenetic diagenesis includes: formation of mosaic dolomites; cementation by blocky equant calcite; neomorphism; pressure-solution; fracturing; halite cementation; and hydrocarbon emplacement.
Late mesogenetic hydrocarbon generation occurred within basal Dawson Bay carbonates and/or was the result of migration into the formation. Middle to late mesogenetic anhydrite, halite, and calcite cementation partly limits reservoir potential. Hydrocarbon occurrence in the Dawson Bay carbonates is primarily associated with porosity within the stromatoporoid buildups, multiple fracturing events over topographic and structural highs, and multiple dolomitization events.
Pound, Wayne R., "The geology and hydrocarbon potential of the Dawson Bay Formation carbonate unit (Middle Devonian), Williston Basin, North Dakota" (1985). Theses and Dissertations. 233.