Photograph by Rebecca Hale
Published July 30, 2009
The standard recipe for oil and natural gas is simple: Take animal or plant remains, bury them under layers of Earth's crust, turn up the pressure and temperature, and set a very, very long timer.
But a new study suggests that Earth could be cooking up the same finished products using a few substitutions.
By mimicking the extreme conditions found deep inside Earth, scientists have created the chains of carbon and hydrogen that make up so-called fossil fuels—without the fossils.
The feat may be a boost to an unorthodox theory that Earth could hold significant amounts of abiotic, or life-free, fuels far below conventional oil reserves.
Experts caution, however, that even if such reserves exist, exploiting them commercially could pose a challenge.
Diamonds and Lasers
Most of today's oil comes from deposits found a mere three to five miles (five to eight kilometers) below Earth's surface.
But Vladimir Kutcherov and colleagues wanted to know if fossil fuels could form where no organic matter exists: the upper mantle, 40 to 95 miles (65 to 150 kilometers) underground.
For their raw material, the team started with methane—a component of natural gas—that had been previously produced in the lab from only water and minerals.
The scientists crushed the "artificial" methane between two diamonds and heated it with a laser to re-create conditions thought to exist in Earth's mantle—although with a much shorter "cooking time" than what would be needed in nature.
The lab technique created pressures more than 20,000 times those found at sea level and temperatures topping 2,240 degrees F (1,227 degrees C).
Under these conditions, the methane reacted to produce a blend of ethane, propane, butane, molecular hydrogen, and graphite. "This is not just an artificial hydrocarbon mixture," said Kutcherov, of Sweden's Royal Institute of Technology.
"This mixture is very similar to the composition of natural gas."
Lab-made ethane subjected to the same temperatures and pressures produced methane, the team reports this week in the online version of the journal Nature Geoscience.
The reversibility suggests a hydrocarbon production cycle could be at work deep inside Earth's mantle, Kutcherov said.
New Energy Source?
The study addresses the controversial view, first proposed by Soviet geologists in the 1950s, that deep Earth holds reserves of oil made from just minerals and water.
Some scientists have even suggested that material from these deeper reservoirs occasionally migrates to the surface and may help to replenish known oil fields.
Henry Scott, of Indiana University South Bend, was part of a 2004 team that made inorganic methane from marble under simulated "deep Earth" conditions.
Production of heavier hydrocarbons from methane is a "big step forward," said Scott, who was not part of the new study.
The work makes it seem increasingly likely that some abotic hydrocarbons can form in the deep Earth.
However, he cautioned, there is little to suggest that commercially important amounts of oil, gas, and other hydrocarbons have fossil-free origins.
"There is simply an overwhelming body of evidence suggesting [commercial deposits] form from the decay of once-living things," he said.
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