Natural gas remains a widely used fossil transition fuel as the world transitions from coal to cleaner energy sources. While considerable conventional reserves exist on various continents, since the beginning of this century a significant part of this natural gas has been produced from so-called unconventional hydrocarbon deposits such as black shale, particularly in North America.
Historically, there have been three origins in the formation of natural gas. Microorganisms that feed on organic material produce gas microbial formed in oxygen-free environments and at low temperatures. When the black shales are buried at depths or exposed to high temperatures during their geological history, the same organic matter decomposes into something called gas thermogenic. Finally, when certain rocks weather upon contact with water, complex chemical reactions involving natural hydrogen gas are produced abiotic.
The study, conducted by BRGM researchers and Canadian and Swiss experts, shows that there is a fourth source of natural gas produced by radiolysis organic matter in shale. In fact, black shales often contain radioactive elements such as uranium in addition to organic material. Their natural radioactivity can interact with organic material, often over hundreds of millions of years, to create this new type of gas “radiolytic”, with methane, ethane and propane. Without being radioactive itself, it can mix with thermogenic and possibly microbial gases in the shale. At first glance, this gas cannot be distinguished from other types of natural gas. Only a detailed analysis of its chemical nature and the isotopic signatures of the carbon that makes it up allowed the research team to highlight its existence in many shale formations around the world.
A study carried out as part of the European H2020 SECURE project
This scientific study, carried out within the framework of the European project SECURe (Horizon 2020 program), in addition to a better understanding of the evolution of gases from unconventional reservoirs, could help to better determine the possible existence of leaks towards near-surface geology. Indeed, when groundwater and soil monitoring in the vicinity of shale gas operations shows high concentrations of methane, these can be linked to the migration of gas from various underlying geological formations, but also to the environmental background. In particular, this can be biogenic gas produced close to the surface, which is not related to the gas being pumped. Considering the radiolytic origin of some of the gas in certain shale formations allows for a better identification of the deep reservoir gases, which is an important question for assessing the risks and potential impacts of the exploitation of natural gas in unconventional reservoirs.
Natural Gas of Radiolytic Origin: An Overlooked Component of Shale Gas (Proceedings of the National Academy of Sciences, 2022) – by Naumenko-Dèzes M., Kloppmann W., Blessing M., Bondu R., Gaucher E., Mayer B.
Founded in 1914, PNAS (Proceedings of the National Academy of Sciences) is the official journal of the United States Academy of Sciences. It is recognized as one of the most prestigious peer-reviewed multidisciplinary scholarly media, publishing several thousand articles annually on cutting-edge research.