Methane in the subsurface can come from three sources: microbes, the high-temperature breakdown of organic matter, which yields thermogenic methane, or abiotic chemical processes that are independent of life. For decades, scientists have distinguished between these three “genetic fields” based on a gas sample’s isotopic composition, and the relative amounts of methane, ethane, and propane it contains. In the 1970s and 1980s, gas geochemists used these characteristics to create handy diagrams to determine the origin of gas samples, but these tools were in need of an update.
The original diagrams were based on the very small number of gas samples available at the time – sometimes just tens or hundreds of measurements for each field – and leave out thousands of recent natural gas measurements. DCO Deep Energy Community member Giuseppe Etiope (Istituto Nazionale di Geofisica e Vulcanologia, Italy) worked with Alexei Milkov (Colorado School of Mines, USA) to update these diagrams to include measurements from more than 20000 gas samples. The new diagrams more closely reflect the full range of natural gas sample composition and also take into account not just conventional petroleum reserves, but other methane sources such as coal beds, freshwater sediments, and surface seeps, from around the world. The researchers published the revised diagrams in a new paper in Organic Geochemistry .
“These diagrams are a tool used by everybody – geologists, microbiologists, and gas geochemists for gas exploration – to report whether the gas is microbial, thermogenic or abiotic,” said Etiope. “However, we have seen that many, many data points fall outside these fields, so this means that the fields defined so far are based on limited data.”
The diagrams work on the principle that gases from the same genetic field tend to have similar characteristics. The characteristics useful for discerning a gas sample’s origin are the concentrations of carbon and hydrogen isotopes, which are atoms of an element with a different numbers of neutrons in the nucleus, and the ratios of different gases. By plotting these characteristics for numerous samples, scientists can create diagrams of the known range for each field. Then when researchers analyze a new gas sample, they can map the results onto the diagram and get a good idea of its origin.
Etiope’s colleagues and mentors, Martin Schoell and Michael Whiticar, developed the earlier diagrams in the 1980s. Since then, a handful of researchers, including Milkov, have expanded the fields and added subcategories in the diagrams, such as different types of microbial methane, but until the recent study, no one had attempted a comprehensive revision.
Etiope and Milkov compiled data from 20621 natural gas samples collected from multiple geological habitats across 76 countries. They used this database of gas measurements to redraw the genetic diagrams to include the total known variability in methane samples. In the new diagrams, the ranges for abiotic and thermogenic methane have both expanded. With the addition of recent data points from China and Canada, the field for thermogenic methane now has greater overlap with the microbial field. Newer sampling also shows that abiotic methane, which was once thought to have the highest concentration of the carbon-13 isotope, sometimes has lower 13C concentrations that looks more like thermogenic gas.
“We are complicating this story,” admits Etiope. While the updated diagrams more accurately reflect the natural variety of these gases, the greater overlap means that the charts cannot always provide a clear-cut answer for a sample’s origin. Etiope thinks that scientists should take a “holistic gas geochemistry” approach. “We cannot use these gas diagrams as a standalone tool,” he said. “We should also examine the geological context of the gas sample. Only a holistic approach can give us a clear picture of the origin of methane.”
The new diagrams are the most comprehensive genetic diagrams yet published and are still a useful starting point for gas and petroleum explorations, basic research in geology and microbiology, and even for forensic analysis, when investigating the possibility of environmental contamination.
“We are confident that this diagram will help methane scholars in future studies,” said Etiope.
Main image: The new genetic field diagrams for natural gas include data from more than 20000 samples, collected from traditional oil and gas reservoirs, as well as other geological habitats such as mud volcanoes, marine sediments, hydrothermal vents, and gas hydrates. Credit: USDA NRCS Texas; Michael C. Rygel; Rogers AD, Tyler PA, Connelly DP, Copley JT, James R, et al., via Wikimedia Commons