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Hydrocarbons in the deep Earth

A computer modeling study published in the journal Proceedings of the National Academy of Sciences shows that at deep Earth pressures and temperatures, longer hydrocarbons may be formed from the simplest one, the methane molecule.

Hydrocarbon molecules are the main building blocks of crude oil and natural gas, and determining their thermochemical properties is important to understand carbon reservoirs and fluxes in the Earth.

Geologists and geochemists believe that nearly all of the hydrocarbons in commercially produced crude oil and natural gas are formed by the decomposition of the remains of living organisms buried under layers of sediments in the Earth's crust, a region that extends five to 10 miles below the Earth's surface.

But "abiogenic" hydrocarbons of purely chemical deep crustal or mantle origin could occur in some geologic settings, such as rifts or subduction zones, said Giulia Galli, professor of chemistry and of physics at UC Davis and senior author on the study.

"Our simulation study shows that methane molecules can combine to form larger hydrocarbon molecules when exposed to the very high temperatures and pressures of the Earth's upper mantle. We don't say that higher hydrocarbons actually occur under the realistic 'dirty' Earth mantle conditions, but the pressures and temperatures are right," she said.

Galli and her colleagues used the University of California's Mako computer cluster in Berkeley and computers at the Lawrence Livermore National Laboratory to simulate the behavior of carbon and hydrogen atoms at the enormous pressures and temperatures found 40 to 95 miles deep inside the Earth.

They used sophisticated techniques based on first principles (the basic properties of carbon and hydrogen atoms) and the computer software system Qbox, developed at UC Davis by Francois Gygi, a professor in the Department of Computer Science.

The researchers found that hydrocarbons with multiple carbon atoms can form from methane, (a molecule with only one carbon and four hydrogen atoms) at temperatures greater than 1,500 K (2,240 degrees F) and pressures 50,000 times those at the Earth's surface, conditions found about 70 miles below the surface.

"In the simulation, interactions with metal or carbon surfaces allowed the process to occur faster; they act as 'catalysts'," said Leonardo Spanu, assistant researcher at UC Davis and the first author of the paper.

The research does not address whether hydrocarbons formed that deep in the Earth could migrate closer to the surface and contribute to exploitable oil or gas deposits. However, the study is fundamentally important because it points to possible microscopic mechanisms of hydrocarbon formation under very high temperatures and pressures.

Galli and some of her collaborators at UC Davis are part of a larger project, the Deep Carbon Observatory, supported by the Alfred P. Sloan Foundation; Galli is co-chair of the observatory's Physics and Chemistry of Carbon directorate. The aim of the observatory is to study the Earth's carbon cycle, including the presence of hydrocarbons and the possibility of microbial life deep in the planet.

Galli's co-authors are Davide Donadio at the Max Planck Institute in Mainz, Germany; Detlef Hohl at Shell Global Solutions, Houston; and Eric Schwegler, Lawrence Livermore National Laboratory.

The research was supported by Shell.

Andy Fell | EurekAlert!
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