Petroleum-fuelled methanogenesis and microbial hydrocarbon oxidation at an abandoned oil well

Here, we present a geochemical study of the soil above a plugged oil well (‘Elwerath’), which was drilled in the early 1920s and is located near Hannover (Germany). At this site ∼40 mg methane per hour is emitted. Geochemical analyses of the soil gas confirm the presence of thermogenic natural gas (with up to 8 % methane and 600 ppm ethane in the soil gas) and the δ 13 C of the methane indicates that most of the emitted methane is thermogenic (δ 13 C −47.1 ‰ VPDB). In addition to natural gas, we also found petroleum in the soil, accounting for up to 80 % of the total soil organic carbon. Detailed soil gas and organic geochemical analyses of extracts from the soil confirm a strong similarity with the old petroleum reservoir. Our data suggest a complex mosaic of gas-altering effects that cannot be explained by admixture with biogenic methane alone, but are rather dominated by microbial degradation of oil and natural gas components (e.g. propane oxidation). Most likely O 2 availability controls the microbial degradation of petroleum close to the ‘Elwerath’ well, because the strongest degree of degradation, with the lowest petroleum content, is found in the upper, well aerated soil horizons. The gas geochemistry and isotopic composition of the soil gases also indicate anaerobic processes, such as methanogenesis, probably with petroleum as carbon source. These processes must have taken place significantly deeper than the top meter sampled, where O 2 was still detectable. CO 2 emissions appear to be negatively affected by the oil contamination, most likely due to the inhibition of natural microbial respiration by toxic effects. While processes in the deeper biosphere appear to play a role here, it is clear that the microbial processes in the soil surrounding the well also regulate the composition and quantity of oil and gas. In conclusion, (i) the high degree of degradation of natural gas and oil in the soil, and (ii) the low methane emissions, indicate that only little hydrocarbon is leaking and that a ‘microbial hydrocarbon filter’ is established and active. • ∼8 % of thermogenic methane present in soil gases, but emission rates low (∼40 mg CH 4 h −1 ). • Propane concentration and δ 13 C indicate strong microbial degradation at depth. • Highly degraded oil-derived higher hydrocarbons detected. • H 2 partly abundant in soil gases and positively correlating with methane and ethane. • Degradation in hydrocarbons argue against a shallow origin.