Migration also explains another commonly offered piece of evidence for organic petroleum, depletion of the carbon 13C isotope, according to Gold.

Photosynthesis and other organic activity favor the stable 12C isotope over the stable 13C isotope. The resulting 13C deficiency is taken as an indicator of organic processes.

Petroleum shows the 13C depletion to an even greater degree than its supposed organic source matter, but in a ratio similar to that of the lipid fractions of those organisms.

Gold theorizes that carbon-bearing molecules diffusing through a porous mass, in any process, results in fractionation that favors the lighter 13C isotope.

"Biology is not a nuclear reactor. It can't make carbon-13 or carbon-12. But it's treated in the literature that the 12C-13C preference is strictly a plant matter," Gold said. "It's quite clear that there is an isotopic fractionation occurring in the migration path."

More evidence of upward hydrocarbon migration from great depth comes from the prevalent occurrence of helium with petroleum, Gold said.

"We have two conflicting pieces of evidence. Petroleum contains helium, which the plants cannot have concentrated," he said. "Petroleum also contains purely biological molecules, which petroleum-fed biology deep in the ground could concentrate.

"This (upward migration from great depth) is the only explanation I've ever heard of to account for the amount of helium brought up with petroleum."

Petroleum explorationists have good reason to care about the true origin of hydrocarbons, Gold noted.

"For one thing, they always avoid drilling into the basement rock," he said. "They've probably avoided drilling into a large amount of very productive rock."

Conversely, the pro-biogenic camp can look to this study, from the same article:

A new perspective on isotopic analysis of abiogenic hydrocarbons appeared in a letter to Nature magazine in April 2002, "Abiogenic formation of alkanes in the Earth's crust as a minor source for global hydrocarbon reservoirs."

Barbara Sherwood Lollar and four co-authors from the Stable Isotope Laboratory at the University of Toronto reported their analysis of gas from the Kidd Creek mine in Ontario, typical of hard rock mines operating throughout the Canadian Shield.

"These gases had been known historically in the mines for a very long period, up to 100 years, but nobody had investigated them until the 1980s. In Precambrian rock, it's not intuitively obvious where these hydrocarbons come from," said Sherwood Lollar, a professor of geology at the university.

According to the authors, the Kid Creek gases were composed of methane, ethane, H2 and N2, with minor concentrations of helium, propane and butane.

"We knew that these were unusual in composition. They don't look like thermogenic. They don't look like microbial," Sherwood Lollar said.

An unusual pattern of d13C values among C1-C4 alkanes provided evidence of abiogenic formation. Additional support came from study of d2H values.

"The inverse relationship of 13C isotope depletion and2H isotope enrichment between C1 and C2 for the Kidd Creek samples supports a polymerization reaction as the first step in the creation" of higher hydrocarbons, the authors concluded.

Because the isotopic signature differed markedly from that of thermogenic or bacteriogenic hydrocarbons, Sherwood Lollar theorized an origin in water-rock interactions.

"The gases are found intimately associated with these saline groundwaters and brines, with up to 10 times the saline content of oceans," she said.

Identification of the 13C-2H inverse relationship in abiogenic gas allowed comparison with isotopic ratios in commercial gas reservoirs. The study found no meaningful presence of abiogenic hydrocarbons in commercial natural gas production.

"Based on the isotopic characteristics of abiogenic gases identified in this study, the ubiquitous positive correlation of d13C and d2H values for C1-C4 hydrocarbons in economic reservoirs worldwide is not consistent with any significant contribution from abiogenic gas," the authors said.

"The key point is that abiogenic hydrocarbons have been talked about for a long time, but until now we didn't have a very good constraint on what they looked like," Sherwood Lollar observed.

Katz said Western science recognizes that abiogenic hydrocarbons can result from natural processes, including the possibility of hydrocarbons originating at great depth.

"I don't think anybody's arguing that gas couldn't be generated from the mantle," he said.

However, even the Russian scientists he has worked with accept the organic origin of petroleum found in large, commercial accumulations.

"I've worked with geochemists and basin modelers at what was the Soviet Union's Institute for Foreign Geologic Studies. They were working with the same concepts we were," he said.

If abiogenic petroleum exists in amounts large enough for economic production, he hopes details of the science involved will be presented at the London Hedberg .

"I have yet to have anyone show me that there are commercial quantities of these hydrocarbons," Katz said.