WITHIN- AND BETWEEN-MEMBER LITHOLOGICAL AND GEOCHEMICAL VARIATIONS IN THE WOODFORD SHALE IN SOUTHERN OKLAHOMA

The Woodford Shale in southern Oklahoma is an important petroleum source rock above the cutoff value (0.5 %) for the total organic carbon (TOC) for unconventional play. Most of the previous studies that evaluated Woodford Shale mineralogy treated it as a single unit, but much variability exists within and between the members. The field component of this research delineated Woodford Shale outcrops in Murray and Carter Counties, Oklahoma into three units based on high-resolution lithological data and gamma ray results. Vertical thickness, bed orientations, subtle lithofacies changes, thinning and thickening of the units, and the ratio of shale to other lithologies were documented in the form of detailed stratigraphic sections and gamma-ray logs. The results were also incorporated with the observations from previously studied Woodford Shale sections in this area to define intervals between the members of this formation. Grab samples collected at 0.25-meter intervals vertically were used to assign each unit to one of six kinds based on the thickness of beds, amount of TOC, ratio of siliceous to argillaceous minerals, and stiffness of rocks. Samples were further processed in the laboratory to develop a multi-proxy approach to geochemical analysis. Loss-in-ignition (LOI) was conducted to evaluate source rock in terms of TOC and carbonate content. Samples were powdered and their bulk mineralogy was determined by X-ray diffractometry (XRD). For clay mineralogy, the fines from the powder were separated, then air-dried, and treated with glycol. The mineralogy in each sample was determined by comparing the spectrogram results with standard spectrograms. X-ray fluorescence (XRF) was conducted to determine major and trace elemental data which were later used in two different ways. First, observed elemental data were analyzed by principal component analysis (PCA) to define the data structures and assemblages in each unit. Second, mineralogical data from XRD were used as a reference to develop an XRF mineralogical model through normalized approaches. Integrated results were used to establish chemical stratigraphy and geological interpretations in each member of the Woodford Shale.