STABLE ISOTOPIC EVIDENCE FOR MICROBIAL OXIDATION OF NATURAL GAS IN SOILS NEAR LEAKING PETROLEUM WELLS OF WESTERN CANADA

We present stable isotopic evidence for the existence of unique microbial communities capable of oxidizing high concentrations of natural gas (>90%) in soils surrounding leaking oil and gas wells. Our study is based on a large number of soil gas samples as well as permanent soil vapour probes installed at regular intervals from leaking oil/gas wells. Results demonstrate that leaking natural gas (C₁ - nC₄) is least altered close to well bore and immediately above the water table. Significant microbial oxidation starts only tens of centimeters away from well bore causing both a dramatic drop in leaking gas concentrations and an increase in isotopic ratios of the residual hydrocarbon gases. At some sites microbial oxidation is so efficient that practically no hydrocarbon gases exit the soil. Methane is by far the most altered gas with measured isotopic enrichment of 94 and 37 ‰, for H and C respectively. Both d¹³C and dD of methane follow a Rayleigh law distribution. Other hydrocarbon gases (C₂ - nC₄) exhibit lesser degrees of isotopic enrichment. Soil CO₂ near some leaking wells reaches a concentration of 20 % v/v and d¹³C of -80.6 ‰ indicating that it is a product of microbial oxidation of methane. Away from well bore CO₂ concentration falls to ~3000 ppm and its d¹³C approaches that of the natural soil CO₂.

Our observations indicate that natural gas oxidation is controlled by oxygen availability which depends on several soil parameters such as soil type, permeability, moisture, local heterogeneities, organic carbon content, and/or hydrocarbon contamination. Regular monitoring for more than 9 months indicated no significant variance in soil gas concentrations and stable isotopic ratios in vapour probes installed close to the water table which suggests steady state conditions. An order of magnitude decrease of all hydrocarbon and CO₂ gas concentrations was observed at some distant shallower probes during the winter months perhaps related to ice formation in the soils.

An intricate interplay between biogenic and abiogenic processes in the soils near the leaking wells is demonstrated by the precipitation of pedogenic carbonates with d¹³C as low as -57.2 ‰ and d¹⁸O of -18.8 ‰ showing that carbonate carbon is derived exclusively from the bacterial oxidation of methane whereas oxygen is derived from local soil moisture.