Abstract: Identifying the Sources of Stray Methane Using Geochemical and Isotopic Fingerprinting

BALDASSARE, F. J. and CHRISTOPHER D. LAUGHREY*

Under certain conditions, the uncontrolled migration of natural gas in the subsurface can result in an explosion with the potential for significant property damage and loss of life. In western Pennsylvania, there are numerous possible sources of stray natural gas including producing and abandoned wells, transportation pipelines, subsurface storage reservoirs, coal mines, and municipal waste landfills. Landfill gases are strictly of microbial origin whereas the others are largely thermogenic in origin.

Baseline geochemical data from gases generated in municipal landfills and those produced from gas wells and coals in western Pennsylvania help to identify sources of methane. Analyzed landfill gases exhibit C₁/(C₂ + C₃) ratios greater than 3.8 x 10³, delta¹³CO₂ values of -19.7 to +17.4 permil, deltaD_CH4 values of -197 to -353 permil, and delta¹³CH₄ values of -33.2 to -57.4 permil. Isotope fractionation due to methane oxidation complicates the interpretation of stable isotopes in landfill methanes. ¹⁴C activity in the landfill methanes ranges from 125 to 145 % of modern. Thermogenic methane in the subsurface rocks of the region have delta¹³C values of about -27 to -55 permil and deltaD values of -150 to -303 permil.

Chemical and isotopic analyses, combined with a thorough site investigation, provide a comprehensive approach for differentiating microbial from thermogenic gases and thermogenic gases from different sources. We used such an approach to confirm the offsite migration of methane from a municipal landfill, and the migration of methane from an abandoned gas pool to the soils adjacent to a commercial office building.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky