Bacteriogenic Ethane in Near-Surface Aquifers:  Implications for Leaking Hydrocarbon Well Bores

The δ13C values of methane and ethane from two near-surface aquifers in western Canada are depleted in 13C as compared to the carbon isotopic compositions of the underlying economic natural gas reservoirs and are indicative of in situ bacteriogenesis. The δ13CC2 values ranged from −45.4 to −73.9‰ (PDB), among the most depleted ever reported. These data can be used to resolve the longstanding uncertainty concerning the isotopic signature of bacteriogenic ethane and to differentiate between bacteriogenic and thermogenic hydrocarbon gas. For well bore leakage cases in the Western Canadian Sedimentary Basin, the characterization of a bacteriogenic end-member permits the quantitative resolution of the contribution of shallow aquifer gas versus deeper commercial reservoir gas to well bore leakage. Bacteriogenic near-surface aquifer ethane can account for 4−28% of the ethane from the production casing and for 29−36% of the ethane in gas bubbles traveling to the surface along the outside of the surface casing. The ability to quantify the contribution to well bore leakage from near-surface aquifer gases versus deeper commercial gas reservoirs using stable carbon isotope signatures is critical to implementing successful remediation strategies for soils and groundwaters contaminated due to aging and abandoned oil and gas wells.