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Researchers Produce Ethane and Other Heavier Hydrocarbons From Methane Under Deep Earth Conditions; Further Deep Carbon Research

There has been some scientific debate for years over whether oil and natural gas hydrocarbons could have been created deeper in the Earth and formed without organic matter (abiogenic petroleum), in addition to their formation from living organisms that died, were compressed, and heated under heavy layers of sediment in the Earth’s crust. Now, a team of researchers has found that ethane and heavier hydrocarbons can be synthesized from methane under the pressure-temperature conditions of the upper mantle—the layer of Earth under the crust and on top of the core.

The research was conducted by scientists at the Carnegie Institution’s Geophysical Laboratory, with colleagues from Russia and Sweden, and was published in the 26 July advanced on-line issue of Nature Geoscience.

Other studies have found some evidence of abiogenic methane, but not in significant quantities. Methane (CH4) is the main constituent of natural gas, while ethane (C2H6) is used as a petrochemical feedstock. Both of these hydrocarbons, and others associated with fuel, are called saturated hydrocarbons because they have simple, single bonds and are saturated with hydrogen.

Using a diamond anvil cell and a laser heat source, the scientists first subjected methane to pressures exceeding 20 thousand times the atmospheric pressure at sea level and temperatures ranging from 1,000 K to 1,500 K (1,300 F° to 2,240 F°; 727 °C to 1,227 °C). These conditions mimic those found 40 to 95 miles deep inside the Earth.

The methane reacted and formed ethane, propane, butane, molecular hydrogen, and graphite. The scientists then subjected ethane to the same conditions and it produced methane. The transformations suggest heavier hydrocarbons could exist deep down, according to the research team. The reversibility implies that the synthesis of saturated hydrocarbons is thermodynamically controlled and does not require organic matter, they said.

The scientists ruled out the possibility that catalysts used as part of the experimental apparatus were at work, but they acknowledge that catalysts could be involved in the deep Earth with its mix of compounds.

We were intrigued by previous experiments and theoretical predictions. Experiments reported some years ago subjected methane to high pressures and temperatures and found that heavier hydrocarbons formed from methane under very similar pressure and temperature conditions. However, the molecules could not be identified and a distribution was likely. We overcame this problem with our improved laser-heating technique where we could cook larger volumes more uniformly. And we found that methane can be produced from ethane.

—Carnegie’s Alexander Goncharov, co-author

The hydrocarbon products did not change for many hours, but the tell-tale chemical signatures began to fade after a few days.

The notion that hydrocarbons generated in the mantle migrate into the Earth’s crust and contribute to oil-and-gas reservoirs was promoted in Russia and Ukraine many years ago. The synthesis and stability of the compounds studied here as well as heavier hydrocarbons over the full range of conditions within the Earth’s mantle now need to be explored. In addition, the extent to which this “reduced” carbon survives migration into the crust needs to be established (e.g., without being oxidized to CO2). These and related questions demonstrate the need for a new experimental and theoretical program to study the fate of carbon in the deep Earth.

—Professor Kutcherov, co-author

This research was supported by the US Department of Energy, the National Nuclear Security Agency through the Carnegie/DOE Alliance Center, the National Science Foundation, the WM Keck Foundation, and the Carnegie Institution.

Deep Carbon Observatory. On 1 July, the Alfred P. Sloan Foundation awarded the Carnegie Institution a $4 million grant over three years to initiate the Deep Carbon Observatory—an international, decade-long project to investigate the nature of carbon in Earth’s deep interior.

Headquartered at the institution’s Geophysical Laboratory, the Deep Carbon Observatory will coordinate the efforts of hundreds of researchers from more than two dozen countries. Their multi-disciplinary research will focus on Earth’s poorly understood deep carbon cycle, including the largely unknown role of deep biology and the possible influences of this cycle on critical societal concerns related to energy, environment and climate.

Carbon plays an unparalleled role in our lives, yet we remain largely ignorant of the carbon-bearing systems more than a few hundred meters beneath our feet. We do not know how much carbon is stored within the Earth, nor do we know the nature of those deep reservoirs. We have only vague hints of an extensive deep microbial ecosystem, which by some estimates rivals the total surface biomass.

—Robert Hazen, the observatory’s Principal Investigator

The project’s self-described initial goal is to galvanize the diverse international scientific community to engage in a transformative study of Earth’s deep carbon cycle that will impact many areas of Earth, environmental, and energy science.

The Observatory’s exploration of carbon within the planet will likely open up new windows on the Earth’s interior as a whole, while addressing major global energy and environmental issues here on its surface.

— Russell Hemley, director of the Geophysical Laboratory



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