Geology and Hydrocarbon Potential of Dawson Bay Formation Carbonate Unit (Middle Devonian), Williston Basin, North Dakota

Wayne Pound

The Middle Devonian Dawson Bay Formation carbonate unit is present in the subsurface of North Dakota except where truncated by postdepositional 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 3,798 m (12,460 ft) below the surface in McKenzie County. In North Dakota, a submarine hard-ground 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.

Lithofacies analysis of the Dawson Bay carbonates suggests a shallowing-upward succession of depositional environments and associated energy zones as follows: shallow epeiric 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 stromatoporid buildups, multiple-fracturing events over topographic and structural highs, and multiple-dolomitization events.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988