Stratigraphy and palynomorphs composition of the Woodford Shale in the Wyche Farm Shale Pit, Pontotoc County, Oklahoma

Abstract

This work presents a detailed lithofacies description and sequence stratigraphy model for the fine grained rocks cored at the Wyche-I well, in the Lawrence Uplift, Pontotoc County, Oklahoma. The detailed core description, thin sections, XRD mineralogy, geochemical analysis and wire-line log data have led to the identification of nine lithofacies: siliceous laminate and finely laminated mudstones (lithofacies A and B), and calcareous mudstones (lithofacies C) are associated with the Mississippian "Pre-Welden Shale" at 0 to 24 m (0-80 ft); argillaceous mudstones with phosphate and pyritic nodules (lithofacies D and E), are generally identified within the upper Woodford interval at 24 to 37 m (80-122 ft). The organically richest shales (lithofacies Fl and F2) are associated with the middle Woodford at 37 to 53 m (122-174 ft); and siliceous/argillaceous mudstone with thin clay laminations (lithofacies G) and argillaceous siltstone with abundant angular detrital quartz (lithofacies H) are commonly related to the lower Woodford at 53 to 65 m (174-212 ft). The Woodford Shale in the Wyche-I well is present between two unconformities which are associated with 2nd order sequence stratigraphy boundaries. The highest Gamma Ray (GR) log values and organically richest deposits in the middle Woodford comprise a 2nd order Condensed Section (CS), limited at the top by a maximum flooding surface (mfs). The GR log transgressive and regressive patterns, the palynological assemblages (Pollen Index) and the Ti/Al and Si/Al ratios illustrate that the lower and middle Woodford correspond to 2nd order Transgressive System Tract(TST) deposits and the upper Woodford to 2nd order Highstand System Tract (HST) deposits. The lithofacies stacking patterns in the Woodford and "Pre Welden" Shales recognized during the detailed core lithofacies description were coupled with GR log variations and grouped into 3rd order cycles. These cycles are bounded by marine­-flooding surfaces, characterized by high GR values (kick) and turnaround points of the lithostratigraphic stacking patterns. The lower and middle Woodford contain together five 3rd order cycles and the upper Woodford contains six 3rd order cycles. The 3rd order cycles were also subdivided into ductile, organic-rich transgressive hemicycles and brittle, quartz-rich regressive hemicycles. The hypothetical best brittle intervals for "fracking" are related to middle Woodford 3rd order regressive hemicycles, characterized by a high biogenic quartz content, high acoustic (P-wave) impedances, high anisotropy, high Young's modulus and low Poisson's ratio. Finally, this work also suggests that erosion and paleogeography features during the early Mississippian, possibly affected the Woodford Shale thickness and the distribution of the Woodford play in the Arkoma Basin. The Woodford Shale is thicker toward the southwest, but this trend is interrupted in the center of the basin, where the Mississippian sequence overlying the Woodford Shale is thicker and where the Misener-Woodford sequence unconformably truncates the underlying Sylvan Shale and older units.