Hydrocarbons in the deep Earth?


National Science Foundation, NASA Astrobiology Institute, US Department of Energy, National Nuclear Security Administration, Carnegie/DOE Alliance CenterWashington, D.C. In an era of rising oil and gas prices, the possibility that there are untapped reserves is enticing. Since the first U.S. oil well hit pay dirt in 1859, commercially viable wells of oil and gas commonly have been drilled no deeper than 3 to 5 miles into Earth's crust. "These experiments point to the possibility of an inorganic source of hydrocarbons at great depth in the Earth--that is, hydrocarbons that come from simple reactions between water and rock and not just from the decomposition of living organisms," stated Dr. Russell Hemley of the Carnegie Institution's Geophysical Laboratory, and co-author of a study published in the September 13-17, early, on-line edition of the Proceedings of the National Academy of Sciences.*

Methane is the most abundant hydrocarbon in the Earth's crust and it is the main component of natural gas. Often, gas reserves are accompanied by liquid petroleum. However these reserves, at 3 to 5 miles beneath the surface, exist in relatively low-pressure conditions. Whether hydrocarbons exist deeper--and could even be formed from non-biological matter--has been the subject of much debate. As depth increases in the Earth, the pressures can become so crushing that molecules are squeezed into new forms and the temperature conditions are like an inferno making matter behave much differently. The team of scientists performed a series of experiments at Carnegie, the Carnegie-managed High Pressure Collaborative Access Team (HPCAT) at Argonne National Laboratory, and at Indiana University South Bend--together with calculations performed at Lawrence Livermore National Laboratory--to mimic conditions that occur in Earth's upper mantle, which underlies the crust at depths of about 12 to 37 miles (20 to 60 km) beneath the continents.

With a diamond anvil cell, the scientists squeezed materials common at Earth's surface--iron oxide (FeO), calcite (CaCO3) and water-- to pressures ranging from 50,000 to 110,000 times the pressure at sea level ( 5 to 11 gigapascals). They heated the samples using two techniques--focused laser light and the so-called resistive heating method--to temperatures up to 2,700 degrees F (1500 degrees C). The researchers found that methane formed by reducing the carbon in calcite over a wide range of temperatures and pressures. The best conditions were at temperatures and pressures of about 1000 degrees F and less than 70,000 times atmospheric pressure.

Dr. Henry Scott, of Indiana University South Bend, related the significance of the experiments to conventional hydrocarbon resources: "Although it is well-established that commercial petroleum originates from the decay of once-living organisms, these results support the possibility that the deep Earth may produce abiogenic hydrocarbons of its own."

"This paper is important," remarked Dr. Freeman Dyson, professor emeritus at the Institute for Advanced Study at Princeton who reviewed the study. "Not because it settles the question whether the origin of natural gas and petroleum is organic or inorganic, but because it gives us tools to attack the question experimentally. If the answer turns out to be inorganic, this has huge implications for the ecology and economy of our planet as well as for the chemistry of other planets."

Source: Eurekalert & others

Last reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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