Understanding the origins of a methane reservoir can reveal a lot about what’s happening in the subsurface. Scientists have used isotopes, which are atoms with different numbers of neutrons, to determine whether gas is microbial (from microorganisms), thermogenic (from the breakdown of old organic matter at elevated temperatures), or abiogenic (from chemical reactions in rocks independent of life). But when gases from different sources migrate, mix or escape, the task of teasing apart their origins is especially challenging.
New analytical tools that detect rare molecules of methane with multiple carbon and hydrogen isotopes, called clumped isotopologues, can yield even more information about methane’s origins, but researchers are still figuring out how to interpret the data. A new paper in Geochimica et Cosmochimica Acta, advances that goal by looking at clumped isotopologues from previously studied methane reservoirs in two sedimentary basins with a mix of microbial and thermogenic gas. The study, performed by Thomas Giunta (formerly at University of Toronto, Canada; now at Université de Bretagne Occidentale, France), with DCO members Oliver Warr, Barbara Sherwood Lollar (both at University of Toronto, Canada), Edward Young, Issaku Kohl, Jeanine Ash (all at the University of California, Los Angeles, USA), Douglas Rumble (Carnegie Institution for Science, USA), Ileana Pérez-Rodríguez (University of Pennsylvania, USA), and Doug LaRowe (University of Southern California, USA), and colleagues, further clarifies the relationship between clumped isotopologue measurements and the origins and evolution of methane in sedimentary basins.
“Tracing methane’s origin is not always trivial, particularly when deciphering if it has a biogenic or abiogenic source,” said Giunta, noting that methane is not only key to understanding the deep biosphere and the global carbon cycle, but also has great economic importance.
Giunta and his colleagues collected gas and water samples from active natural gas wells at two sites, one in Canada and one in the U.S., believed to contain a mix of microbial and thermogenic methane. Gas from the Michigan Basin comes from the Antrim shale, which dates back to the Devonian Period (about 419 to 359 million years ago) and may contain up to 80 percent microbial gas. In the adjacent Southwest Ontario basin, the methane comes from older layers dating back to the Cambrian Period (about 541 to 485 million years ago), the Ordovician Period (about 485 to 443 million years ago), and the Silurian Period (about 443 to million 419 million years ago).
After collecting gas samples, the researchers separated methane from other gases, such as carbon dioxide and water vapor, using a gas chromatograph, and injected it into Panorama , a high-resolution mass spectrometer developed by Young and colleagues. Panorama can measure concentrations of methane with “light” isotopes (CH4), and “heavy” isotopes with extra neutrons. More importantly, it can differentiate between rare clumped isotopologues, such as methane molecules that include a carbon atom with an extra neutron and a heavier atom of hydrogen called deuterium (13CH3D), and molecules with two atoms of deuterium (CD2H2). DCO helped support the creation of Panorama and also provided funding for operating the instrument in the current study.
In a methane sample, the relative abundance of these clumped isotopologues can be used to distinguish whether or not the methane is at thermodynamic equilibrium with its environment. More generally, these novel tracers can indicate where a methane sample came from, whether it mixed with other sources or underwent degradation, and in some cases, the temperature when it formed.
Panorama’s measurements of clumped isotopologues supported previous findings that both basins contain microbial and thermogenic gas. But samples from the basins each had their own distinct signatures.