PALEOMAGNETIC AND MAGNETIC FABRIC INVESTIGATION OF THE FAYETTEVILLE SHALE, AR

For this study, we have incorporated paleomagnetic, anisotropy of magnetic susceptibility (AMS), and petrographic observations to determine the magnetic fabric and constrain the timing of diagenetic events in the Fayetteville shale. The Fayetteville shale is a black, organic rich, unconventional reservoir located in northern Arkansas. Carbonate beds, composed of calcite, dolomite, ferroan dolomite, pyrite, and organic matter, in the upper Fayetteville Shale were sampled. AMS data indicate oblate, prolate, and linear fabrics. Samples with vertical prolate fabrics contain zoned ferroan dolomite. We interpret that the ferroan dolomite is causing an inverse fabric. Oblate fabrics are compactional in origin whereas the linear fabrics could be caused by paleocurrents and alignment of magnetic grains. New paleomagnetic analyses are consistent with a previous paleomagnetic study (Farzaneh et al., 2012) which found that the carbonate beds contain a characteristic remanence magnetization (ChRM) with southeasterly declinations and shallow down inclinations. A tilt test indicates that the ChRM is pretilting and is interpreted to be chemical remanence magnetization (CRM) based on maximum unblocking temperatures of 500°C and low burial temperatures (~100°C). The CRM the paleopole for the CRM plots in the early-Permian on the apparent polar wander path. Modeled timing for the oil window of the Fayetteville occurred prior to the acquisition of the CRM. This suggests that the CRM could be related to diagenetic processes such as maturation of organic matter or smectite to illite transformation.