New York Water Science Center
The Marcellus Shale of the Appalachian Basin is one of the largest unconventional gas plays in the United States. The Marcellus Shale is the most extensive of a series of Appalachian black-shale formations deposited in sediment-starved subsiding foreland basins during the Devonian Period. The targeted black shales in the Marcellus, which are delineated on geophysical logs by their elevated gamma radiation and low density, are organic rich and pyritic.
The Marcellus play is being developed in Pennsylvania, West Virginia, and Ohio at depths between 5,000 and 9,000 feet below land surface. Typically, six horizontal legs 4,000 feet long are drilled in the basal Marcellus per multi-well pad and 3 to 5 million gallons of water are used to hydraulically fracture each leg.
Water-resource issues associated with development of the Marcellus play include the 1) impact of surface-water and groundwater withdrawals for hydraulic fracturing during low-flow periods, 2) disposal of black-shale drill cuttings that may produce acidic, metals-rich drainage with elevated radioisotopes, and 3) treatment of hydraulic-fracturing flowback that contains elevated dissolved solids and radioisotopes. Surface spills of hydraulic-fracturing fluids and flowback and problems with casing and grout seals that allow migration of gas to freshwater aquifers pose the greatest threats to the water resources.
First posted December, 2011
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