Abiogenic petroleum origin

The theory of abiogenic petroleum origin states that petroleum is produced by non-biological processes deep in the Earth. This stands in contrast to the more widely held view that it is created from the fossilization of ancient organic matter. According to this theory, petroleum is formed by non-biological reactions deep in the Earth's crust. The constituent precursors of petroleum (mainly methane) are commonplace and it is possible that appropriate conditions exist for oil to be formed deep within the Earth.

Although this theory has support by a large minority of geologists in Russia, where it was intensively developed in the 1950s and 1960s, it has only recently begun to receive attention in the West, where the biogenic theory is still believed by the vast majority of petroleum geologists. Although it was originally denied that abiogenic hydrocarbons exist at all on earth, this is now admitted by Western geologists. The orthodox position now is that while abiogenic hydrocarbons exist, they are not produced in commercially significant quantities, so that essentially all hydrocarbons that are extracted for use as fuel or raw materials are biogenic.

A variation of the abiogenic theory includes alteration by microbes similar to those which form the basis of the ecology around deep hydrothermal vents.

One prediction of this theory is that other planets of the Solar system or their moons have large petroleum oceans, either from hydrocarbons present at the formation of the Solar system, or subsequent chemical reactions.

That this theory is receiving increasing attention from Western geologists is indicated by the fact that the American Association of Petroleum Geologists scheduled a to meet in Vienna in July 2004 entitled "Origin of Petroleum—Biogenic and/or Abiogenic and Its Significance in Hydrocarbon Exploration and Production". The conference had to be canceled, however, due to financial considerations. Instead, AAPG will be holding a one-day session on the topic at the June 2005 annual meeting in Calgary, Alberta.

Comparison of theories

There are two theories on the origin of carbon fuels: the biogenic theory and the abiogenic theory. The two theories have been intensely debated since the 1860s, shortly after the discovery of widespread petroleum. There are several differences between the biogenic and abiogenic theories.

Raw material

• Biogenic: remnants of buried plant and animal life.
• Abiogenic: deep carbon deposits from when the planet formed or subducted material.

Events before conversion

• Biogenic: Large quantities of plant and animal life were buried. Sediments accumulating over the material slowly compressed it and covered it. At a depth of several hundred meters, catagenesis converts it to bitumens and kerogens.
• Abiogenic: At depths of hundreds of kilometers, carbon deposits are a mixture of hydrocarbon molecules which leak upward through the crust. Much of the material becomes methane.

Conversion to petroleum and methane

• Biogenic: Catagenesis occurs as the depth of burial increases and the heat and pressure breaks down kerogens to form petroleum.
  • Significant advances in the understanding of chemical processes and organic reactions and improved knowledge about the effects of heating and pressure during burial and diagenesis of organic sediments support biogenic processes.
• Abiogenic: When the material passes through temperatures at which extremophile microbes can survive some of it will be consumed and converted to heavier hydrocarbons.

Formation of coal

• Biogenic: Coal is organic material which was buried and compressed but did not undergo catagenesis into kerogens.
• Abiogenic: Coal is organic material which was filled with hydrocarbons which seeped into the deposit. This can happen on the surface, such as in a swamp with methane and petroleum seeps.

Evidence supporting abiogenic theory

Cold planetary formation

In the late 19th century it was believed that the Earth was extremely hot, possibly completely molten, during its formation. One reason for this was that a cooling, shrinking, planet was necessary in order to explain geologic changes such as mountain formation. A hot planet would have caused methane and other hydrocarbons to be outgassed and oxidized into carbon dioxide and water, thus there would be no carbon remaining under the surface. Planetary science now recognizes that formation was a relatively cool process until radioactive materials accumulate together deep in the planet.

Known hydrocarbon sources

Carbonaceous chondrite meteorites contain carbon and hydrocarbons. Heated under pressure, this material would release hydrocarbon fluids in addition to creating solid carbon deposits. Further, at least ten bodies in our solar system are known to contain at least traces of hydrocarbons. In 2004, the Cassini spacecraft confirmed methane clouds and hydrocarbons on Titan, a moon of Saturn.

Unusual deposits

Hydrocarbon deposits have been found in places which are poorly explained by biogenic theory. Some oil fields are being refilled from deep sources, although this does not rule out a deep biogenic source rock. The White Tiger field in Vietnam and many wells in Russia, in which oil and natural gas are being produced from granite basement rock. As this rock is believed to have no oil-producing sediments under it, the biogenic theory requires the oil to have leaked in from source rock dozens of kilometers away.

Deep microbes

Microbial life has been discovered 4.2 kilometers deep in Alaska and 5.2 kilometers deep in Sweden.

Evidence supporting biogenic theory

It was once argued that the abiogenic theory does not explain the detection of various biomarkers in petroleum. Microbial consumption does not yet explain some trace chemicals found in deposits. Materials which suggest certain biological processes include tetracyclic diterpane, sterane, hopane, and oleanane. Although extremophile microorganisms exist deep underground and some metabolize carbon, some of these biomarkers are only known so far to be created in surface plants. This shows that some petroleum deposits may have been in contact with ancient plant residues, though it does not show that either is the origin of the other.

See also

• Fossil fuel