Lisa J. Molofsky
John A. Connor
Shahla K. Farhat
GSI Environmental Inc.
Houston
Albert S. Wylie Jr.
Tom Wagner
Cabot Oil & Gas Corp.
Pittsburgh
Results from more than 1,700 water wells sampled and tested prior to proposed gas drilling in Susquehanna County, Pa., show methane to be ubiquitous in shallow groundwater, with a clear correlation of methane concentrations with surface topography.
Specifically, water wells located in lowland valley areas exhibit significantly higher dissolved methane levels than water wells in upland areas, with no relation to proximity of existing gas wells. The correlation of methane concentrations with elevation indicates that, on a regional level, elevated methane concentrations in groundwater are a function of geologic features, rather than shale gas development.
Technical literature and historical publications confirm the presence of methane gas in natural seeps and water wells in this region for many decades, long before shale gas drilling operations were initiated in 2006.
Potential sources of this naturally occurring methane include thermogenic gas-charged sandstones in the Catskill formation, which are tapped by most water wells in this region. These sandstones exhibit an extensive network of fractures, joints, and faults that serve as principle conduits of groundwater flow and potential pathways for the movement of shallow-sourced dissolved methane.
Biogenic methane, which is produced by the natural decomposition of organic material within thick valley alluvium and glacial drift deposits in the area, may also be found in water wells that draw water from shallower sediment deposits.
The source of this dissolved methane is important with regard to understanding the potential effects of ongoing shale gas development and the appropriate measures for protection of water resources.
In 2009 and 2010, the Pennsylvania Department of Environmental Protection (DEP) conducted stable isotope analyses of methane in gas wells and water wells in Susquehanna County, which indicated that the isotopic signature of thermogenic gas from Upper and Middle Devonian deposits overlying the Marcellus shale can be distinguished from that of Marcellus shale gas.
The ability to distinguish between different formation gases has important implications with regards to the findings of the recent study by researchers from Duke University,1 which suggested that the thermogenic signature of elevated methane concentrations in water wells in Susquehanna County was consistent with an origin in deep shale gas deposits, such as the Marcellus and Utica formations, that are currently targeted by hydraulic fracturing activities.
The present study, however, shows that the isotopic signatures of the Duke study's thermogenic methane samples were more consistent with those of shallower Upper and Middle Devonian deposits overlying the Marcellus shale. This finding indicates that the methane samples analyzed in the Duke study could have originated entirely from shallower sources above the Marcellus that are not related to hydraulic fracturing activities.
The apparent misinterpretation of the origin of the observed thermogenic methane underscores the need for a multiple lines-of-evidence approach for proper characterization of methane gas sources, with careful integration of the relevant geologic, historical, well construction, and isotopic data.
Methane in Susquehanna County groundwater
From May 2008 through 2009, in accordance with current Pennsylvania DEP guidelines, Cabot Oil & Gas Corp. collected water samples from all existing water wells within 1,000 ft of proposed gas well drilling sites.
From 2010 to the present, the sampling program was extended to include all water wells within 2,500 ft of proposed gas well drilling sites in anticipation of revised Pennsylvania DEP guidelines.
Collectively, these samples were submitted for "predrill" analysis of water quality parameters including concentrations of dissolved gases (methane, ethane, propane) and general chemistry analyses pertaining to primary and secondary drinking water standards.
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