Analysis of the Late Devonian to Early Carboniferous (Frasnian-Tournaisian) Woodford Shale in the KGS-OGS Current #1, Southwestern Arkoma Basian and Comparison to Northern Shelf

Lynn Watney
Kansas Geological Society

A continuous 540 ft long wireline slim hole core of Late Devonian to Early Carboniferous shale was acquired in the OGS-KGS Current #1 borehole in 2008. Site is located in the southwestern portion of the Arkoma Basin on a well defined structural block called the Lawrence Uplift in south-central Oklahoma. The corehole encountered 230 ft of Late Devonian-Early Carboniferous Woodford Shale, 20 ft of Osag-eanWeldon Limestone and Kinderhookian pre-Weldon Shale, and 290 ft of Meramecian and Chesterian Caney Shale. The succession consists of basinal shales, siltstone, and minor limestones. The rela-tively shallow depth of the coring location has minimized diagenesis and the Woodford Shale lacks extensive silificification. The corehole was taken to serve as a regional reference section to be tied to global stratotypes using chronostratigraphic methods to develop a robust, process-based understanding of the strata.

Biostratigraphic, petrophysical, geochemical, and sequence strati-graphic information were derived from the core and borehole wireline logs to infer controls on deposition. Primary lithofacies were de-scribed from slabbed core supplemented by SEM and thin-section petrography. High resolution (0.1 ft sampling) slim hole logging tools including spectral gamma, neutron-density porosity, and resistivity logs were used with the core description to develop a provisional sequence stratigraphy.

Meter- to decimeter-scale depositional sequences and parase-quences are defined comprising oxic (high Th/U ratio) to euxinic con-ditions (low Th/U ratio). Sequence boundaries are sharply defined by scoured surfaces with basal beds including phosphate nodule lags and lenticular and often cross-laminated detrital siltstone with abrupt lower contacts. Without lateral, context the distinction of forced re-gression or simply scouring by bottom currents is often unclear. Sedi-ment condensation is characterized by organic- and uranium-rich, phosphatic and pyritic hard dark-gray shales with pelagic fauna domi-nated by radiolarians. Condensed intervals are associated low resis-tivity and lower density. The condensed section is overlain by less organic-rich green or gray claystone or silty claystone containing ben-thic fauna, trace fossils, pellets, and minor glauconite. The regres-sive lithofacies is characterized by high Th/U ratios, attributed to more alumina-rich terrestrial clays. Oxic claystones in the Chesterian Caney Shale are notably soft and bioturbated. Siltier clays in the Woodford are hard and exhibit discontinuous microfractures. Abrupt contacts between condensed and regressive lithofacies in a rhythmic succession comprise the upper Chesterian Caney Shale, clearly suggest forced regression. Wavelet analysis of the Woodford Shale using of the natural gamma log indicates four bundles of ~100 ft scale cycles comprised of shorter wavelength (~12 ft) cycles. This is contrasted with distinct 6 to 12 ft long cycles of the Chesterian inter-val. These cycles in general reflect alternating current energy, fluctu-ating levels of oxygen and biotic activity and changing bottom- and pore-water chemistry. It has yet to be determined what the broader significance is of the apparent cyclicity in the Woodford Shale.

AAPG Search and Discovery Article #90184 © AAPG Woodford Shale Forum, Oklahoma City, Oklahoma, April 11, 2013