Lines-of-Evidence for the Investigation of Regional Groundwater Quality in Areas of Active Marcellus Shale Gas Extraction, Pennsylvania, USA
Molofsky, Lisa J.; Connor, John A.; Wylie, Albert S.; Wagner, Tom; Farhat, Shahla K.
When studying potential stray gas impacts, the ability to differentiate between multiple sources of thermogenic gas (e.g., shallow vs. deep reservoir gas, pipeline vs. storage gas) is important in order to correctly identify the origin of migrating gases and associated transport mechanisms. Isotopic and compositional analyses have been successfully used in the oil and gas industry for a number of decades to characterize and differentiate gas reservoirs. However, the variable application of these analyses to investigate the sources of methane in groundwater, often without resolution, underscores the need for a systematic approach to geochemical fingerprinting and the collection of historical and geological background information in the consideration of stray gas impacts.
Recent media attention has focused on potential impacts from shale-gas extraction in Susquehanna County in northeastern Pennsylvania, where hydraulic fracturing of the Marcellus shale first began in 2006. This talk will address the lines of evidence utilized to evaluate whether northeastern Pennsylvania has experienced regional impacts in the form of elevated methane concentrations or compromised groundwater quality due to recent shale-gas extraction activities.
Specifically, we will evaluate extensive historical documentation regarding the presence of shallow natural gas shows, as well as geologic evidence for the occurrence of shallow thermogenic Upper Devonian gases. The results of pre-drill sampling of water wells in Susquehanna County between 2008 and 2011 will also be compared to historical groundwater data to assess trends and potential changes in groundwater quality since the onset of local shale-gas extraction activities. Lastly, regional trends in isotopic analyses of hydrocarbon gases from the Marcellus shale and overlying gas-charged deposits will be related to changes in thermal maturity throughout the Appalachian Basin. Based on this evaluation, we will show that the use of site-specific isotopic analyses is critical for identifying minor yet discernible differences in different thermogenic gas sources, such as those found in the Marcellus shale and overlying Upper Devonian deposits in Susquehanna County.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013