--> Microbially Influenced Waulsortian Mound Reservoirs in the Lower Mississippian (Tournasian) Lodgepole Formation, Dickinson Field Complex, Williston Basin, North Dakota, USA

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Microbially Influenced Waulsortian Mound Reservoirs in the Lower Mississippian (Tournasian) Lodgepole Formation, Dickinson Field Complex, Williston Basin, North Dakota, USA

Abstract

Waulsortian mounds in the Lodgepole Formation of the Williston basin and Montana trough occur in both outcrop and the subsurface. They are Type 1 buildups consisting mainly of fenestrate bryozoan cementstones and peloidal muds. Skeletal wackestones, packstones, and grainstones are common as interbeds. Other skeletal components include crinoids, ostracods, forams, and sponge spicules. Centimeter-scale stromatactis vugs that may contribute significantly to total reservoir porosity are characteristic of the mounds. The stromatactis cavities have radiaxial calcite linings (commonly on fenestrate bryozoan sheets), geopetal peloids, cavity-dwelling microfossils that are absent from coeval, level-bottom beds, and internally resedimented peloids of multiple origins. The microbial signature within the mounds consists of small peloids that commonly form a clotted texture and are endemic, in contrast to larger peloids found locally within the mounds and in coeval, off-mound sediments. Dickinson mounds exhibit normal Mississippian carbon isotopic signatures and mound fossils exhibit high species richness and moderate dominance and are not chemoautotrophic. The mounds appear to have grown below wave base, and neither shallowing-upward trends nor evidence of subaerial exposure have been recognized in the mound succession. At Dickinson, the Lodgepole Formation ranges upwards of 300 m thick and the mounds are located within the oldest two of three 3rd-order (2+ My each) Lodgepole sequences that form part of the transgressive phase of the 16 My Madison Group 2nd-order sequence. The Dickinson mounds appear to have nucleated on a subtle paleohigh situated some 80 km basinward of the toe-of-slope (TOS), unlike mounds elsewhere that typically grew within a few kms of the TOS. Individual Dickinson mounds are typically 100 m thick. As inferred from seismic data, the smallest mounds, which are 800 m in diameter, coalesced to form circular and loaf-shaped complexes as large as 2300 m by 7500 m. Porosity within the mounds is dominated by fractures and diagenetically enhanced depositional porosity that includes stromatactis vugs and interparticle porosity in grainstones. Fractures appear to have formed both syn- and post-depositionally. Average porosity in the mounds is 5%, and reservoir permeability ranges from approximately 200 to 2000 md. Waulsortian mounds and closely associated facies in the Dickinson field complex had approximately 104 MMBOOIP at the time of discovery in 1993.