Microfacies analysis, employing thin section petrography coupled with X-ray diffraction (XRD) analysis, and implemented in conjunction with computed tomography (CT) scans provides a methodology for characterizing mudrock heterogeneity. This analysis is a primary component of an integrated method that captures lateral, temporal, textural and compositional variability. It allows for documentation of variations in composition, texture, lithofacies and of organic-rich intervals and enables the development of basin-specific models.
This workflow was applied in order to characterize the lithologic heterogeneity of the gas- and liquids-rich, (Upper Devonian) Woodford Shale fairway in the Anadarko Basin in western Oklahoma. Continuous cores from three vertical wells represent a proximal to distal transect across the Anadarko Basin. Organic matter (OM)-rich, biogenic accumulations that dominate the major mudrock lithofacies are characterized by 3 major groups: entactiniid radiolaria (siliceous zooplankton), Tasmanites (marine algae) and benthic agglutinated foraminifera. These variations reflect varying hydrodynamic and depositional processes including changing sea level, wave base position, and circulation intensity. Ichnofacies distributions indicate general dysoxic settings reflecting vertical oxygen-level gradients prevailed at the sediment-water interface, with lateral oxygenation gradients across the basin. When combined with CT scan interpretations of the cores, the variations in type, abundance and preservation of these biogenic components, rock textures and associated ichnofauna reveal significant local and regional variations in depositional settings.
Additional lithofacies with limited lateral distributions reflect the influx of extrabasinal sediments, localized changes in current intensity and diagenetic effects. Although distributed basin wide, clay-dominated laminasets (claystones) are rare, only millimeters thick, and most likely, were the result of 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. The addition of extrabasinal sediments negatively impacts (by dilution) OM concentrations. OM-rich, biosiliceous deposits with lowest detrital (extrabasinal) silt contents were deposited during times of maximum flooding and provide optimal targets for exploiting this resource.