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Source Rock of Woodford Tight Oil Play on the Cherokee Platform (Oklahoma)


Tight oils recently produced from the Woodford Formation on the Central-Northern Cherokee Platform (Oklahoma) are light API-gravity oils (40-45oAPI). The producing formation is the Woodford Shale of Late Devonian to Early Mississippian age (~385 Ma to ~360 Ma), an organic-rich (4~12% TOC) black shale widely distributed in the southern mid-continent with very low porosity (<1.25%) and extremely low permeability (<1mDa). All of these characteristics have led to the presumption the oils being produced in this area were generated in-situ. Based on the previous vitrinite reflectance measurements (VRo) the thermal maturity of the Woodford Shale in the study area is in the range of 0.52% to 0.58% VRo corresponding to the immature-to-early-mature stage of oil generation. A detailed geochemical study on oil samples and Woodford cores extracts has been conducted to investigate possible sources for these oils. Results obtained to date have led to the following observations and deduction: (i) oils produced close to the Nemaha Uplift (Logan and West Payne Counties) were not only Woodford sourced but also had a Mississippian source contribution based on the presence of abundant extended tricyclic terpanes and other source specific biomarker fingerprint characteristics; (ii) oils sampled from the East of the Cherokee Platform (Central-East Payne County) share strong Woodford source characteristics but were not generated in-situ from the Woodford Shale, which is not mature enough in that area but probably migrated from the Woodford Shale in the deeper Anadarko Basin. The preliminary results of this research is consistent with some new findings reported by the Devon geologists that abundant marine coarse-grained biogenic silica (radiolarian-rich chert facies) found in Woodford cores (Central-East Payne County) in this area may be a contributor to good reservoir petrophysical properties suggesting the Woodford Formation may not be the source rock in this area but simply a tight reservoir.