Mineral and Organic Carbon Heterogeneity in the Upper Devonian-Lower Mississippian Woodford Shale, Anadarko Basin, Oklahoma—Implications for a World-Class Shale-Gas System
Increasing focus on “shale-gas” systems in North America as prolific, unconventional (continuous) gas reservoirs has sparked interest in the production potential of the Upper Devonian-Lower Mississippian Woodford Shale, Anadarko Basin, Oklahoma. Our detailed investigation of the mineralogy (bulk rock and clay) is integrated with a companion geochemical study of organic matter (total organic carbon (TOC) and Rock-Eval pyrolysis) in fresh outcrop samples of the formation that were collected from locations previously characterized by detailed stratigraphic and spectral gamma surveys. This work helps to 1) establish the nature of Woodford mineralogy (stratigraphically), and 2) document any possible mineralogic controls on source and (or) reservoir rock properties of this potentially important shale-gas system.
Preliminary quantitative X-ray diffraction analyses of the outcrop samples reveal varying weight percentages (wt %) of quartz, illitic clay, pyrite (unaltered), potassium feldspar (K-spar), dolomite, ankerite, kaolinite, apatite, and marcasite. Clay-rich, finely laminated, black fissile shale intervals (38-81 wt % quartz, 15-40 wt% clay minerals) contain an average of 13.3% TOC with mean hydrogen index (HI) values of 642 mg hydrocarbon/g (HC/g TOC), whereas finely laminated siliceous (chert) intervals (>90 wt % quartz, <5 wt % clay minerals) are leaner, with an average TOC of 4.5%, and a mean HI of 606 HC/g TOC. There is similarity among HI values of the different lithologies (shale vs. chert) that may indicate a common source of organic matter. Importantly, illitic clay content correlates positively with TOC contents, whereas quartz content correlates negatively with TOC, illitic clay, K-spar, and kaolinite. K-spar correlates positively with kaolinite and illitic clay.
The positive correlation between TOC and illitic clay points to the possibility that TOC content was controlled by illitic clay (sorption of organic compounds on clay surfaces during deposition?). Conversely, the negative correlation between quartz and TOC indicates that highly siliceous intervals may not be prone to host high TOC accumulations, possibly because organic matter was diluted by an increase in (biogenic?) silica accumulation during deposition of cherty intervals. Nevertheless, organic/clay-rich (ductile) and quartz-rich (brittle) intervals together are likely to be necessary components in any attempt to successfully produce gas from the Woodford.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009