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Geochemistry and Depositional History of the Union Springs Member, Marcellus Formation in Pennsylvania

Abstract

The black shales of the Appalachian Basin have gained renewed and widespread attention over the last several years. While gas has been produced from the Appalachian Basin for over one hundred years (Curtis, 2002), low recovery had made these source rocks uneconomical to companies in the past. However, technological advances in horizontal drilling and hydraulic fracturing, as well as an increase in global demand, have made extraction from these unconventional plays attractive. More recent interest has been directed at the Middle Devonian Marcellus Formation, which extends throughout much of the Appalachian Basin, and may contain up to 500 trillion cubic feet of gas (Engelder and Lash 2008). There is still much debate over the conditions that persisted when the Marcellus Formation was deposited, specifically, over the relative importance of primary productivity versus preservation of organic matter (Arthur and Sageman, 2005). A solid understanding of the nature and origin of the Marcellus source rock, including sediment-water chemistry and organic carbon flux, is required to most effectively and efficiently explore and develop the play. Our research is interested in testing the euxinic basin model after Sageman et al. (2003). Rock samples have been collected from three cores located in the Appalachian Basin Black Shale Group core lab at The Pennsylvania State University, at a resolution of three one-inch samples per foot to provide a geochemical transect from more proximal to more distal in the basin. Samples have been collected across the basal Union Springs Member of the Marcellus Formation for two of the cores, and across the Union Springs Member and into the Cherry Valley Member for the additional core, and analyzed for major, minor and trace elements, and for total organic carbon. Preliminary results show that at the onset of deposition of the Union Springs Member the sediment-water interface, and possibly, a portion of the water column was anoxic to euxinic, as suggested by high concentrations of uranium, molybdenum and other redox sensitive elements. Small-scale parasequences can be seen as overall deepening of the basin occurred, and oxic conditions at the sediment/water interface appear to punctuate the anoxic to euxinic water conditions. As deposition of the Marcellus Formation continued, a greater influx of clastic material is observed, as indicated by the aluminum and associated elements.