Detailed Outcrop Gamma-Ray Characterization of the Woodford Shale, South-central Oklahoma

Alischa M. Krystyniak¹, Stanley T. Paxton², and William S. Coffey^3
1 School of Geology, Oklahoma State University, Stillwater, OK
² Oklahoma State University, Stillwater, OK
³ Devon Energy Corporation, Oklahoma City,

Detailed outcrop gamma-ray characterization of the Woodford Shale, south-central Oklahoma.

The Devonian - Mississippian Woodford Shale of Oklahoma is a hydrocarbon-prone source rock that contains a heterogeneous mix of lithofacies with marine affinities. We are currently constructing detailed lithofacies profiles and measuring the spectral gamma-ray response in outcrop. Outcrops vary in thickness from about 30 to 150 m. The intent of this work is to provide documentation for the variations in gamma-ray response relative to the different lithofacies. Lithofacies recognized to date include black, highly fissile paper shale, black siliceous shale, gray/brown shale, intervals of shale containing phosphate nodules, some chert layers, and rare limestone.

TOC, XRD, whole rock chemistry, and thin-section data are being collected from each of the lithofacies in order to more fully document the controls on gamma-ray response. Relative to potassium and thorium, uranium is responsible for the bulk of the variation in outcrop gamma-ray response. Gamma-ray readings in Carter and Murray Counties have values of uranium that range from 7 to 80 ppm. Paper shales tend to have the highest concentrations of uranium. XRD reveals that the Woodford contains a high percentage of illite and mixed-layered illite / smectite. Thin sections show that the quartz in the siliceous shale is both detrital silt and diagenetic chert. Some packages of the Woodford appear to contain alternating layers, 2-3 cm in thickness, of highly paper shale with siliceous shale. API gamma-ray values, calculated using a standard equation and plotted with the vertical lithofacies descriptions, reveal characteristic “finger-prints” for each of the lithofacies.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005