Researchers Delineate Factors Controlling Formation of Subsurface Methane

The groundwaters bathing crystalline bedrock, shaped by high temperatures and extreme pressures, can have surprisingly high concentrations of methane, even in places where ancient organic carbon has transformed into graphite or other inorganic forms. Researchers are exploring the sources of this methane to figure out how it is produced and its role in the deep carbon cycle.  

Riikka Kietäväinen, a member of DCO’s Deep Energy and Deep Life Communities, along with Lasse Ahonen, also a member of the Deep Life Community, (both at the Geological Survey of Finland, in Espoo), and colleagues investigated factors that control methane formation at two sites in Finland: the Outokumpu Deep Drill Hole and the Pyhäsalmi copper-zinc mine, which both extend about 2.5 kilometers into Precambrian bedrock.

In a new paper in Geochimica et Cosmochimica Acta, the researchers combine isotopic measurements from water, gas, and minerals sampled from the two sites with existing chemical and microbiological analyses [1]. They find that methane levels depend on the surrounding rock type, with metasedimentary rocks yielding more methane than volcanic or igneous ones. Their results also suggest that methane at deeper levels likely came from chemical processes such as the interactions between hydrogen and inorganic intermediates like graphite, and that methane generated by microbes mainly occurs up to 1.5 kilometers deep.

“I think it’s surprising to see how common it is to have hydrocarbons in crystalline rocks,” said Kietäväinen. While methane occurs in both sites, she points out, “Lithology really matters. The gas compositions, concentrations and isotopic compositions are different depending on what rock types we had.”

The Outokumpu Deep Drill Hole is the deepest scientific drill hole in Finland, reaching a depth of 2,516 meters. It is located in eastern Finland within 1.9 billion year old bedrock, which is primarily metasedimentary. The second site is the Pyhäsalmi copper-zinc mine, located about 180 kilometers northeast of Outokumpu. The mine itself reaches 1,430 meters below the surface, but an additional borehole descends 2,400 meters deep and runs through mostly metamorphic volcanic rocks. 

Researchers collect samples in the Pyhäsalmi mine. Credit: Arto Pullinen, GTK

Microbe-generated methane occurred most clearly at depths above 1.5 kilometers, which may be the result of high hydrogen concentrations deeper in the sites. Though many methanogens use hydrogen to form methane, excess hydrogen inhibits their activities. The researchers suspect that the methane farther down stemmed from chemical processes involving carbon that was once organic, such as graphite.  Many microbes live at depths of 2.5 kilometers, but because scientists know little about their capabilities, if they are making biogenic methane, it may be very different from typical biogenic methane, and the methods from the study would not detect it.

Besides expanding our understanding of methane formation in crystalline bedrock, the findings also will be useful for the Finnish Research Program on Nuclear Waste Management, which co-funded the work with the Academy of Finland and Geological Survey of Finland. Finnish officials have made plans to store nuclear waste in a repository located 500 meters below the surface, which is similar to the Outokumpu site. They hope to avoid any problems related to natural gas dissolved in the groundwater. “Knowing where the methane comes from helps us to plan how to avoid these mine gas problems,” said Kietäväinen.

In future work, the researchers plan to analyze other gases at locations across Finland and to further investigate the contributions of crystalline bedrock to the deep carbon cycle and to gas flux.

 

Images: Top: A groundwater sampling tube descends into the Outokumpu Deep Drill Hole. Credit: Riikka Kietäväinen. Middle: Researchers collect samples in the Pyhäsalmi mine. Credit: Arto Pullinen, GTK

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