Lithologic Heterogeneity in the Upper Devonian Woodford Shale (Anadarko Basin, Oklahoma USA)

Spaw, Joan M.

Integrated petrologic analysis of continuous cores from 3 vertical wells representing a proximal to distal transect across the Anadarko Basin, Oklahoma was undertaken to characterize the heterogeneity of the Frasnian-Famennian (U. Devonian) Woodford Shale. Although global mechanisms of shale deposition were active in the Devonian, influences of local sedimentary processes and hydrodynamic conditions are evident.

Ten major mudrock lithofacies and 4 areally limited lithofacies are recognized based on CT (computed tomography) scan interpretations combined with thin section- and XRD- (x-ray diffraction) based microfacies analysis. Organic matter (OM)-rich, biosiliceous accumulations dominate, characterized by 3 major groups: entactiniid radiolaria, Tasmanites algae and benthic agglutinated foraminifera. Vertical and lateral variations in types, abundance and preservation of these biogenic components reveal significant local and regional variations in depositional settings.

Locally abundant, radiolaria grainstone laminasets record high energy events. Once deposited, these grainstones underwent early diagenetic alteration and commonly contain dolomite cement-filled ptygmatic veins. Although distributed basin wide, clay-dominated laminasets (claystones) are rare, only millimeters thick, and most likely, were generated by storm-generated benthic sediment clouds. Quartz- and feldspar-rich silts form very fine, upwards fining, parallel laminasets. These sediments are extrabasinal sediment influxes and occur at discrete intervals.

CT scan and thin section analyses reveal the persistent presence of biogenic structures throughout the basin. Reduced chondrites ichnofacies, characterized by short, thin vertical burrows and grazing trails are indicative of dysoxic conditions at the sediment-water interface. Changes within this ichnofacies assemblage reflect relative stages of dysoxia with the most dysoxic environment prevailing at the most distal setting.

Multiple scenarios can give rise to the production and dilution of organic-rich sediments. OM preservation is affected by OM supply and type, bottom water oxygen levels, rate of OM burial and fine sediment volume. Variations in biofacies and lithofacies affect rock textures and fabrics that appear to affect OM connectivity. The addition of extrabasinal sediments negatively impacts (by dilution) OM concentrations.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013