Lithologic and geochemical assesment of hydrocarbon producing capability of the Woodford Shale in southern Oklahoma

Scope and Method of Study. The late Devonian and early Mississippian Woodford shale was studied throughout the Cumberland syncline in Love, Johnston, Marshall, Bryan, Atoka, and Carter Counties, Oklahoma. The purpose of this study was to determine if the Woodford could be subdivided based on variations in core characteristics, wireline response, production history, vitrinite reflectance, and electron microprobe response to produce generalized subunits that can be traced through the subsurface. Furthermore the subunits were classified into either reservoir or source rock intervals within of the Woodford Shale.Findings and Conclusions. Based on the examination and interpretation of rock, petrophysical, and mapping data from the Ardmore Basin, the following conclusions are proposed. (1)The Woodford Shale is differentiated into five units with distinct lithologic characteristics and wire line log responses. (2) These subunits could be classified as either reservoir or source rock. It was determined that units containing greater amounts of shale are source rocks because they have lower permeability determined by the absence of primary or secondary fracture patterns. Within the Woodford there were two shale intervals that fall into this classification, the Black Shale subunit and the Lower Shale subunit. Reservoir rocks all demonstrated several key characteristics. They contain chert and occasionally phosphate nodules that correspond with more highly fractured intervals. The presence of chert and or phosphate decreases ductility and increase the brittleness of the shale causing it to fracture during deformation. The three subunits that were characterized as reservoirs were the Chert subunit, the Interbedded subunit, and the Lag subunit. Geochemical analysis determined that kerogens were type II, which denotes a marine source. Vitrinite reflectance measurements yielded an average value of 0.6%, which is far below the condensate gas window value of 1%.