Seasonal Effects on the Carbon Stable Isotope Compositions of Natural gas Subjected to Microbial Oxidation in Soils Near Leaking oil Wells in Western Canada

The molecular and carbon stable isotope compositions of leaking natural gas in the unsaturated zone near oil wells in Saskatchewan have been monitored for 12 to 20 months. Leaking gas originates in the Upper Cretaceous Colorado Group shales and is a mixture of bacteriogenic and incipient theromgenic gas that consists of more than 99 percent methane, 2400 ppm ethane, and lesser propane, butane, and pentane. Long term monitoring of soil gas concentrations and stable isotopic compositions demonstrates that microbial oxidation reduces hydrocarbon gas concentrations at the soil surface to less than 1000 ppm and results in the production of large volumes of carbon dioxide with significantly depleted in ¹³C carbon stable isotope signature. Rapidly dropping natural gas concentrations, rising carbon dioxide concentrations, and elevated carbon, nitrogen, and sulphur soil contents at depths of 100 to 150 cm indicate that microbial oxidation is confined to a relatively narrow zone around the well bore. The δ ¹³C of light hydrocarbon gases and carbon dioxide collected from a soil gas probe installed at 100 cm depth and 50 cm distance from well bore exhibit significant seasonal variance. Hydrocarbon gases generally have lowerδ ¹³C during the summer and higher δ ¹³C during the winter whereas CO₂ exhibits the opposite trend. Kinetic fractionation factor associated with the microbial oxidation of methane estimated from carbon isotope measurements of CH₄ and CO₂ varies from 8 per mil during the summer to 28 per mil during the winter. Soil temperatures at 100 cm depth vary from 1.7° C in March to 17.3\deg C in late August and exhibit significant negative correlation (R² = 0.89) with the estimated kinetic fractionation factor. The negative correlation likely reflects higher methane consumption rates during the summer and lower rates during the winter. Lower oxidation rates in the winter may be related to environmental stress associated with the formation of a several tens of centimeters thick frozen layer in the upper soil horizon from November to April or May. The frozen layer may obstruct oxygen and natural gas transport thereby affecting the openness of the system.