A PALEOMAGNETIC AND DIAGENETIC STUDY OF THE WOODFORD SHALE, OKLAHOMA

An unoriented Woodford core from the Anadarko Basin (Grady County, Oklahoma) was sampled in order to identify and interpret the origin of the magnetic components present in the shale as well as to investigate the diagenetic history. Thermal demagnetization reveals a complex magnetization with four components. A viscous remanent magnetization (VRM) obtained has a steep down inclination (73.1°). The inclination in the study area is 63.1° and Q-factor analysis was performed to determine if the component was drilling induced. The average Q-factor of the outside of the core is greater than that of the inside, thus the VRM is interpreted to be influenced by a drilling-induced remanent magnetization (DIRM). Two intermediate components are found, one with a steep up inclination that is interpreted to reside in pyrhottite, and one with a steep down inclination that is interpreted to reside in magnetite, based on unblocking temperatures (320-360°C and 450°C, respectively). A characteristic remanent magnetization (ChRM) unblocked at a higher temperature (480°C) with a shallow inclination (-4.5°) is interpreted to reside in magnetite. The VRM method of orienting core was unsuccessful, probably because of the DIRM. Therefore, an inclination only plot for the study area was produced to determine the age of the magnetizations. The intermediate components were acquired during normal and reversed epochs and are Paleogene to Neogene in age. The ChRM is Permian to Triassic in age. The stable remanent magnetizations are likely chemical remanent magnetizations (CRM). The higher temperature CRM could be related to maturation of organic matter, the smectite to illite transformation, or alteration by external fluids. Preliminary petrographic evidence reveals that some intervals contain multiple, cross-cutting veins mineralized by calcite, dolomite, and barite, suggesting the shale has undergone a complex diagenesis. The presence of saddle dolomite in vein-filled fractures indicates the shale has been diagentically altered by hydrothermal fluids. Further work includes scanning electron microscopy, as well as rock magnetic studies to determine the magnetic mineralogy of the shale and determine the origins of the CRMs.