The Origin of the Magnetic Susceptibility Signal in the Woodford Shale, Southern Oklahoma

The Upper Devonian-Mississippian Woodford Shale is a petroleum source rock and a gas reservoir in southern Oklahoma. This study uses geochemical, p X-ray diffraction, petrographic, and rock magnetic data to investigate the origin of the magnetic susceptibility (MS) signal and to understand better the diagenetic history of the Woodford Shale, Arbuckle Mountains, Oklahoma. The MS in the Woodford shows considerable variability vertically with fissile shales having the highest values and siliceous intervals the lowest values. Rock magnetic studies indicate that the MS is dominantly controlled by paramagnetic minerals. Hysteresis properties, as well as analysis of demagnetization characteristics, the presence of the verwey transition in low temperature measurements, and analysis of isothermal remanent magnetization- cumulative log-Gaussian curves, indicates that magnetite is present in some intervals. Paleomagnetic results indicate that the magnetite contains a Late Paleozoic chemical remanent magnetization acquired during burial diagenetic processes such as maturation of organic matter and/or clay diagenesis. Contributors to the paramagnetic signal include diagenetic illite, pyrite, and ferroan dolomite. X-ray diffraction analysis indicates that illite is present and a preliminary Kubler Index measurement of the < 1 ƒÝm fraction shows the presence of diagenetic illite. Pyrite is abundant in some intervals and could locally be a contributor to the MS. Zoned ferroan dolomite is present in some intervals and also contributes to the MS signal. The dolomite has depleted stable C isotope values relative to marine values, suggesting that organic carbon was incorporated into the dolomite when it formed. The results of this study suggest that MS signal in the Woodford resides predominantly in diagenetic phases.