CONSTRAINING THE TIMING AND ORIGIN OF DIAGENETIC EVENTS IN THE MARCELLUS SHALE; AN INTEGRATED PETROGRAPHIC AND PALEOMAGNETIC APPROACH

The Devonian Marcellus Shale is both a source rock and reservoir for hydrocarbons in the Appalachian Basin. An integrated paleomagnetic and petrographic study of two cores in Pennsylvania was undertaken to determine the timing and nature of the diagenetic events, and to test if externally derived fluids have altered the Marcellus Shale. Early diagenetic events include compaction, fracturing, and calcite precipitation and dissolution. Middle events in the paragenetic sequence include precipitation of silica, pyrite, dolomite, and plagioclase. Late diagenetic features include fracturing and vein mineralization by calcite, quartz, and pyrite.

Anisotropy of Magnetic Susceptibility (AMS) was measured to analyze the magnetic fabrics in an oriented and unoriented core. The data for both cores indicates predominately an oblate AMS fabric. The K_(max) values for the oriented core have a N-S trend. The paleomagnetic data from an unoriented core indicate that there are two magnetic components: a viscous remanent magnetization (VRM) removed at low demagnetization temperatures and a characteristic remanent magnetization (ChRM) commonly removed at 300-500ºC. An attempt was made to orient the ChRM based on rotation of the VRM direction to the modern direction for the locality. This method resulted in a direction with southerly declinations and shallow inclinations but there is a streak in the data; more data is needed to evaluate if the VRM method will work in this core. The core is vertical and the ChRM has an inclination of -7.8, which corresponds to a time of acquisition between 295-305 Ma based on a comparison with the expected inclinations for the study location. The ChRM is interpreted as a chemical remanent magnetization (CRM) that resides in magnetite. Studies are currently underway to determine whether the CRM is related to burial diagenetic processes or alteration by fluids.