Pressure makes hydrocarbons

UC DAVIS (US) — At deep Earth pressures and temperatures, longer hydrocarbons may form 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, says Giulia Galli, professor of chemistry and of physics at the University of California, Davis, and senior author of the study published in the journal Proceedings of the National Academy of Sciences.

“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 says.

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 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’,” says 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.

Collaborators include researchers from the Max Planck Institute in Mainz, Germany; Shell Global Solutions, Houston; and Lawrence Livermore National Laboratory. 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.

The research was supported by Shell.

More news from UC Davis: http://news.ucdavis.edu/