As the pace of drilling activity in the Marcellus Formation in the northern Appalachian 21 Basin has increased, so has the number of alleged incidents of stray natural gas migration to two shallow aquifer systems. For this study, more than 1,900 gas and water samples were analyzed for molecular composition and stable isotope compositions of methane and ethane. The samples are from Quaternary to Middle Devonian-age strata in a five-county study area in northeastern Pennsylvania. Samples were collected from 181 gas wells during mudgas logging programs and from 67 private water supply wells during baseline groundwater-quality testing programs.

Evaluation of this database reveals that microbial, mixed microbial/thermogenic, and thermogenic gases occur in some shallow aquifer systems, and that the gas occurrences pre-date Marcellus Formation drilling activity. Isotope data reveal that thermogenic gases in the regional Quaternary/ Upper Devonian shallow subsurface (average δ13C1 = -42.13 ‰; average δDC1 = - 228.26 ‰) typically are distinct from gases in Middle Devonian strata (average δ13C1 = -32.87 ‰; average δDC1 = -163.45 ‰). Additionally, gas geochemistry at the site-specific level reveals a complex thermal and migration history with gas mixtures and partial isotope reversals (δ13C1>δ13C2) in units above the Marcellus Formation.

Identification of a source for stray natural gas requires the synthesis of multiple data types at the site-specific level. Molecular and isotope geochemistry provides evidence of gas origin and secondary processes that may have affected the gases during migration. Such data provide a focus for investigations where the potential source of the stray gas includes multiple formations.