INTEGRATED PALEOMAGNETIC AND DIAGENETIC STUDY OF THE HAYNESVILLE SHALE, TEXAS

An integrated diagenetic and paleomagnetic study of the Upper Jurassic Haynesville Shale was undertaken to determine the origin and timing of diagenetic events in this shale-gas unit. Thermal demagnetization of two unoriented cores from Harrison County, Texas, uncovered a low temperature component with moderate to steep inclinations interpreted as a viscous remanent magnetization (VRM) in a few specimens, a moderate temperature component (160-320°C) with moderate to steep inclinations interpreted as a chemical remanent magnetization (CRM) residing in pyrrhotite, and a higher temperature component (300-480°C) also interpreted as a CRM, residing in magnetite. A drilling induced remanent magnetization was ruled out based on relatively low inclinations and Q-factor analysis. Because only a few of the specimens had a well-defined VRM, orienting the core was not successful As a result, the inclination values were utilized to infer the timing of remagnetization. Inclinations for the magnetite CRM (49°) suggest acquisition in the late Jurassic/early Cretaceous or late Cretaceous/Paleogene whereas inclinations for the pyrrhotite CRM (64°) coincides with a Late Cretaceous to Paleogene remagnetization. The shale has a complex paragenetic sequence including precipitation of diagenetic minerals such as sphalerite, barite, celestite, and anhydrite. Horizontal veins are interpreted as early and vertical veins and brecciation are interpreted as late diagenetic events. Although there is clear evidence for alteration by fluids, a paleomagnetic contact test around veins in the cores yields no conclusive results. The cores contain total organic carbon values of 2-4 wt % and the origin of magnetite CRM may be attributed to a burial processes such as maturation of organic matter which occurred at ~140 Ma based on modeling studies. Alternatively, late diagenetic externally-derived fluids could have caused a Paleogene/Neogene remagnetization. The pyrrhotite CRM may have formed through thermochemical sulfate reduction.