| dc.description.abstract | The Late Devonian – Early Mississippian Woodford Shale is a significant unconventional play in the Midcontinent region. An approximately 365ft thick exposure of the Woodford Shale is present in the McAlister Cemetery Quarry. Mudrock samples were collected every stratigraphic foot and analyzed for its characteristics using outcrop gamma-ray variations, XRF elemental compositions, XRD mineralogy, hardness variations, petrographic and SEM analysis. Eight major lithofacies were defined for the Woodford Shale section, starting from most clay-rich to most siliceous, and decreasing degree of fissility: clayshales, mixed clayshales – mudshales, argillaceous mudshales, siliceous mudshales, siliceous mudstones, and radiolarian chert. Calcareous lithofacies defined include dolomitic mudstones and dolomitic mudshales.
The technique of chemostratigraphy relies upon the fact that stratigraphic variations in geochemistry can be useful for the interpretation of the changes in minor fluctuations in variables such as facies, paleo-redox conditions, organic paleoproductivity, carbonate paleoproductivity, and depositional cyclicity. Redox-sensitive elements were used for analyzing paleoenvironmental conditions such as bottom water redox conditions, water column metal-oxyhydroxide particulate shuttles, and the degree of water mass restriction. Using high-resolution chemostratigraphy paired with hierarchical clustering analysis (HCA), the study area presents an opportunity to better understand the paleodepositional conditions and to investigate the possibility for the subdivision of the Woodford Shale into more units and to compare their geochemical character. Overall, the Woodford Shale showed stratigraphic geochemical variations associated with a decrease in the degree of basin restriction up-section.
The general sequence-stratigraphic model established for unconventional resource shales was applied to the Woodford Shale section for sequence stratigraphic interpretations. Using gamma-ray (GR) parasequences, the Lower and Middle Woodford Shale were deposited during a 2nd order transgressive cycle represented by increasing GR. The Upper Woodford Shale was deposited during a turn-around point, during a 2nd order regressive cycle. Additionally, fourteen 3rd order regressive-transgressive cycles were identified. In the Lower Woodford Shale, five 3rd order cycles were identified, four 3rd order cycles were defined in the Middle Woodford Shale, and five 3rd order cycles were defined in the Upper Woodford Shale.
Source rock evaluation demonstrates that the Woodford Shale is organic-rich. The Lower, Middle and Upper Woodford Shale possess average TOC values of 12.34 wt%, 11.2 wt%, 5.96 wt%; respectively. Type II kerogen is the most dominant (oil-prone), suggesting marine origin. Type I kerogen occurs in the Lower Woodford Shale, indicating lacustrine type deposition. Results from both calculated and measured Vitrinite reflectance shows that the Woodford Shale has low to moderate thermal maturity, and falls within the immature to early oil window. | en_US |
