INTERPRETING DISSOLVED METHANE IN SHALLOW AQUIFERS ACROSS SIX OIL AND GAS PRODUCING BASINS OF THE UNITED STATES

An upsurge of unconventional oil and gas (UOG) extraction practices after 2007 augmented concerns for production activities affecting groundwater quality and prompted several shallow aquifer studies in UOG production zones throughout the United States. Several of these studies linked impacted shallow aquifer hydrochemistry to UOG activities, detecting constituents that have included, but are not limited to, known hydraulic fracturing fluid components, heavy metals, and methane gas and accompanying aliphatic hydrocarbons(C₂H₆+) (Osborn et al., 2011; Warner et al., 2012; Darrah et al., 2014; Hilderbrand et al., 2015). Yet, public concerns persist and there is still uncertainty regarding the geologic controls on shallow aquifer methane. Detection of thermogenic natural gas is a valuable indicator of UOG-associated fugitive gas migration when coupled with compositional analysis of isotopes of the methane and associated gases, and coproduced waters (e.g. δ¹³CO₂, δ¹³C-CH₄,⁴He, ³⁶Ar, ²⁰Ne, ²¹Ne) (Kresse, et al., 2011; Osborn et al., 2011). Here, publicly available shallow aquifer data was compiled from August 2015 to August 2016 for six UOG producing basins across the United States: Williston, Denver-Julesburg, Arkoma, Fort-Worth, Illinois, and Appalachian. Derived from federal and state databases, inquiries with groundwater researchers, and various peer reviewed studies, data was evaluated for fugitive gases within basins, then further examined for comparisons across geologically different basins.

Assembled data plots revealed evident and discrete data groups that are discernable and unique to each basin. This observation could imply that unique trends in dissolved gases may be facilitated by some geologic characteristic(s) unique to each basin such as redox conditions, density of flow paths, etc. However, factors dictating natural gas source and distribution within shallow aquifers are unquestionably multivariate. Thus, this study is a first order inquiry that is intended to supplement the critically needed, comprehensive understanding of natural gas sources and migration mechanisms in UOG host-basins.