Gas seeps in the French Alps

Tom Bott reports on his research trip to France.

My project is interested in how soil microbiology changes when there is an increase in the amount of methane and other gases such as ethane and propane (collectively called alkanes). Such gases may originate from the geology below the site, for example from oil or gas bearing rock. Within the UK there is a rich legacy of oil and gas extraction onshore, with many of the wells now sealed over 50 years ago. Such wells may be allowing gases to reach the surface more easily due to their ageing engineering. This is a problem as methane, the main constituent of the rising gases, is a potent greenhouse gas. With a knowledge of the microbial changes within a soil exposed to increased gas concentrations, we can potentially monitor these ageing wells at the surface and therefore identify wells with damaged caps. Using microbiology instead of direct gas monitoring should give an indication of the site’s gas emission over the previous months rather than the gas seepage at the time of surveying, as the microbial community is much slower to change than the day to day gas flux.

To explore the relationship between alkanes and the microbial community sites with a strong flux are needed. In the French Alps a series of such seeps have been identified, the gas composition includes methane, ethane and propane, as well as carbon dioxide. Thanks to additional support from the STARS Training Fund, I was able to travel to two of these sites to collect soil samples. I was able to join a field campaign led by the French Geological Survey, who along with other partners, including the British Geological Survey, were investigating these sites as part of the SECURe project (https://www.securegeoenergy.eu/).

The two sites visited were close to Grenoble. La Fontaine Ardent, in Figure 1, has been known about since the Roman era. This site has a high methane content with the remaining portion carbon dioxide. The second site, La Rochasson, has only been more recently discovered. The gas emanating from here is still rich in methane but also has small quantities of ethane and propane. The gas from both sites thought to originate from shallow, black shales. These two sites, with their subtly different gas compositions and high gas flux, are ideal for examining how soil microbial communities might respond to these gaseous alkanes.

The collected soil samples have been transported back to the BGS and undergone DNA extractions. Genes, central to the consumption of methane and propane, will be enumerated and compared to the total size of the bacterial population to give a relative abundance of those ‘functional’ genes. With the two datasets collated and mapped, the microbial data will be compared the gas flux data collected by BRGM enabling a spatial comparison. If the hypothesis is correct, the relative abundances of functional genes will increase in areas with increased alkane flux.

If this work supports my hypothesis, I hope to be completing similar work in the UK. Sites in England have a much smaller flux and this will test the limits of my techniques. There is also the possibility of returning to France to explore seasonal impacts upon the microbiology, for example low temperatures.

Figure 1: La Fontaine Ardent gas seep. Being predominately methane the rising gas burns readily when it reaches the surface and mixes with the atmosphere.

Tom Bott, November 2019