Methane Origins in Shallow Aquifers: Deciphering a Compilation of Publicly Available Data

Abstract

The current inability to address the possible relationship of unconventional oil and gas (UOG) production with discrete concentrations of dissolved thermogenic gases in overlying shallow aquifers represents a knowledge gap existing in today’s research. This study is a first-order effort of its kind and seeks to help address the aforementioned knowledge gap by attempting a cumulative review of publicly available groundwater data from six different basins across the United States that host UOG reservoirs. Here, groundwater data was mined from August 2015 to August 2016. Data was compiled from federal and state databases, inquiries with groundwater researchers, and various peer reviewed scientific studies conducted across the United States. This data compilation was then evaluated for widespread distinctions in dissolved methane (carbon isotopic composition (δ13C1,) molecular composition (C1/C2), concentration (mgL-1)) in shallow groundwaters overlying production zones. Shallow aquifer data from these six basins plotted in discrete groups, indicating variable gas types and implying different gas transport mechanisms. However, factors dictating natural gas source and distribution within shallow aquifers are unquestionably multivariate. Thus, several qualitatively defined parameters including basin type and age, vertical distance between shallow aquifer and produced unit, and zones of structural weakness (e.g., faults and fractured networks) were chosen for an evaluation in juxtaposition with dissolved gas data analysis. This study may be a precursor to a more comprehensive understanding of the natural gas transport mechanisms and sources in UOG-hosting basins. Finally, this study may also shed light on whether risk of gas migration to aquifers may vary basin to basin as a reflection of general variability in basin structures, and what publicly available data sets can reveal regarding dominant types of shallow aquifer methane.