An integrated approach to understanding sedimentary structures and depositional processes in Devonian-Mississippian black shale: The Woodford Shale and associated strata in the southern midcontinent

Depositional processes and redox states in the Woodford Shale were assessed critically by examining lithologic and geochemical variables in core and outcrop to facilitate efficient development of this world-class reservoir. The Woodford was deposited in a Late Devonian epicontinental sea near the southern edge of Laurussia during the Kaskaskia cratonic onlap. The Woodford consists mainly of siliceous and argillaceous mudrock. The siliceous mudrocks contain radiolaria and cryptocrystalline silica, including chert. Paleo-redox indicators show decreasing oxygen levels from the shallow shelf toward the Ouachita embayment and the ancestral Anadarko Basin. Much of the Woodford accumulated under dysoxic conditions, and anoxic conditions apparently existed for extended periods only in the deepest reaches of the shelf. Woodford sediment that accumulated near the edge of the Ouachita embayment in east-central Oklahoma contains more biogenic silica than that higher on the shelf, suggesting that silica and phosphate deposition were associated with oceanic upwelling. Ripple cross-laminae and graded beds indicate bottom currents during deposition. Soft-sediment deformation is common in the Woodford Shale, particularly along the southern margin of the Anadarko Basin. A key outcome of this research is the realization that virtually all features in the Woodford Shale can be explained in terms of the physical and geochemical processes that operate today in oxygen minimum zones.