TESTING WHETHER SHALES ARE OPEN OR CLOSED BASED ON INTEGRATED DIAGENETIC AND PALEOMAGNETIC STUDIES: WOODFORD SHALE AND WOLFCAMP SHALE

We conducted integrated diagenetic and paleomagnetic studies of two shales, the Woodford (OK) and the Wolfcamp (TX) to test if the shales were open or closed to hydrothermal fluids and to determine the timing of alteration. An oriented Woodford Shale core from the Ardmore Basin exhibits a complex paragenesis with multiple hydrothermal minerals, including biotite, magnesite, norsethite, gorceixite, and potassium feldspar present in and around fractures. These results suggest that the Woodford Shale was an open system during part of its diagenetic history. Paleomagnetic analysis reveals a chemical remanent magnetization (CRM) or a thermochemical remanent magnetization (TCRM) with south-southeasterly declinations and shallow inclinations that resides in magnetite and was acquired at 245 ± 10 Ma during the Late Permian based on the pole position. Because the paleomagnetic specimens show evidence of extensive hydrothermal alteration, the CRM/TCRM is interpreted to date the migration of hydrothermal fluids through the shale. The agreement in timing with other studies which report hydrothermal alteration in southern Oklahoma and the Ouachita Mountains in the Late Permian, suggest that there were post collisional fluid-flow events.

In contrast, results from the Wolfcamp Shale in the Midland Basin (TX) are less conclusive regarding alteration by hydrothermal fluids. The shale contains a Jurassic CRM in magnetite. 1D basin modeling suggests that the Jurassic CRM occurred within the oil window (100-120°C) during a tectonically quiescent phase. Electron microscopy and inorganic geochemistry suggest the Jurassic CRM resulted from processes such as the maturation/migration of organic matter/hydrocarbons, illitization, chloritization, and ultimately neoformation of magnetite on the surfaces of chlorite. The presence of barite with high salinity fluid inclusions and sphalerite could be interpreted as evidence for hydrothermal activity and hence, another viable mechanism for the CRM. Fluid inclusion results suggest homogenization temperatures of 80-120°C which falls within the expected burial temperatures predicted by the 1D model. The agreement between these temperatures suggests that a CRM related to hydrothermal fluids is unlikely although alteration by locally derived external fluids is possible.