As economic natural gas accumulations are not known from Finland, gases have often been overlooked in bedrock and groundwater studies. However, gases, including methane, nitrogen, hydrogen and helium, can also be found in the Precambrian crystalline bedrock of the Fennoscandian Shield. The gases are mainly dissolved in water, but bubbling is also seen here and there. For example, the existence of sometimes explosive mine gases has been known for decades. The formation of gases can also potentially mobilize hazardous compounds, cause the migration of fluids and increase microbial activity. Thus, the study of gases also supports the safety assessment of underground construction sites, besides mining environments, including sites for the geological disposal of nuclear waste.
In a recent study led by the Geological Survey of Finland and co-funded by the Academy of Finland and the Finnish Research Programme on Nuclear Waste Management (KYT2014 and KYT2018 grants to projects SALAMI and RENGAS, respectively), the presence and isotopic composition of methane and other hydrocarbons and related fluid and mineral phases were examined at the Outokumpu Deep Drill Hole and the Pyhäsalmi Mine (Figure 1), and the results were compared with previous data from fourteen other sites across Finland and one site in Sweden. The deepest samples were taken from a depth of almost 2.5 km, and more than a hundred new isotope analyses were conducted.
Comparison of gases and regional lithology demonstrated that methane is relatively abundant in metasedimentary regions (Figure 2). The rock types in these areas today comprise schists and gneisses. Indeed, the selected nuclear waste disposal site at Olkiluoto is among the most methane-rich sites found in Finland, as concluded in this study. An especially large amount of methane was found in areas consisting of organic carbon-rich rock types, black schists. In these areas, up to 90% of the gas was composed of hydrocarbons. In contrast, very little methane was found from granitic and ancient volcanic regions.
The emergence of methane is affected by both geological and microbiological processes (Figure 3). Worldwide, organic matter is the main source of methane found in wetlands and sedimentary formations. According to our study, this also applies to ancient crystalline bedrock. However, the formation mechanisms are different. In contrast to the traditional natural gas, which is formed by the break-up of organic matter at elevated pressures and temperatures, hydrocarbons generated in the crystalline bedrock most likely form at low temperatures and probably through inorganic intermediates, mainly graphite. Methanogenic microbes have also been detected deep within the bedrock. Microbially produced methane is particularly found within the upper 1.5 km depth. Deeper down, methane appears to be dominantly abiotic, which is evidenced, among other features, by its exceptionally heavy isotope composition. Based on isotopic analysis of fracture calcites, indications of methane-oxidizing microbes could also be found near the active fracture zones in the upper 1 km of the bedrock at Outokumpu.
This study was published in Geochimica et Cosmochimica Acta (vol. 202, pages 124-145). (http://www.sciencedirect.com/science/article/pii/S0016703716307293)
- Kietäväinen R., 2017. Deep groundwater evolution at Outokumpu, eastern Finland: from meteoric water to saline gas rich fluid. PhD Thesis, Geological Survey of Finland, Espoo. 37 p., 1 appendix, with original articles I-IV.
- Kietäväinen R. & Purkamo L., 2015. The origin, source and cycling of methane in deep crystalline rock biosphere. Frontiers in Microbiology 6, 725, doi: 10.3389/fmicb.2015.00725
- Miettinen H., Kietäväinen R., Sohlberg E., Numminen M., Ahonen L. & Itävaara M., 2015. Microbiome composition and geochemical characteristics of deep subsurface high-pressure environment, Pyhäsalmi mine Finland. Frontiers in Microbiology 6, 1203, doi: 10.3389/fmicb.2015.01203
Text: Riikka Kietäväinen
Dr Riikka Kietäväinen is a research scientist in the Bedrock Construction and Site Assessment unit of the Geological Survey of Finland. In her work, she has specialized in isotope geochemistry and hydrogeology with a focus on deep groundwaters, gases, subsurface ecosystems and water–rock interaction applied to the long-term safety of the geological disposal of nuclear waste.