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A Diagenetic and Paleomagnetic Study of the Woodford Shale, Oklahoma


A Woodford shale core from the Anadarko Basin, as well as outcrop samples from the Arbuckle Mountains in southern Oklahoma, were obtained in order to develop a paragenetic sequence and determine the timing of events. Detailed analysis of a three additional Woodford shale cores is currently underway. The shale is extensively altered and contains a complex paragenesis. Early events include precipitation of quartz and pyrite. The dominant clay is illite. Multiple fractures, both early and late, as well as brecciated intervals, are common. X-Ray computed tomography results indicate complex fracture morphologies. The fractures contain multiple generations of calcite, dolomite, ferroan dolomite and quartz. Other minerals such as authigenic albite, barite, and anhydrite occur in or around the fractures. Sphalerite, saddle dolomite, witherite, and magnesite suggest alteration by hydrothermal fluids. Anisotropy of magnetic susceptibility (AMS) data shows vertical fabrics associated with complex mineralized fracture networks. Future research will compare AMS fabric parameters to distributions of organic material in order to evaluate the control of petrofabrics on migration pathways. Paleomagnetic analysis of outcrop samples indicates a chemical remanent magnetization (CRM) residing in magnetite acquired in the late Paleozoic. Results from one core has a more complex magnetization with four components. In addition to a viscous remanent magnetization, two intermediate temperature components occur in the core, one with a steep up inclination (-64.5°) interpreted to reside in pyrrhotite, and another with a steep down inclination (64.2°) that is interpreted to reside in magnetite. These are interpreted as CRMs that were acquired during normal and reversed epochs in the Cretaceous to Neogene. A few samples contain a higher temperature CRM with shallow inclinations that is similar to the outcrop results. The intermediate temperature CRMs are interpreted to be related to altering fluids whereas the higher temperature CRM could be related to the maturation of organic matter, the smectite to illite transformation, or altering fluids. Multiple magnetic components coupled with the presence of hydrothermal minerals indicates an open diagenetic system, however, many other authigenic minerals can be explained by internal or “closed processes” such as shale dewatering and clay diagenesis.