A snapshot taken from a first-principles molecular dynamics simulation of liquid methane in contact with a hydrogen-terminated diamond surface at high temperature and pressure. The spontaneous formation of longer hydrocarbons are readily found during the simulations. Image: Lawrence Livermore National Laboratory
new computational study published in the Proceedings of the National Academy
of Sciences reveals how hydrocarbons may be formed from methane in deep
Earth at extreme pressures and temperatures.
thermodynamic and kinetic properties of hydrocarbons at high pressures and
temperatures are important for understanding carbon reservoirs and fluxes in
work provides a basis for understanding experiments that demonstrated
polymerization of methane to form high hydrocarbons and earlier methane forming
reactions under pressure.
are the main building block of crude oil and natural gas. Hydrocarbons
contribute to the global carbon cycle (one of the most important cycles of the
Earth that allows for carbon to be recycled and reused throughout the biosphere
and all of its organisms).
team includes colleagues at UC Davis, Lawrence Livermore National Laboratory
and Shell Projects & Technology. One of the researchers, UC Davis Professor
Giulia Galli, is the co-chair of the Deep Carbon Observatory’s Physics and
Chemistry of Deep Carbon Directorate and former LLNL researcher.
and geochemists believe that nearly all (more than 99%) of the hydrocarbons in
commercially produced crude oil and natural gas are formed by the decomposition
of the remains of living organisms, which were buried under layers of sediments
in the Earth’s crust, a region approximately 5-10 miles below the Earth’s
hydrocarbons of purely chemical deep crustal or mantle origin (abiogenic) could
occur in some geologic settings, such as rifts or subduction zones said Galli, a
senior author on the study.
simulation study shows that methane molecules fuse to form larger hydrocarbon
molecules when exposed to the very high temperatures and pressures of the
Earth’s upper mantle,” Galli said. “We don’t say that higher
hydrocarbons actually occur under the realistic ‘dirty’ Earth mantle
conditions, but we say that the pressures and temperatures alone are right for
it to happen.
and colleagues used the Mako computer cluster in Berkeley and computers at
Lawrence Livermore 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 and the computer
software system Qbox, developed at UC Davis.
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 Fahrenheit) and pressures 50,000 times those at the
Earth’s surface (conditions found about 70 miles below the surface).
the simulation, interactions with metal or carbon surfaces allowed the process to
occur faster—they act as ‘catalysts,’ ” said UC Davis’ Leonardo Spanu, the
first author of the paper. The research does not address whether hydrocarbons
formed deep in the Earth could migrate closer to the surface and contribute to
oil or gas deposits. However, the study points to possible microscopic
mechanisms of hydrocarbon formation under very high temperatures and pressures.
Galli’s co-authors on the paper are Spanu; Davide Donadio at the Max Planck
Institute in Meinz, Germany; Detlef Hohl at Shell Global Solutions, Houston;
and Eric Schwegler of Lawrence Livermore National Laboratory.