abiogenic petroleum origin

The theory of abiogenic petroleum origin states that petroleum (or crude oil) is primarily created from non-biological sources of hydrocarbons located deep in the Earth. The theory stands in contrast to the more widely held conventional view that petroleum is created from the remains of ancient living matter. The constituent precursors of petroleum (mainly methane) are commonplace and it is possible that appropriate conditions exist for hydrocarbons to be formed deep within the Earth. Although this theory is supported 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. Planetary scientist Thomas Gold was one of the abiogenic theories greatest proponents in recent years. 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 conference 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. If the theory is correct, then it could greatly change future energy development.

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 deposits. 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 organic matter derived from dead 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.
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 kerogen-like 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. Meteorite ALH84001, believed to be from Mars, contains carbonate minerals which were formed about 3.9 billion years ago. The deposits are in igneous rock. On Earth, microorganisms often create carbonates. However, the material could also have been formed by water carrying carbon dioxide, or by hot carbon-dioxide-bearing fluids. Kerogen-like material has also been detected in interstellar clouds and dust particles around stars.

Methane not on Earth

Methane has been detected or is believed to exist in several locations of the solar system. It is believed to have been created by abiotic processes, except possibly on Mars.

Neptune

Triton

Uranus

Comet Halley
Comet Hyakutake

In 2004, the Cassini spacecraft confirmed methane clouds and hydrocarbons on Titan, a moon of Saturn. Traces of methane gas also are in the thin atmosphere of the Earth's Moon. Methane has also been detected in interstellar clouds.

Unusual deposits

Hydrocarbon deposits have been found in places that are said to be 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. In the White Tiger field in Vietnam and many wells in Russia, 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. Methanophile organisms have been known for some time, and recently it was found that microbial life in Yellowstone National Park is based on hydrogen metabolism. Other deep and hot extremophile organisms continue to be discovered. U.S. Geological Survey (USGS) scientist Frank Chapelle and his colleagues from the USGS and the University of Massachusetts have discovered a potential analog for life on other planets. A community of Archaea is thriving deep in the subsurface source of a hot spring in Idaho. Geothermal hydrogen, not organic carbon, is the primary energy source for this methanogen-dominated microbial community. This is the first documented case of a microbial community completely dominated by Archaea.

Ambiguous results

Ongoing research has changed the status of some information. For example, some biomarkers which were interpreted as evidence supporting the biogenic theory have been undermined by finding similar materials in thermophilic situations which are part of abiogenic theories.

Biomarkers

Chemicals of biological origin have been found in many geologic hydrocarbon deposits. These biomarkers were believed to be from known surface sources. Due to the difficulty in culturing and sampling deep heat-loving bacteria, thermophiles, little was known of their chemistry. As more is learned of bacterial chemistry, more biomarkers seem likely to be due to bacterial action. Hopanoids, called the 'most abundant natural products on Earth', were believed to be indicators of oil derived from ferns and lichens but are now known to be created by many bacteria, including archaea. Sterane was thought to have come from processes involving surface deposits but is now known to be produced by several prokaryotes including methanotrophic proteobacteria.

Deep hot carbon sources

Carbonate lava

Carbonatites are intrusive carbonate-mineral-rich igneous rocks. Although they are deposits of carbon from an igneous source, the geology behind their creation is not understood.

Hydrothermal vents

Hydrothermal vents expel mineral-rich geothermally heated water.

Carbon dioxide abiogenically produced from magma: As magma outgasses helium and carbon dioxide at depths less than 60 km, there should be deep carbon fluids present in areas such as oceanic ridges where the magma is able to heat surface waters.
Microbes can create methane: Extremophile methanogens such as Methanopyrus can convert CO2 to methane.
Methane can also be created chemically: Iron in rock can release hydrogen from water, then carbon dioxide can combine with the hydrogen to produce methane and water. University of Minnesota researchers discovered that rocks rich in chromium minerals can encourage chemical methane production, while also producing the more complex hydrocarbons ethane and propane.
Methane and carbon dioxide may be dissolved in water which enters hydrothermal vent systems.
Hydrothermal vents might release methane and carbon from deposits of biological origin, although this is less likely in vents at spreading oceanic ridges.

Evidence supporting biogenic theory

Unusual deposits

While it is true that some oil fields do not conform to the standard model of a fixed amount of oil trapped in a sedimentary basin, these examples are accomodated by the biogenic model. For example, the White Tiger field cited above is located in an area where significant normal faulting brings relatively young sedimentary rocks into contact with older, fractured igneous rocks. The produced oil is described as typically lacustrine (derived from lake deposits), consistent with the migration of hydrocarbons from the organic-rich sediments into the fractured basement. One type of unusual deposit that one might to expect to find if the abiogenic hypothesis is true would be hydrocarbons trapped in sedimentary basins that do not contain conventional source rocks. Evidence for such deposits is lacking. Many economically disappointing wells have been drilled into geologic structures that would be expected to trap hydrocarbons, but there are no hydrocarbons present. Oil companies have learned that an appropriate source rock is necessary before they will commit to drilling a well in a potential new field.

Biomarkers

It has been 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 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 evidence is consistent with the biogenic hypothesis, although it might be true that these hydrocarbons have merely been in contact with ancient plant residues. There also is evidence that low-temperature relatives of hyperthermophiles are widespread, so it is also possible for biological deposits to have been altered by low-temperature bacteria which are similar to deeper heat-loving relatives.