New research
is expanding the range of the formation of fossil fuels. Experiments in the
laboratory have produced abiogenic methane — gas not produced by living
organisms — at temperatures and pressures similar to that of the upper
mantle. The findings could change the way scientists view hydrocarbons on this
planet and others.
Looking into a diamond anvil using a microscope, the extreme heat from the creation
of methane is visible. Researchers subjected common minerals to temperatures
and pressures similar to those in the upper mantle to create “fossil-less”
fuel in a lab. Image courtesy of Henry Scott.
A group of researchers led by Henry Scott at Indiana University, South Bend,
placed iron oxide, calcite and water into a diamond anvil cell and cranked up
the pressure to more than 5 gigapascals (50,000 times the pressure at sea level).
Using a variety of methods, including focused laser light, the researchers heated
the sample to scalding temperatures exceeding 1,000 degrees Celsius — producing
temperatures and pressures analogous to the upper mantle, around 100 kilometers
below Earth’s surface. The results, presented in the Sept. 14 Proceedings
of the National Academy of Sciences, include the formation of methane, which
is generally believed to be biological in origin.
“There has been a longtime assumption that the only source of hydrocarbons
is biogenic — hydrocarbons created by microbes and decaying organic matter,”
says Barbara Sherwood Lollar, a geochemist at the University of Toronto, who
is not involved with the research. But “abiogenic hydrocarbons are real,”
she says.
Conventional wisdom suggests that hydrocarbons are not stable at the mantle’s
high temperatures and pressures. The new experiment, however, shows that “methane,
at least, is thermodynamically stable and could exist for long periods of time
in the Earth,” says Laurence Fried, a computational chemist at Lawrence
Livermore National Laboratory in California, who collaborated on the project.
That finding is consistent with previous work by a handful of Russian and American
scientists who have championed the highly contentious hypothesis that all hydrocarbons
are abiogenic in origin. Scott says, however, that the new study does not confine
hydrocarbons’ origin to abiogenic sources and does not address the question
of how widespread abiogenic methane is.
Methane is the main component of natural gas and is the most abundant hydrocarbon
in Earth’s crust. Because the new experiment uses common minerals and could
occur across a range of temperature and pressure conditions, Fried says that
“there are potentially very large deposits of methane.” Drilling to
the mantle is implausible; however, hydrocarbons might travel upward into the
shallow crust where extraction is possible. “We know fluids like carbon
dioxide can make it up,” Scott says, “so theoretically methane can
come up too.”
Although abiogenic hydrocarbon is still controversial, most researchers believe
that some abiogenic methane exists in small quantities in the crust. Sherwood
Lollar says that any abiogenic methane resources are probably too small to warrant
exploration and production. “There are a few small mining companies using
abiogenic methane locally for their own purposes,” Sherwood Lollar says.
“But I don’t think this is the next answer to the oil crisis.”
Compared to conventional hydrocarbon deposits, abiogenic methane traps are just
a drop in the bucket, she says.
Although abiogenic methane may not be a new energy source for people, it could
fuel chemosynthetic bacteria in the crust. “We used to think that life
stopped very quickly with depth,” Sherwood Lollar says. “In the last
10 to 20 years, we’ve discovered that Earth, in fact, has a deep biosphere.”
If other planets or moons are producing abiogenic methane, she says, it will
help scientists design strategies and techniques for sampling in the hunt for
life on other planets.
Jay Chapman
Geotimes contributing writer
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