EVALUATION OF MAGNETIC SUSCEPTIBILITY RELATIVE TO SPECTRAL GAMMA-RAY RESPONSE IN THE UPPER DEVONIAN - LOWER CARBONIFEROUS WOODFORD SHALE OF SOUTH-CENTRAL OKLAHOMA

Economic interest in the occurrence and distribution of shale gas has heightened interest in tools and techniques available for the characterization and correlation of shale. One promising tool is magnetic susceptibility (MS). Previous researchers have measured the magnetic susceptibility (MS) of the Woodford Shale as a means to correlate marine strata and to define important geologic boundaries on a global basis. Likewise, gamma-ray (GR) response is commonly used for gross regional stratigraphic correlation in the Woodford Shale. MS literature reveals no instances of basin-specific shale studies employing outcrops with well-control data in the form of logs or core. In the present study, we quantify the correspondence between MS and GR at two Woodford Shale exposures and in select subsurface cores. Recent research concludes that MS response in rocks is controlled by input of detrital iron that varies with fluctuations in global sea level. Likewise, GR response in hot shales is strongly related to U sequestration under conditions of anoxia and starved sedimentation. Consequently, one might expect MS to vary inversely with GR. Preliminary results based on high-resolution paired MS and GR measurements suggest that lithology of the Woodford Shale (siliceous shale or dolomite versus fissile shale) exerts strong control on both MS and GR response. The data also suggest that some Woodford Shale contains an MS signal that varies directly with GR response. Although GR response and correspondence to lithology has been widely documented by the petrophysics community, the lack of documentation of physical and chemical conditions involved in the transportation and selective sequestration of magnetically susceptible mineral matter in shale is problematic. The facility and economy of MS measurements in shale could prove to be powerful prospecting tool when used in combination with GR.