28 July 2009

Hydrocarbons go deep

Oil that fuels homes and cars and natural gas may exist even deeper below the Earth’s crust.

For years, scientists debated whether hydrocarbons could exist deeper in the Earth, forming without organic matter. Scientists now found ethane and heavier hydrocarbons can undergo synthesis under the pressure-temperature conditions of the upper mantle—the layer of Earth under the crust and on top of the core.

The Earth’s interior shows hydrocarbons forming in the upper mantle and transported through deep faults to shallower depths in the Earth’s crust.

Methane is the main constituent of natural gas, while ethane is a petrochemical feedstock. These hydrocarbons, and others associated with fuel, are saturated hydrocarbons because they have simple, single bonds “soaked” with hydrogen, said researchers at the Carnegie Institution’s Geophysical Laboratory, with colleagues from Russia and Sweden. 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 1,300°F to over 2,240°F. These conditions mimic those found 40 to 95 miles deep inside the Earth.

As it turns out, 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. The reversibility implies the synthesis of saturated hydrocarbons is thermodynamically controlled and does not require organic matter.

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

“We were intrigued by previous experiments and theoretical predictions,” said Carnegie’s Alexander Goncharov a coauthor of a paper on the subject.

“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,” Goncharov said. “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.”

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

Professor Vladimir Kutcherov of the Royal Institute of Technology, Moscow State Academy for Fine Chemical Technology, a co-author of a paper on the subject, put the finding into context.

“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,” Kutcherov said. “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.”

For related information, go to www.isa.org/environment.