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Outcrop-Based Correlation Of Magnetic Susceptibility With Spectral Gamma-Ray Spectrometry In The Woodford Shale Of South-Central Oklahoma

Aufill, Michael Gordon
In the present study, we quantify the correspondence between magnetic susceptibility (MS) and spectral gamma-ray measurements (SGR) in measured sections of the Woodford Shale. Paired MS/SGR measurements were made at 15 cm intervals. Shale samples were compositionally analyzed by inductively coupled plasma-mass spectrometry (ICP-MS). Iron/aluminum ratios (Feppm/Alppm) from the ICP-MS data were compared to accepted shale standards in order to estimate the relative proportions of detrital iron and authigenic iron. The data suggest that lithology exerts primary control on MS and SGR. Moreover, MS from some Woodford Shale sections varies with SGR. Fe/Al ratios calculated from ICP-MS suggest that Fe in the Woodford Shale is predominantly authigenic. The concomitant occurrence of diagenetic FeS2 (pyrite) nodules, MS/SGR maxima, uranium concentrations over 100 ppm, and large quantities of dispersed pyrite appears to be common in distal anoxic environments characteristic of petroleum source rocks where Fe oxides are chemically unstable.