This debate began decades ago: How much of the planet’s natural gas is abiotic – made up of methane with nonbiological origins? At times, the scientific back-and-forth argument has resembled a slow-motion tennis match, with a new volley coming every few years.
Now, Daniel Xia thinks he has helped deliver a winning smash across the net.
Tiny amounts of abiotic methane may exist on Earth, but “it’s much less than previously believed,” he stated. In fact, he said, almost all methane on the planet has biological origins.
Xia is a geochemist and geological adviser for Apache Corp. in Houston and a research volunteer for the University of Texas at San Antonio. His years of research findings lead to his current view of nonbiological methane.
Those findings were most clearly laid out in the November 2021 review article “Validity of geochemical signatures of abiotic hydrocarbon gases on Earth,” by Xia and coauthor Yongli Gao, published in the Journal of the Geological Society.
“Case studies demonstrate that previously proposed abiotic hydrocarbon gases in continental serpentinite-hosted seepages, continental or oceanic hydrothermal vents, volcanic emissions, gas fields in volcanic reservoirs and fluid inclusions in alkaline or granitic rocks were formed by various biotic processes,” the authors wrote.
“The occurrence of abiotic hydrocarbon gas with decisive evidence is limited to fluid inclusions in some mantle minerals,” they said.
Conspicuous Examples
In their paper, Xia and Gao first considered the proposed sources of abiotic hydrocarbon gas, then the proposed supporting geochemical evidence, and then specific locations of the world where abiotic methane is thought to be produced.
And then, they basically rebutted and whacked down every one of the proposals for abiotic methane origin, demonstrating in each case how a biological origin appeared more likely and more logical.
One of the locations addressed was the Kidd Creek metal mine within the Abitibi greenstone belt in Ontario, Canada. Kidd Creek gas “is a seepage in continental serpentinization, one of the two poster children for the abiogenesis hypothesis. The other type is hydrothermal fields, such as Lost City in the mid-Atlantic ridge,” Xia noted.
“For 20 years it has been recorded as a poster-child example of abiotic gasses. But they didn’t give any consistent mechanical explanation,” he said.
In fact, the Kidd Creek methane and abiotic gas theories were detailed by the EXPLORER 20 years ago – this is a very, very slow tennis match – in a November 2002 article titled “Gas origin theories to be studied.”
Kidd Creek gas displays several anomalies, including isotopic irregularity, that some researchers interpreted as evidence of abiotic origin. But Xia and Gao found the gas compositional features “point to a generation of the gaseous alkanes through microbial hydrogenolysis of long alkyl chains.”
They said their geochemical interpretation “shows that some hydrocarbon gases from pre-GOE (the Great Oxidation Event in the Paleoproterozoic era) serpentinization sites are most probably the oldest preserved microbial gas” in the world.
Explanation of the Error
Xia commented that much of abiotic gas theory has arisen from empirical rather than analytical observation, based on methane characteristics that weren’t predicted, or methane presence that wasn’t expected.
“We know the empirical explanation tends to ignore the microbial activity, so that is not solid,” he said, and added, “You find that a lot of empirical information for abiotic gas is circular logic.”
Geochemists studying early unconventional natural gas production were surprised to discover a reversal of the expected isotopic sequence in some gas. Researchers had identified a normal sequence of carbon isotopes, but this was different, with reversed carbon isotopic maturity trends for ethane, propane and carbon dioxide – where, for example, δ13C-CH₄(methane)> δ13C-C₂H₆(ethane)> δ13C-C₃H₈(propane).
“In the early 2000s in the United States, they systematically studied the isotopic composition of shale gas. At that moment, we knew that isotopic reversal was more common than previously thought,” Xia said.
Studies of production from the Barnett and Fayetteville shales started to show this isotope sequence reversal, sometimes referred to as “rollover,” in high thermal maturity gas. That finding eventually led to Xia’s interest in refuting abiotic gas theory.
“I started to work in this subject in the 1990s during my graduate study in the Chinese Academy of Sciences, when some gas fields in the Songliao Basin with reverse carbon isotopic order – methane is more enriched in 13C than ethane, and so on – were regarded as abiotic commercial gas fields,” Xia explained.
“Only, after the shale gas boom, people found that the reverse order exists in the Arkoma and Appalachian basins, which is a character of high thermal maturity of organic matter and has nothing to do with abiogenesis. Since then, I aimed to explain all the abnormal isotopic phenomena in gas reservoirs and gas seeps,” he said.
Other proposed theories of abiotic methane generation have relied on naturally occurring Fischer-Tropsch-type catalysis or the results of low-temperature serpentinization, the hydrothermal alteration of mantle rock. Xia firmly rejects both ideas.
Conditions for effective reaction with metal catalysts can be created in the lab, but not so easily in nature, he noted.
“If you want to have free metal, iron or nickel, you really need a very reduced condition. The more serious problem is the effectiveness of the catalyst. We all know we don’t want sulfur in our FTT processes, but in hydrothermal fluids, sulfur compounds exist commonly,” he said.
So far, experiments in long-term, low-temperature serpentinization effects have failed to produce any meaningful link with methane production, Xia said, and observed, “this argument really has been denied several decades ago. There is no evidence that methane can be generated from the low-temperature serpentinization.”
Additionally, more unorthodox theories of abiotic gas generation also have been proposed, generally by contrarian scientists or by ultra-deep drillers hoping for a steady supply of nonbiological methane. Those ideas gained some currency years ago because of an almost desperate desire to believe the world was not running out of natural gas.
Younger scientists might find it hard to imagine, but at one time the industry, and even organizations like the U.S. Geological Survey, predicted a looming shortage of natural gas. Based on projected output numbers, some experts theorized that around the year 2020 the United States would produce only a few trillion cubic feet of gas a year, with a small reserve remaining.
Forget the suspense. U.S. dry natural gas production in 2020 totaled about 33.5 trillion cubic feet and the country had close to a 100-year reserve of technically recoverable gas, according to the U.S. Energy Information Agency.
People now are more likely to talk about an over-abundance of methane: fugitive emissions and gas flaring, methane from landfills, methane from agricultural practices and farm animals, methane everywhere.
And all of it, as Xia would note, derived from life.
Extraterrestrial Methane
Today, interest in methane production and content has shifted to other parts of the solar system, to Jupiter and Saturn and especially one of Saturn’s moons, Titan, thought to be rich in methane. Xia and Gao address this briefly in their paper:
“Primordial CH₄ was formed from carbon hydrogenation on interstellar dust grains … higher alkanes were formed through CH₄ irradiation, C-C chain addition and C-C chain hydrogenation on the dust grains ... These hydrocarbons exist in the outer solar system,” they wrote.
Earth, too, might have once had its share of primordial methane – now long gone, according to Xia. Any early volatiles “should have escaped because of the (early-Earth) high-temperature bombardment” and degassing, he said.
Xia views his research into abiotic gas as more than an academic interest, and important to his work in the industry. If “it is commonly admitted that abiotic gas can’t form commercial reservoirs, why does abiogenesis still matter to hydrocarbon exploration?” Xia asked.
He gave three reasons:
- “It affects the interpretation of geochemical data in the presence of high-maturity source rocks.”
- “It influences the understanding of chemical process of late-gas generation, which further impacts the evaluation of gas resources in high-maturity source rocks.”
- “It is related to the distribution of ‘gold hydrogen’, the free hydrogen gas in geological reservoirs.”
A Final Verdict?
Summarizing their research into abiotic gas theories, Xia and Gao found no evidence to support any meaningful presence of nonbiological methane on the planet.
“For all the cases and occurrences discussed in this paper, abiogenesis has not been found necessary to explain the atypical geochemical signatures in hydrocarbons. Therefore, abiotic CH₄ is far rarer than previously believed,” they wrote.
“The only occurrence of abiotic hydrocarbons with relatively solid evidence is some CH₄ in primary fluid inclusions of some mantle minerals (such as diamond), but its contribution to gas seepages is negligible compared with hydrogenotrophic microbial CH₄ during serpentinization,” they concluded.
Debate about the potential for abiotic methane might not be over quite yet. In a few years, someone could still lob a scientific study or two back over the net in rebuttal.
But for now, the ball is in the other court.