Rocks into Gas

Geologists have long believed that the world's supply of oil and natural gas came from the decay of primordial plant and animal matter, which, over the course of millions of years, turned into petroleum.

Two diamond anvils, each about 3 millimeters high, in a diamond anvil cell. They compress a small metal plate that holds the sample. The device can generate pressures greater than those in the center of the earth (3.6 million atmospheres) The methane generation experiments use pressures in the 50-100,000 atmosphere range, corresponding to the earth's upper mantle.

Photograph courtesy of Dudley Herschbach

But new research coauthored by Dudley Herschbach, Baird research professor of science and recipient of the 1986 Nobel Prize in chemistry, questions that thinking. Published last fall in the Proceedings of the National Academy of Sciences, the study describes how investigators combined three abiotic (non-living) materials -- water (H2O), limestone (CaCO3), and iron oxide (FeO) -- and crushed the mixture together with the same intense pressure found deep below the earth's surface. This process created methane (CH4), the major component of natural gas. Herschbach says this offers evidence, although as yet far from proof, for a maverick theory that much of the world's supply of so-called fossil fuels may not derive from the decay of dinosaur-era organisms after all.

Herschbach became interested in the origins of petroleum hydrocarbons while reading A Well-Ordered Thing, a book about the nineteenth-century Russian chemist Dmitri Mendeleev, who developed the periodic table. Written by Michael Gordin '96, Ph.D. '01, a current Junior Fellow, the book mentions a theory long held by Russian and Ukrainian geologists: that petroleum comes from reactions of water with other abiotic materials, and then bubbles up toward the earth's surface. Intrigued, Herschbach read further, including The Deep, Hot Bio-sphere by the late Cornell astrophysicist Thomas Gold. An iconoclast, Gold saw merit in the Russian and Ukrainian view that petroleum has nonliving origins. He theorized that organic materials found in oil -- which most scientists took as a sign that petroleum comes from living things -- may simply be waste matter from microbial organisms that feed on the hydrocarbons generated deep in the earth as these flow upward.

Another of Gold's assertions about methane and oil really caught Herschbach's attention. "He said there wasn't much chance that you could do a laboratory experiment to test this," Herschbach reports. "And I thought, 'Holy smoke! We could do this with the diamond anvil cell.'" Long interested in how molecules behave under high-pressure conditions, he contacted Russell Hemley, Ph.D. '83, a former student now at the Geophysical Laboratory at the Carnegie Institution of Washington, to suggest the methane experiment. Together with Henry Scott of Indiana University and other researchers, Herschbach sought to create the same conditions found 140 miles below the earth's surface, where temperatures are scorching and pressures mount to more than 50,000 times those at sea level. "It's a great pressure cooker," he explains.

The diamond anvil cell, developed at the Carnegie Institution, can create the same pressures found as far as 4,000 miles beneath the earth's surface. The cell employs two diamonds, each about three millimeters (roughly one-eighth-inch) high, which sit with their tips facing each other in hardened precision frames that are forced together, creating intense pressure in the small space between the tips. Diamonds are an ideal material for such experiments, Herschbach explains. As one of the hardest substances on earth, they can withstand the tremendous force, and because they're transparent, scientists can use beams of light and X-rays to identify what's inside the cell without pulling the diamonds apart. He notes that previous experiments by Russian scientists arrived at different conclusions because they used an old-fashioned press that had to be opened before any products inside could be analyzed, potentially changing the results.

"The experiment showed it's easy to make methane," Herschbach says. The new findings may serve to corroborate other evidence, cited by Gold, that some of the earth's reservoirs of oil appear to refill as they're pumped out, suggesting that petroleum may be continually generated. This could have broad implications for petroleum production and consumption, and for our planet's ecology and economy.

But before we begin to think of petroleum as a renewable resource, Herschbach urges caution. "We don't know if a globally significant or commercially significant portion of methane might be formed abiotically from this pressure-cooker business," he says. "Even if we did convince ourselves that a lot of hydrocarbons are formed that way, we don't yet know how long it takes for it to percolate up and refill the reservoirs."

For Herschbach, these exciting research questions have "given me a second scientific childhood." He and his colleagues are eager to return to the lab and find out if even higher pressures will create more complex hydrocarbons, such as butane or propane. The research raises fundamental questions about how scientists determine if a material has living or nonliving origins. It also validates the work of previous scientists. "The fair conclusion," Herschbach says, "is that the views of Thomas Gold and Russian scientists all the way back to Mendeleev need to be taken more seriously than they have been in the Western world."

Dudley Herschbach e-mail address: herschbach@chemistry.harvard.edu