INTEGRATED DIAGENETIC AND PALEOMAGNETIC STUDY OF THE HAYNESVILLE SHALE

Four unoriented cores of the Jurassic Haynesville Shale were investigated to determine the origin and timing of diagenetic events and to assess if the unit was open or closed to external fluids. Facies present include siliceous, carbonate-rich, and burrowed mudstones, as well as dolostone. Petrographic analysis indicates a complex paragenesis. Pyrite, ferroan dolomite, dedolomite, and authigenic chlorite occur in the matrix, and hydrocarbons are present in allochems. Healed fractures in multiple orientations contain calcite (blocky and beef), dolomite, pyrite, sphalerite, barite, celestine, anhydrite, double terminated quartz, and albite. Horizontal veins are interpreted as early events, and vertical veins and brecciation are interpreted as late events. Vein heterogeneity suggests multiple diagenetic fluids. Most ⁸⁷Sr/⁸⁶Sr values are elevated compared to coeval seawater which suggests alteration by external fluids although dissolution of internal feldspars during illitization could be an explanation for the elevated values.

Paleomagnetic analysis revealed one component with positive inclinations (61°) that resides in pyrrhotite (160-320°C) and a second with reversed inclinations (-50°) that resides in magnetite (300-480°C). Both are interpreted as chemical remanent magnetizations (CRM). Inclinations of the CRMs were utilized to determine their age but expected inclinations do not vary by much since the late Jurassic, therefore both CRMs can only be inferred to be Late Jurassic to Cenozoic in age. The inclination of the pyrrhotite CRM is similar to the inclination at the core location (61°). Assuming this CRM is Modern, the core was oriented by rotating this CRM to the present-day direction (0°) and then correcting the magnetite CRM. Based on the rotation, the magnetite CRM has a good grouping and a Paleogene/Neogene pole position on the apparent polar wander path. The cores contain moderate total organic carbon values (~2-4 wt %) and the origin of magnetite CRM may be attributed to maturation of organic matter. Alternatively, late external fluids could have caused a Paleogene/Neogene CRM. Although some minerals could have been self-sourced, the results suggest the Haynesville was at least partially open.