Microbial turnover of hydrocarbons at a leaking abandoned oil well in Germany

With a contribution of about one third, methane is the second most important greenhouse gas in the climate system. In addition to a biogenic formation, e.g. in wetlands, methane also is emitted during anthropogenic industrial activities. BGR is investigating abandoned onshore oil and gas wells in Germany, which are generally plugged and buried, for their relevance as sources of methane. Initial results from studied wells examined so far (about 75 wells) indicated no or very low methane emissions at very few sites. A controlling process for low methane emissions for the wells could be microbial methane oxidation, which is an important process in organic-rich soils overlying wells in Northern Germany (Jordan et al., accepted).We present here data from soil above a plugged oil well, drilled in the early 1920s and located at Nienhagen near Hannover (Germany). At the well ~40 mg CH4 h-1 were emitted (average range for plugged US oil wells ~50 to 170 mg h-1 per well; Williams et al. 2021). Gas geochemical analyses of the soil gas confirm the presence of natural gas (up to 8 % methane and 600 ppm ethane) and the δ13C of the methane supports that the majority is thermogenic (-47.1 ‰). In addition to natural gas, we also found petroleum in the soil, which reached up to 80 % soil total organic carbon. Our data suggest a complex mosaic of hydrocarbon-altering effects dominated by products from the microbial degradation of well-derived oil and natural gas (e.g., propane oxidation). It is likely that O2 availability controls the degradation of petroleum in the soil under investigation, because the strongest degree of degradation was found in the upper soil horizons. The properties of the formerly produced oil exclude biodegradation in the reservoir, so the degraded oil must have been formed during the ascent or in the topsoil. The gas geochemical composition of the soil gases indicates also deeper, anaerobic processes, such as methanogenesis, probably with petroleum as the carbon source. Soil microcosms from different depths showed, indeed, a rapid onset of microbial degradation of added oil both under aerobic and anaerobic conditions in the lab. Although processes in a deeper biosphere appear to play a role here, it is likely that mostly the microbial processes in the soil surrounding the well regulate the composition and quantity of oil and gas. In conclusion, the (i) high degree of degradation in the natural gas components in the soil and petroleum, as well as the overall (ii) only low methane emissions, indicate that the Nienhagen well is only leaking relatively little and that a “microbial hydrocarbon filter” is established and active.ReferencesJordan, S.F.A., Schloemer, S., Krüger, M., Heffner, T., Horn, M.A., Blumenberg, M., (accepted) Preprint. Interferences caused by the microbial methane cycle during the assessment of abandoned oil and gas wells. EGUsphere. doi:10.5194/egusphere-2024-1461Williams J. P., Regehr A. and Kang M. (2021) Methane Emissions from Abandoned Oil and Gas Wells in Canada and the United States. Environmental Science & Technology 55, 563–570.

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