AN INTEGRATED DIAGENETIC AND PALEOMAGNETIC STUDY OF THE MARCELLUS SHALE WITHIN THE PLATEAU PROVINCE OF THE APPALACHIAN BASIN

An integrated paleomagnetic and diagenetic study of two cores of the Devonian Marcellus Subgroup in the Appalachian Basin in Pennsylvania was undertaken to constrain the timing and nature of the diagenetic events and to test if externally derived fluids have altered the Marcellus Subgroup. The paragenetic sequence for the two cores is complex and is grouped into 26 diagenetic events. Key events include replacement of allochems by silica and calcite cements, precipitation of framboidal and cubic pyrite (early and late diagenesis), generation of hydrocarbons and formation of bitumen, localized veining with calcite, barite, and sphalerite mineralization, and formation of diagenetically altered and veined breccias in the Cherry Valley Limestone that contain dolomite, sphalerite, barite, and late sylvite. Analysis of CH₄ inclusions in calcite veins indicated homogenization temperatures that ranged from -88.3 to -98.6^oC. Anisotropy of Magnetic Susceptibility (AMS) results indicate that the cores contain a predominately oblate fabric although diagenetically altered intervals contain a different AMS fabric that is interpreted as altered. The cores contain two magnetizations: a viscous remanent magnetization (VRM) removed at low demagnetization temperatures and a characteristic remanent magnetization (ChRM). Orienting one core using the VRM produced directions with streaked southerly declinations and shallow inclinations. The streak may have been caused by a drilling induced remanent magnetization. Specimen directions from the oriented core are also streaked. The ChRM in both cores has shallow inclinations (-10.5 and -3.9), which corresponds to a 285-305 Ma time of acquisition 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, that likely formed as a result of fluids activated during the Alleghanian orogeny. The petrographic and paleomagnetic results suggest that the Marcellus Subgroup was a complex open diagenetic system. The altering fluids probably had multiple origins including tectonic (basinal saline brines), overpressuring caused by water expelled from compaction of sediments, and from the generation of hydrocarbons.