PROVENANCE OF THE MARCELLUS SHALE: GEOCHEMICAL INSIGHTS INTO THE SOURCE TO SINK RELATIONSHIPS WITHIN THE MIDDLE DEVONIAN ACADIAN FORELAND BASIN

Little is known of the provenance of the Marcellus Shale, one of the most prolific unconventional shale gas plays in the world. As part of the Marcellus Shale Energy and Environmental Laboratory (MSEEL) project, 56 sidewall plugs of the Marcellus Shale were taken from the MIP-3H well in Morgantown, WV in order to evaluate controls on clay provenance through geochemical proxies. Bulk mineralogy, determined through RIR interpretation of XRD data, show that the rocks are composed of illite/muscovite (approximately 60-80%), quartz (~10-20%), pyrite (~5-10%), and chlorite (~5%) with calcite, dolomite, albite, and barite as accessory phases. The chemical index of alteration (CIA), chemical index of weathering (CIW), and the index of compositional variability (ICV) were all calculated from elemental compositional data acquired through laboratory XRF analysis of fused disks. CIA (~45-75) and CIW values (~55-90) generally increase up-section. The ICV values range from 0.70-1.62, consistent with a granitic source weathering trend, and decrease up-section. These indices indicate increased weathering of detrital clay deposited in the Marcellus Shale over time. Ratios of Si/Al decrease up-section, suggesting greater clay influx through time; however this could reflect decreased biogenic silica deposition. Ratios of Ti/Al increase up-section, indicating increasing terrestrial clastic influence. TIMS analysis of three samples produced Sm¹⁴⁷/Nd¹⁴⁴ values from 0.1281-0.1245, ε_Nd from -10.183 to -10.514, and depleted mantle model ages (T_DM) of 1791 to 1850 Ma. Three potential provenance models could be supported by these data: 1) mixed sediment from the Canadian Superior Craton (>2.8 Ga) and Grenville orogenic rocks (1.3-1.0 Ga) to the N/NW, 2) sediments sourced from the Penokean and/or Plains/Trans-Hudson orogenic rocks (>1.8 Ga) to the W, or 3) sediment influx from Lower Paleozoic rocks of the adjacent Acadian fold-thrust belt. However, cross-plots of trace elements (Zr/Sc vs. Th/Sc) do not indicate abundant dense mineral phases related to recycling sedimentary rock, but rather support erosion of an old, chemically juvenile continental crust terrain type (ε_Nd vs. Th/Sc). Thus, together these data suggest sediment of the Marcellus Shale primarily represents mixed detritus from Superior Craton and Grenville rocks.