Gas Origin Theories to be Studied

Hedberg to Get to the Nub of the Matter

The debate about cooking up hydrocarbons keeps getting hotter.

Some scientists insist that all petroleum comes from abiogenic processes, with hydrocarbon development occurring in the Earth's mantle.

Most geochemists and petroleum geologists remain convinced that crude oil and natural gas have organic origins.

Look for this dispute to intensify in 2003, with new heat coming from an unexpected venue. In June, AAPG's typically sleepy Hedberg Conference could be the spark that sets off scientific fireworks.

Hedberg conferences address topics proposed by AAPG's Research Committee. They take place in informal settings, with attendance limited to 80-100 persons.

On June 9-12, however, a Hedberg Conference will be held in London with the theme "Origin of Petroleum — Biogenic and/or Abiogenic and Its Significance in Hydrocarbon Exploration and Production."

"The timing is right," said Barry Katz, a ChevronTexaco Fellow in Houston and a member of the conference's program committee. "Historically, what has been the big issue is that there's essentially a Western and an Eastern school of thought.

"On the Western side, we've gone through what you've typically done in the scientific method," he noted. "The Russian arguments have been just that, arguments. We have yet to get them in a room to see what they have on the table."

Katz said he hopes the leading theorists from both sides will attend, so "we can have a balanced view and get everybody to talk to each other. That's what the Hedberg conferences are all about."

Is It Commercial?

An explorationist might dismiss the entire controversy over petroleum origination, except for two key points:

  • Theorists of abiogenic petroleum tend to see hydrocarbons as not just abundant but super-abundant, with no possibility of constrained supply.
  • Petroleum generated by abiogenic processes could occur anywhere, so exploration need not be limited to sedimentary basins, or to depths of only a few miles.

Modern theory directly links petroleum origination to organic detritus, according to Michael Lewan, a research geochemist for the U.S. Geological Survey in Denver.

"The modern, organic theory of the origin of petroleum states that a portion of the lipid fraction of micro-organisms deposited in anaerobic sediments is the original source of petroleum," he said.

Proteins and carbohydrates make up 85-95 percent of the weight of these micro-organisms, and are rapidly degraded by microbial activity, Lewan said.

The remaining 5-15 percent can be preserved in anaerobic sediments, representing "unique depositional conditions that result in organic-rich, sedimentary-rock intervals in some stratigraphic sequences," he explained.

Lipid material preserved in the original sediments polymerizes into kerogen, an insoluble organic material, Lewan said.

"As these organic-rich rock intervals are heated with burial in sedimentary basins, the hydrocarbon polymers within the kerogen thermally crack through a free-radical mechanism to yield liquid and gaseous petroleum hydrocarbons," he said.

Research in the lab and in the field demonstrates that petroleum development can and does take place in the earth's crust, he stated.

"I feel we've done a very good job of simulating production of petroleum in the laboratory," Lewan said. "Between the lab work and the fieldwork, we've put together a very good picture."

Image Caption

Burning water? This liquid came from a water well at the site.
Photo courtesy of Thomas Gold.

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The debate about cooking up hydrocarbons keeps getting hotter.

Some scientists insist that all petroleum comes from abiogenic processes, with hydrocarbon development occurring in the Earth's mantle.

Most geochemists and petroleum geologists remain convinced that crude oil and natural gas have organic origins.

Look for this dispute to intensify in 2003, with new heat coming from an unexpected venue. In June, AAPG's typically sleepy Hedberg Conference could be the spark that sets off scientific fireworks.

Hedberg conferences address topics proposed by AAPG's Research Committee. They take place in informal settings, with attendance limited to 80-100 persons.

On June 9-12, however, a Hedberg Conference will be held in London with the theme "Origin of Petroleum — Biogenic and/or Abiogenic and Its Significance in Hydrocarbon Exploration and Production."

"The timing is right," said Barry Katz, a ChevronTexaco Fellow in Houston and a member of the conference's program committee. "Historically, what has been the big issue is that there's essentially a Western and an Eastern school of thought.

"On the Western side, we've gone through what you've typically done in the scientific method," he noted. "The Russian arguments have been just that, arguments. We have yet to get them in a room to see what they have on the table."

Katz said he hopes the leading theorists from both sides will attend, so "we can have a balanced view and get everybody to talk to each other. That's what the Hedberg conferences are all about."

Is It Commercial?

An explorationist might dismiss the entire controversy over petroleum origination, except for two key points:

  • Theorists of abiogenic petroleum tend to see hydrocarbons as not just abundant but super-abundant, with no possibility of constrained supply.
  • Petroleum generated by abiogenic processes could occur anywhere, so exploration need not be limited to sedimentary basins, or to depths of only a few miles.

Modern theory directly links petroleum origination to organic detritus, according to Michael Lewan, a research geochemist for the U.S. Geological Survey in Denver.

"The modern, organic theory of the origin of petroleum states that a portion of the lipid fraction of micro-organisms deposited in anaerobic sediments is the original source of petroleum," he said.

Proteins and carbohydrates make up 85-95 percent of the weight of these micro-organisms, and are rapidly degraded by microbial activity, Lewan said.

The remaining 5-15 percent can be preserved in anaerobic sediments, representing "unique depositional conditions that result in organic-rich, sedimentary-rock intervals in some stratigraphic sequences," he explained.

Lipid material preserved in the original sediments polymerizes into kerogen, an insoluble organic material, Lewan said.

"As these organic-rich rock intervals are heated with burial in sedimentary basins, the hydrocarbon polymers within the kerogen thermally crack through a free-radical mechanism to yield liquid and gaseous petroleum hydrocarbons," he said.

Research in the lab and in the field demonstrates that petroleum development can and does take place in the earth's crust, he stated.

"I feel we've done a very good job of simulating production of petroleum in the laboratory," Lewan said. "Between the lab work and the fieldwork, we've put together a very good picture."

Although hydrocarbons can be produced from inorganic sources, a 1993 study based on helium isotopes found that abiogenic hydrocarbons account for less than 200 parts per million of cumulative global production to date, Lewan said.

"Is it so diffuse that it never really accumulates? Is it focused in certain areas where it can be accumulated?" he asked.

"I don't think anybody has ever doubted that there is an inorganic source of hydrocarbons. The key question is, 'Do they exist in commercial quantities?'"

From Russia, With Love

Various other theories oppose the organic-origin explanation. The principal counter-theory is often called the abyssal, abiotic Russian-Ukrainian theory of petroleum.

In 1951, a group of Russian scientists issued a challenge to the theory of organic petroleum origination. They claimed that hydrocarbons are produced from inorganic materials, at upper-mantle to lower-crust depths.

New controversy over that proposal resulted from a paper published in August 2002 in the Proceedings of the National Academy of Sciences (U.S.), "The evolution of multi-component systems at high pressures: IV. The thermodynamic stability of the hydrogen-carbon system: The genesis of hydrocarbons and the origin of petroleum."

This paper, written by J.F. Kenney of Gas Resources Corp. in Houston and three Russian co-authors, rejects the proposal that petroleum can derive from "highly oxidized biotic molecules of low chemical potential."

Drawing on scaled particle theory and simplified perturbed hard-chain theory, Kenney et al. present an evaluation of the chemical potentials and related thermodynamic affinities for n-alkanes. They conclude:

"The H-C system does not spontaneously evolve heavy hydrocarbons at pressures less than about 30 kbar, even in the most favorable thermodynamic environment. The H-C system evolves hydrocarbons under pressures found in the mantle of the Earth and at temperatures consistent with that environment."

They also briefly describe the experimental production of petroleum hydrocarbons using only wetted marble (CaCO3) and solid iron oxide (FeO), in an apparatus allowing investigation at pressures up to 50 kbar and temperatures up to 1,500° Celsius.

Kenney said there is no real debate about petroleum origination.

"There has not been any 'debate' about the origin of hydrocarbons for over a century," he stated. "Competent physicists, chemists, chemical engineers and men knowledgeable of thermodynamics have known that natural petroleum does not evolve from biological material since the last quarter of the 19th century."

In their paper, Kenny et al. contrast the H-C system with the H-C-O system, "which manifests consistently decreasing chemical potentials with increasing polymerization."

They then discuss reactions involving C6H12O6, or glucose, as a "typical biotic reagent." In response, Lewan noted that neither carbohydrates nor proteins are now thought to have a part in petroleum formation.

"Significant scientific advances over the last 40 years have tested, modified and refined the organic theory for petroleum formation in the Earth's crust," Lewan said.

"It is unfortunate that Kenney et al. have chosen to ignore these efforts of other competent scientists, and elevate their inorganic theory on the misconception that the organic theory is based on carbohydrates being the source of petroleum."

The Golden Touch

No one in the United States has been more associated with the theory of abiogenic petroleum than Thomas Gold, a professor of astronomy at Cornell University, now retired.

"The only real opponents to this story (of abiogenic origin) are in Western Europe and in the United States, and they are the professional petroleum geologists," he said.

"The subject of organic chemistry was wrongly taken by petroleum geologists long ago to mean chemistry of biologic origins. You can still have a book of organic chemistry that has nothing to do with organisms at all."

Gold most recently explained his theories of the origin of petroleum in his 1999 book The Deep Hot Biosphere, which presents the view that life on Earth exists to a depth of many miles.

That helps him explain the apparent organic constituents of petroleum. In Gold's view, hydrocarbons form at a depth of 100 to 300 kilometers and take on some organic attributes as they migrate upward.

"Oil is a very good nutrient for microbiology. In 1972, I began to realize that the oil had soaked up biological molecules that the petroleum itself had fed," he said.

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."

Also, in Gold's theory hydrocarbons continue to well up from the mantle. He believes depleted petroleum reservoirs are refilling, all over the world.

Seeing Is Believing?

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.

"I'm a scientist, so I have to keep an open mind. But I need to see some evidence."

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