DISSECTING BLACK SHALES AND ASSOCIATED FLUIDS FROM THE MIDDLE DEVONIAN MARCELLUS FORMATION, APPALACHIAN BASIN, USA

The Middle Devonian Marcellus Formation contains organic-rich black shales that constitute one of the world’s largest unconventional shale gas plays, currently producing both methane and natural gas liquids. It is one of several black shale units deposited under restricted marine conditions in the Appalachian, Michigan, and Illinois basins during the Paleozoic Era. Recent studies of core and produced water have generated new information relevant to the development of more accurate models for basin deposition and thermal evolution. The Appalachian Basin is structurally and tectonically complex, affected by multiple orogenies and fluid flow events. These have impacted thermal maturation and hydrocarbon development, and resulted in diagenetic and catagenetic redistribution of geochemical and isotopic components. Sampling of late-stage produced water following hydraulic fracturing offers a unique opportunity to study fluids intimately associated with shale in the subsurface environment, and to work out the origin of these fluids and their possible relationship to the post-depositional evolution of the shale. We are combining fluid studies with chemical extractions of selected geochemical components from core samples of the Marcellus Formation and surrounding units using sequential leaching techniques. Targeted solutions extract water-soluble components (e.g., salts), exchangeable cations, carbonate minerals, organic matter and sulfides, and residual silicates. We have found that different components of black shales can maintain Sr and U isotope disequilibrium among carbonate, water soluble and exchangeable sites, and that these reservoirs reflect different stages of depositional, diagenetic and catagenetic evolution. We are combining these data with whole rock major and trace element compositions, element distributions and associations in different reservoirs, and clastic source indicators like Nd isotopes to build a comprehensive picture of Marcellus depositional history and post-depositional rock-fluid interactions.