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DOI:10.1306/08192525013
Stratiform-Dolostone Flow Units in the Williston Basin: Stratigraphic, Porosity-Permeability and Oil-Producing Relationships
David M. Petty
Ahead of Print Abstract
Oil-productive stratiform-dolostone flow units in the Williston Basin occur in Mississippian, Devonian, and Ordovician formations. These flow units share similar characteristics despite occurring at depths of 5,085 to 14,440 ft (1,550–4,400 m). Sediment deposition occurred in low-energy, epeiric-shelf settings characterized by muddy carbonate and evaporite accumulations. The stratiform-dolostone flow units of this study are mappable stratigraphic units with laterally and vertically continuous porosity that parallel fourth and fifth-order stratal surfaces and are characterized by predictable rock properties. Continuous porosity covers areas ranging from 2 to >100,000 km2 (2 to >40,000 mi2) in 0.6 to 6 m (2 to 20 ft) thick flow units. Sample porosity and air permeability vary from 6 to 39% and 0.1 to 711 md Ka. Porosity variability within individual flow units is dominantly controlled by the degree of dolomite crystal intergrowth (planar-e vs planar-s dolomite), the amount of inter-dolomite micrite, microspar and pseudospar, and the amount of vuggy porosity. The flow-unit average dolomite crystal size varies from 8 to 61 μm and is the dominant control on flow-unit porosity-permeability and oil-producing relationships. These flow units formed by dolomitization of synchronous, mud-rich stratigraphic units that now have dolomite intercrystal porosity in the matrix of mudstone, wackestone, packstone and stromatolitic boundstone. Regional porosity distribution was controlled by both precursor sediment composition and regional diagenetic patterns. The stratiform dolomitization is interpreted to have occurred during shallow burial, dominantly by magnesium-rich brines expelled downward and seaward from calcium sulfate deposits that now form capping anhydrite beds.
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