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[BBG02-04] Microbial cycling of hydrocarbons revealed by new isotope tracers
Keywords:Hydrocarbons, Stable isotopes, Microbial oxidaition
Here, we present new data on position-specific isotope analysis of propane from sedimentary basins in North America (Michigan basin and Southern Ontario), where economic reservoirs of hydrocarbons accumulate . We use a method we recently developed allowing the determination of d13C values of terminal (CH3) and central (CH2) position of propane with a precision of 1‰ . The δ13C value of the central position of propane increases by around 10‰ with increasing bulk δ13C of propane, while the δ13C value of the terminal position is uniform for all the samples measured. The results are not consistent with production models for thermogenic natural gas generation , nor with experimental results obtained from thermogenic propane generation. However, the results are consistent with the proposed mechanism for anaerobic bacterial oxidation of propane .
When propane is biodegraded by BuS5 – a sulfate-reducing anaerobic bacteria that uses propane as a carbon source – the remaining propane is relatively 13C-enriched in the central position. The position-specific fractionation factor measured for the bacteria is consistent with that observed for canadian natural gas samples, suggesting anaerobic bacterial propane oxidation occurs in the basin. In addition, data from the Michigan basin suggests that anaerobic C3/C4 bacterial oxidation is coupled to the production of ethane. Whether the production is biological or not remains to be elucidated.
Overall, the data suggests that position-scpeific isotope composition can help refine the origins of hydrocarbons, in particular microbial production and consumption.
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