“Unconventional” petroleum reservoirs represent important, but still enigmatic, depositional systems. In this paper, Egenhoff and Fishman describe and interpret organic-rich mudstone of the upper member of the Upper Devonian-Lower Mississippian Bakken Formation in the intracratonic Williston basin of North Dakota. Detailed sedimentological characterization including ichnology and analysis of TOC reveals several facies associations, and facilitate a conceptual model for processes governing the deposition of these strata. The results show spatial and temporal changes in facies associations dominated by bed-load transport, and ichnofossil characteristics that demonstrate a distinct gradient from higher to lower diversities towards the basin center. These data are interpreted reflect deposition of upper Bakken shales in a dysoxic environment, on a highly differentiated deep shelf that was still influenced by storm-induced currents. This interpretation that the basin was dysoxic (not anoxic) challenges the conventional model of basins in which many unconventional reservoirs occur, and may be applicable to other black shale units that form world-class source rocks worldwide.
Traces in the dark—sedimentary processes and facies gradients in the Upper Devonian–Lower Mississippian upper shale member of the Bakken Formation, Williston Basin, North Dakota, U.S.A. by Sven O. Egenhoff and Neil S. Fishman