Spectral Gamma-Ray Character of the Chattanooga Shale, Tennessee and Oklahoma

Daniel Hurst.  School of Geology, Oklahoma State University, 105 NRC, Stillwater, OK  74078, phone: (405) 744-6358, fax: (405) 744-7841  [email protected]

The economic importance of shale gas has fueled interest in the Woodford/Chattanooga interval in Oklahoma and equivalent strata in other petroleum-producing provinces. Changes in spectral gamma-ray patterns across Woodford/Chattanooga outcrops reflect changing depositional settings and water depths. In this study, a spectral gamma-ray survey was taken across the Chattanooga Shale at the Hurricane Bridge outcrop in Dekalb County, Tennessee. The Chattanooga at Hurricane Bridge contains two members: the Dowelltown Shale and Gassaway Shale. The Dowelltown rests unconformably on Ordovician Leipers carbonate. The Dowelltown is further subdivided into a lower and upper unit based on color and weathering properties. Generally, the upper unit is lighter-colored (gray vs. black) and forms a recessed weathering profile. A bentonite marks the boundary between the Dowelltown and overlying Gassaway. The Gassaway is a more resistant and massive black shale without conspicuous subdivisions. The Gassaway is subdivided into lower, middle, and upper units using subtle weathering characteristics, faunal assemblages, and phosphate nodules. The spectral gamma ray identified changes in uranium, thorium, and potassium concentrations that corresponded to the lithostratigraphic units. The darker-colored lower unit in the Dowelltown contains higher uranium concentration than the lighter-colored upper unit. Uranium concentrations increase dramatically in the Gassaway, and the three units are recognized by varying uranium concentrations. Potassium and thorium concentrations respond inversely to uranium and increase in lighter-colored (gray) shale. Similar spectral gamma-ray responses are recorded for the Chattanooga in eastern Oklahoma as darker-colored units have higher uranium concentration and lower thorium and potassium to uranium ratios.