Soft-sediment deformation structures as a tool to recognize synsedimentary tectonic activity in the middle member of the Bakken Formation, Williston Basin, North Dakota

Abstract

Abstract The Bakken Formation (uppermost Devonian-Lower Carboniferous) in North Dakota consists of black shales with a middle member composed of mixed siliciclastics and carbonates. Millimeter to decimeter-scale soft-sediment deformation structures (SSDS) are common in the upper third to quarter of the middle member, having been observed in sixty-three out of ninety cores studied. The nine categories identified herein reflect brittle and ductile deformation and loss of sediment shear in compressional and extensional settings. They are interpreted to form due to seismically-induced folding, liquefaction and fluidization of unconsolidated sediment, as well as microfaulting and brecciation of consolidated sediment, and reflect synsedimentary tectonic movements, e.g. along faults that were active during deposition. The distribution of SSDS follows the orientation of the major structural elements rooted in the basement of the Williston Basin. The stratigraphic and spatial distribution of SSDS suggests variable tectonic activity: cores near the most active fault systems have the highest net thickness of units with SSDS, whereas those farther from the most active faults exhibit only a few deformed units. The predominance of SSDS in the upper portion of the middle member suggests there was a change from relative tectonic quiescence to a more active regime which in turn led to modifications of basin geometry and new accommodation patterns. Synsedimentary deformation might have had a positive effect on reservoir quality in deformed units by increasing porosity and permeability as a result of cohesion loss and rearrangement of grains. Additionally, multiple injection structures and complexes of small-scale normal and reverse faults could contribute to better connectivity of reservoir units.