Regional Variability in the Gas Geochemistry of the Appalachian Basin and Implications for Groundwater Investigations

The occurrence of stray gas (methane) in aquifer systems can be a natural condition or due to anthropogenic activity. Investigations of stray gas migration require data collection and evaluation at the site-specific level. Analyses of gas geochemistry provide evidence of gas origin and focus for the investigation that must be integrated with other data types including geological and engineering to conclude a stray gas source.

Most gases in the Paleozoic strata of the Appalachian Basin were either cracked directly from kerogen or cracked directly from oil that was generated in those source rocks. These gases were subjected to further thermal stresses, geologic upheaval, and migration. Other sources of gas include the regionally prominent coal beds of the bituminous coalfields and microbial gas in glacial drift found in the northern area of the basin. Any of these gases may have migrated to shallower formations on geologic timescale or contemporaneously due to anthropogenic activity. Gas migration of both orders may yield mixing of gases on both a regional and local scale. Understanding the complexity associated with the interplay of gases of different origins found in the shallow system is instructive for investigations of stray gas migration.