Commonly the thermal maturity of the Late Devonian to Early Mississippian Woodford shale found on the flanks of the Arbuckle Mountains of Oklahoma is determined by vitrinite reflectance, values ranging from 0.3-1.5%. Using phyllosilicate minerals, specifically diagenetic mixed layer illite/smectite and diagenetic illite, an understanding of the extent and processes leading to the thermal maturation may be developed. Analysis by XRD of the clay mineralogy of the Woodford shale found kaolinite and mixed layer illite/smectite with <5% smectite and R≥3 stacking order. Modeling of the Woodford shale also suggests the percentage of smectite present in mixed layer illite/smectite to be <5% and commonly <2.5%. Deconvolution of the illite (001) peak supports the low smectite content and high illite crystallinity. The long range ordered illite, R≥3, and high illite crystallinity values are indicative of diagenesis to anchizone conditions suggesting a higher thermal maturity relative to previously measured values of vitrinite reflectance.