In a new research, scientists have found for the first time that ethane and heavier hydrocarbons can be synthesised under the pressure-temperature conditions of the upper mantle of the Earth.
The research was conducted by scientists at the Carnegie Institution’s Geophysical Laboratory, with colleagues from Russia and Sweden.
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 1,300 Fahrenheit to over 2,240 Fahrenheit.
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.
The reversibility implies that 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 that catalysts could be involved in the deep Earth with its mix of compounds.
“We were intrigued by previous experiments and theoretical predictions,” said Carnegie’s Alexander Goncharov, a co-author.
“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,” he 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,” he added.
According to Professor Kutcherov, a coauthor, “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,” he said.