Extreme Heat and Pressure May Yield Hydrocarbons in Earth’s Mantle

All scientists strive to shed light on difficult questions, but researchers who study deep carbon face the additional challenge of illuminating a very dark place indeed: hundreds of kilometers beneath Earth’s surface. With the deepest boreholes reaching little farther than 12 kilometers, researchers must rely on theoretical calculations and lab simulations to understand the high-temperature and high-pressure reactions occurring deep within the mantle.

In a new review in ChemistrySelects, three Deep Carbon Observatory researchers synthesize theory, computer modeling, and lab experiments to explain the chemical and physical processes that carbon undergoes in Earth’s upper mantle. Anton Kolesnikov (Gubkin Russian State University of Oil and Gas in Moscow) Vladimir Kutcherov (Royal Institute of Technology in Stockholm and Gubkin Russian State University of Oil and Gas), and John Saul (Swala Gem Traders, Arusha, Tanzania), draw on decades of research to explain the formation of hydrocarbons under “thermobaric” conditions, with extremely high temperatures and pressure levels. They conclude that it is possible for methane and heavier hydrocarbons to form from chemical reactions deep within the mantle and percolate up to the surface, suggesting that fossil fuels may be a renewable resource.

“There’s a fundamental question that many people think has been answered, but in fact, hasn’t. And that is whether the ultimate origin of petroleum is biological or geological,” said Saul.

The idea that fossil fuels can form in the upper mantle is still controversial in many scientific circles, but Russian and Ukrainian scientists have discussed these ideas for at least 50 years, says Kutcherov.

In their paper, the authors review numerous studies that use standard thermodynamic approaches and computer modeling to calculate how temperature and pressure affect carbon in the subsurface.

The researchers also summarize results from experiments in Kutcherov’s lab and others that mimic conditions in the mantle using equipment such as diamond anvil cells and large reactive volume devices. These advanced instruments can heat up small volumes of compounds to more than 3,000 degrees Fahrenheit, and apply pressures greater than 80 thousand times the atmospheric pressure felt at Earth’s surface. The experiments suggest that methane and heavier hydrocarbons can form under such thermobaric conditions through nonbiological means from a broad variety of different carbon-containing compounds. 

Saul describes this system as a “great big pressure cooker down deep in the Earth” that leaks hydrocarbons. The researchers think these compounds travel along migration channels through fractured rocks to reach the surface, where they collect into economic oil and gas deposits.

Researchers still don’t know how prevalent these hydrocarbons might be in the subsurface or the rate that these reactions might take place.

Going forward, Kutcherov hopes to better understand how crude oil forms under these conditions and to model the reaction in his lab.

“After the deep carbon cycle project, we need to create the deep hydrocarbon cycle project,” said Kutcherov. “This is our dream.”

High-pressure equipment at Gubkin Russian State University of Oil and Gas
Photo provided by Vladimir Kutcherov

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