Definitions

Abiogenic petroleum origin

The hypothesis of abiogenic petroleum origin is an alternative hypothesis to the biological origin theory which was popular in Russia and Ukraine between the 1950s and 1980s. Most petroleum geologists do not consider it to be of commercial value.

The hypothesis states that natural petroleum was formed from deep carbon deposits, perhaps dating to the formation of the Earth. The presence of methane on Titan is held as evidence. Supporters of the abiogenic hypothesis suggest that there may be a great deal more petroleum on Earth than commonly thought, and that petroleum may originate from carbon-bearing fluids which migrate upward from the mantle.

Although the abiogenic hypothesis was accepted by some geologists in the former Soviet Union, most geologists now consider the biogenic formation of petroleum to be supported scientifically. Though evidence exists for abiogenic creation of methane and hydrocarbon gases within the Earth, studies indicate that they are not produced in commercially significant quantities (ie a median abiogenic hydrocarbon content in extracted hydrocarbon gases of only one fiftieth of one percent). The abiogenic origin of petroleum has also recently been reviewed in detail by Glasby, who raises a number of objections, including that there is no direct evidence to date of abiogenic petroleum (liquid crude oil and long-chain hydrocarbon compounds).

Although the biogenic theory for petroleum was first proposed by Georg Agricola in the 16th century, various abiogenic hypotheses were proposed in the nineteenth century, most notably by Alexander von Humboldt, the Russian chemist Dmitri Mendeleev and the French chemist Marcellin Berthelot. Since that time, the abiogenic hypotheses have lost ground to the view that petroleum is a fossil fuel.

Abiogenic hypotheses were revived in the last half of the twentieth century by Russian and Ukrainian scientists, and more interest was generated in the West by the publication in 1999 of The Deep Hot Biosphere by Thomas Gold. Gold cited the discovery of thermophile bacteria in the earth's crust as new support for the hypothesis, postulating that these bacteria could explain the existence of certain biomarkers in extracted petroleum.

History of abiogenic hypothesis

Alexander von Humboldt was the first to propose an inorganic abiogenic hypothesis for petroleum formation after he observed petroleum springs in the Bay of Cumaux (Cumana) on the northeast coast of Venezuela.

In 1804 von Humboldt said, "petroleum is the product of a distillation from great depth and issues from the primitive rocks beneath which the forces of all volcanic action lie."

Abraham Gottlob Werner and the proponents of neptunism in the 1700s believed basaltic sills to be solidified oils or bitumen. While these notions have been proven unfounded, the basic idea that petroleum is associated with magmatism persisted. The chief proponents of what would become the abiogenic hypothesis were von Humboldt, Mendeleev and Berthelot.

Russian geologist Nikolai Alexandrovitch Kudryavtsev proposed the modern abiotic hypothesis of petroleum in 1951. He analyzed the geology of the Athabasca Tar Sands in Alberta, Canada and concluded that no "source rocks" could form the enormous volume of hydrocarbons, and that therefore the most plausible explanation is abiotic deep petroleum. However, humic coals have since been proposed for the source rocks.

Kudryavtsev's work was continued by Petr N. Kropotkin, Vladimir B. Porfir'ev, Emmanuil B. Chekaliuk, Vladilen A. Krayushkin, Georgi E. Boyko, Georgi I. Voitov, Grygori N. Dolenko, Iona V. Greenberg, Nikolai S. Beskrovny, and Victor F. Linetsky.

Astronomer Thomas Gold was the most prominent proponent of the abiogenic hypothesis in the West until his death in 2004. Currently, Jack Kenney of Gas Resources Corporation is perhaps the foremost proponent in the West.

State of current research

Currently there is little direct research on abiogenic petroleum or experimental studies into the synthesis of abiogenic methane. However, several research areas, mostly related to astrobiology and the deep microbial biosphere and serpentinite reactions, continue to provide insight into the contribution of abiogenic hydrocarbons into petroleum accumulations.

• rock porosity and migration pathways for abiogenic petroleum
• ocean floor hydrothermal vents as in the Lost City hydrothermal field;
• Mud volcanoes and the volatile contents of deep pelagic oozes and deep formation brines
• mantle peridotite serpentinization reactions and other natural Fischer-Tropsch analogs
• Primoridal hydrocarbons in meteorites, comets, asteroids and the solid bodies of the solar system
• Primordial or ancient sources of hydrocarbons or carbon in Earth
• Primordial hydrocarbons formed from hydrolysis of metal carbides of the iron peak of cosmic elemental abundance (Cr, Fe, Ni, V, Mn, Co)
• isotopic studies of groundwater reservoirs, sedimentary cements, formation gases and the composition of the noble gases and nitrogen in many oil fields
• the geochemistry of petroleum and the presence of trace metals related to Earth's mantle (Ni, V, Cd, As, Pb, Zn, Hg and others)

Similarly, research into the deep microbial hypothesis of hydrocarbon generation is advancing as part of the attempt to investigate the concept of panspermia and astrobiology, specifically using deep microbial life as an analog for life on Mars. Research applicable to deep microbial petroleum theories includes

• Research into how to sample deep reservoirs and rocks without contamination
• Sampling deep rocks and measuring chemistry and biological activity
• Possible energy sources and metabolic pathways which may be used in a deep biosphere
• Investigations into the reworking primordial hydrocarbons by bacteria and their effects on carbon isotope fractionation

A 2006 review article by Glasby presented arguments against the abiogenic origin of petroleum on a number of counts.

An article on abiogenic hydrocarbon production in the February 2008 issue of Science Magazine used data from experiments at Lost City (hydrothermal field) to report how the abiotic synthesis of hydrocarbons in nature may occur in the presence of ultramafic rocks, water, and moderate amounts of heat.

Foundations of the hypotheses

Within the mantle, carbon may exist as hydrocarbon molecules, chiefly methane, and as elemental carbon, carbon dioxide and carbonates. The abiotic hypothesis is that the full suite of hydrocarbons found in petroleum can be generated in the mantle by abiogenic processes, and these hydrocarbons can migrate out of the mantle into the crust until they escape to the surface or are trapped by impermeable strata, forming petroleum reservoirs.

Abiogenic theories reject the supposition that certain molecules found within petroleum, known as biomarkers, are indicative of the biological origin of petroleum. They contend that some of these molecules could have come from the microbes that the petroleum encounters in its upward migration through the crust, that some of them are found in meteorites, which have presumably never contacted living material, and that some can be generated abiogenically by plausible reactions in petroleum.

The hypothesis is founded primarily upon:

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Proponents Item
Gold The presence of methane on other planets, meteors, moons and comets
Gold, Kenney Proposed mechanisms of abiotically chemically synthesizing hydrocarbons within the mantle
Kudryavtsev, Gold Hydrocarbon-rich areas tend to be hydrocarbon-rich at many different levels
Kudryavtsev, Gold Petroleum and methane deposits are found in large patterns related to deep-seated large-scale structural features of the crust rather than to the patchwork of sedimentary deposits
Gold Interpretations of the chemical and isotopic composition of natural petroleum
Kudryavtsev, Gold The presence of oil and methane within non-sedimentary rocks upon the Earth
Gold The existence of methane hydrate deposits
Gold Perceived ambiguity in some assumptions and key evidence used in the orthodox biogenic petroleum theories
Gold Bituminous coal creation is based upon deep hydrocarbon seeps
Gold Surface carbon budget and oxygen levels stable over geologic time scales
Kudryavtsev, Gold Biogenic theories do not explain some hydrocarbon deposit characteristics
Szatmari The distribution of metals in crude oils fits better with upper serpentinized mantle, primitive mantle and chondrite patterns than oceanic and continental crust, and show no correlation with sea water
Gold The association of hydrocarbons with helium, a noble gas
Gold Deep microbial hypothesis of hydrocarbon generation

Conventional theories

Most petroleum geologists prefer theories of oil formation which hold that oil originated in shallow seas as vast quantities of marine plankton or plant materials died and sank into the mud at the bottom under anaerobic conditions that prevented biodegradation. Under these conditions, anaerobic bacteria converted the lipids (fats, oils and waxes) into a waxy substance called kerogen.

As the source rock was buried deeper, overburden pressure raised temperatures into the oil window, between 60 and 120 °C, in which thermal depolymerization broke up the kerogen molecules into the straight-chain hydrocarbons that make up most of petroleum. This process is called generation kitchen. Once crude oil formed, it became very fluid and migrated upward through the rock strata. This process is called oil expulsion. Eventually it was either trapped in an oil reservoir or oil escaped to the surface and was biodegraded by soil bacteria.

Any oil buried deeper entered the gas window of 120 °C to 220 °C and was converted into natural gas by thermal cracking. Thus, below a certain depth, the theories predict that no oil will be found, only unassociated gas. If it went even deeper, even natural gas would be destroyed by high temperatures.

Proposed mechanisms of abiogenic petroleum

Primordial deposits

Thomas Gold's work was focused on hydrocarbon deposits of primordial origin. Meteorites are believed to represent the major composition of material from which the Earth was formed. Some meteorites, such as carbonaceous chondrites, contain carbonaceous material. If a large amount of this material is still within the Earth, it could have been leaking upward for billions of years. The thermodynamic conditions within the mantle would allow many hydrocarbon molecules to be at equilibrium under high pressure and high temperature. Although molecules in these conditions may disassociate, resulting fragments would be reformed due to the pressure. An average equilibrium of various molecules would exist depending upon conditions and the carbon-hydrogen ratio of the material.

Creation within the mantle

Russian researchers performed the above calculations of thermodynamic equilibrium and concluded that hydrocarbon mixes would be created within the mantle. Experiments under high temperatures and pressures produced many hydrocarbons, including n-alkanes through C10H22, from iron oxide, calcium carbonate, and water. Because such materials are in the mantle and in subducted crust, there is no requirement that all hydrocarbons be produced from primordial deposits.

Hydrogen generation

Hydrogen gas and water have been found more than 6 kilometers deep in the upper crust, including in the Siljan Ring boreholes and the Kola Superdeep Borehole. Data from the western United States suggests that aquifers from near the surface may extend to depths of 10 to 20 km. Hydrogen gas can be created by water reacting with silicates, quartz and feldspar, in temperatures in the 25° to 270°C range. These minerals are common in crustal rocks such as granite. Hydrogen may react with dissolved carbon compounds in water to form methane and higher carbon compounds.

One reaction not involving silicates which can create hydrogen is:

Ferrous oxide + Water → Magnetite + hydrogen

$3FeO + H_2O rarr Fe_3O_4 + H_2$

The above reaction operates best at low pressures. At pressures greater than 5 GPa almost no hydrogen is created.

Serpentinite mechanism

One proposed mechanism by which abiogenic petroleum is formed was first proposed by the Ukrainian scientist, Prof. Emmanuil B. Chekaliuk in 1967. He proposed that petroleum could be formed at high temperatures and pressures from inorganic carbon in the form of carbon dioxide, hydrogen and/or methane.

This mechanism is supported by several lines of evidence which are accepted by modern scientific literature. This involves synthesis of oil within the crust via catalysis by chemically reductive rocks. A proposed mechanism for the formation of inorganic hydrocarbons is via natural analogs of the Fischer-Tropsch process known as the serpentinite mechanism or the serpentinite process .

$CH_4 + begin\left\{matrix\right\} frac\left\{1\right\}\left\{2\right\} end\left\{matrix\right\}O_2 rarr 2 H_2 + CO$
$\left(2n+1\right)H_2 + nCO rarr C_nH_\left\{2n+2\right\} + nH_2O$

Serpentinites are ideal rocks to host this process as they are formed from peridotites and dunites, rocks which contain greater than 80% olivine and usually a percentage of Fe-Ti spinel minerals. Most olivines also contain high nickel concentrations (up to several percent) and may also contain chromite or chromium as a contaminant in olivine, providing the needed transition metals.

However, serpentinite synthesis and spinel cracking reactions require hydrothermal alteration of pristine peridotite-dunite, which is a finite process intrinsically related to metamorphism, and further, requires significant addition of water. Serpentinite is unstable at mantle temperatures and is readily dehydrated to granulite, amphibolite, talc-schist and even eclogite. This suggests that methanogenesis in the presence of serpentinites is restricted in space and time to mid-ocean ridges and upper levels of subduction zones. However, water has been found as deep as 12 km, so water-based reactions are dependent upon the local conditions. Oil being created by this process in intracratonic regions is limited by the materials and temperature.

Serpentinite synthesis

A chemical basis for the abiotic petroleum process is the serpentinization of peridotite, beginning with methanogenesis via hydrolysis of olivine into serpentine in the presence of carbon dioxide. Olivine, composed of Forsterite and Fayalite metamorphoses into serpentine, magnetite and silica by the following reactions, with silica from fayalite decomposition (reaction 1a) feeding into the forsterite reaction (1b).

Reaction 1a:
Fayalite + water → Magnetite + aqueous silica + Hydrogen

$3Fe_2SiO_4 + 2H_2O rarr 2Fe_3O_4 + 3SiO_2 + 2H_2$

Reaction 1b:
Forsterite + aqueous silica → Serpentinite

$3Mg_2SiO_4 + SiO_2 + 4H_2O rarr 2Mg_3Si_2O_5\left(OH\right)_4$

When this reaction occurs in the presence of dissolved carbon dioxide (carbonic acid) at temperatures above 500 °C Reaction 2a takes place.

Reaction 2a:
Olivine + Water + Carbonic acid → Serpentine + Magnetite + Methane

$\left(Fe,Mg\right)_2SiO_4 + nH_2O + CO_2 rarr Mg_3Si_2O_5\left(OH\right)_4 + Fe_3O_4 + CH_4$
or, in balanced form: $18 Mg_2SiO_4 + 6 Fe_2SiO_4 + 26 H_2O + CO_2$$12 Mg_3Si_2O_5\left(OH\right)_4 + 4 Fe_3O_4 + CH_4$

However, reaction 2(b) is just as likely, and supported by the presence of abundant talc-carbonate schists and magnesite stringer veins in many serpentinised peridotites;

Reaction 2b:
Olivine + Water + Carbonic acid → Serpentine + Magnetite + Magnesite + Silica

$\left(Fe,Mg\right)_2SiO_4 + nH_2O + CO_2 rarr Mg_3Si_2O_5\left(OH\right)_4 + Fe_3O_4 + MgCO_3 + SiO_2$

The upgrading of methane to higher n-alkane hydrocarbons is via dehydrogenation of methane in the presence of catalyst transition metals (e.g. Fe, Ni). This can be termed spinel hydrolysis.

Spinel polymerization mechanism

Magnetite, chromite and ilmenite are Fe-spinel group minerals found in many rocks but rarely as a major component in non-ultramafic rocks. In these rocks, high concentrations of magmatic magnetite, chromite and ilmenite provide a reduced matrix which may allow abiotic cracking of methane to higher hydrocarbons during hydrothermal events.

Chemically reduced rocks are required to drive this reaction and high temperatures are required to allow methane to be polymerized to ethane. Note that reaction 1a, above, also creates magnetite.

Reaction 3:
Methane + Magnetite → Ethane + Hematite

$nCH_4 + nFe_3O_4 + nH_2O rarr C_2H_6 + Fe_2O_3 + HCO_3 + H^+$

Reaction 3 results in n-alkane hydrocarbons, including linear saturated hydrocarbons, alcohols, aldehydes, ketones, aromatics, and cyclic compounds.

Carbonate decomposition

Calcium carbonate may decompose at around 500 °C through the following reaction:

Reaction 5:
Hydrogen + Calcium carbonate → Methane + Calcium oxide + Water

$4H_2 + CaCO_3 rarr CH_4 + CaO + 2H_2O$

Laboratory experiments

Some research and laboratory experiments explore possible mechanisms, but there is little related geological evidence.

Carbonate reduction

Methane has been formed in laboratory conditions via carbonate reduction at pressures and temperatures similar to that in the upper mantle, but a large amount of water was provided to the reaction in excess of that which is typical in mantle lithology. Likely reactions include:

Reaction 6a:
Ferrous oxide + Calcium carbonate + Water → Hematite + Methane + Calcium oxide

$8FeO + CaCO_3 + 2H_2O rarr 4Fe_2O_3 + CH_4 + CaO$
and
Reaction 6b:
Ferrous oxide + Calcium carbonate + Water → Magnetite + Methane + Calcium oxide
$12FeO + CaCO_3 + 2H_2O rarr 4Fe_3O_4 + CH_4 + CaO$

Methane formation is favored under 1,200 °C at 1 GPa. At 1,500 °C hydrogen production was prevalent. Methane production is most favored at 500 °C and pressures <7 GPa; higher temperatures are expected to lead to carbon dioxide and carbon monoxide production through a reforming equilibrium with methane.

This is cited as evidence of the plausibility of methanogenesis under mantle conditions.

Calcite decomposition

One carbon compound, carbon dioxide, can be created by calcite decomposition at 1,500 °C: Reaction 7:
Calcium carbonate → Calcium oxide + Carbon dioxide
$CaCO_3 rarr CaO + CO_2$

Calcite is likely molten at these temperatures, being a mixture of CaO ions and CO2.

Ethane and Ethylene synthesis

The synthesis of ethane and ethylene has been done at 800 °C, using electric discharges in laboratory experiments. This experiment was in a hot gas, rather than hot mantle fluids. The calculated reactions are:

Carbon dioxide + Methane → Carbon monoxide + Ethane + Water

$CO_2 + 2CH_4 rarr CO + C_2H_6 + H_2O$
and
Carbon dioxide + Ethane → Carbon monoxide + Ethylene + Water
$CO_2 + C_2H_6 rarr CO + C_2H_4 + H_2O$

Fischer-Tropsch process analogs

The Fischer-Tropsch process and similar reactions can create hydrocarbons through direct reactions or reactions with catalysts. Fischer-Tropsch synthesis proceeds from carbon monoxide and hydrogen, while CO2 hydrogenation proceeds from carbon dioxide and hydrogen. Artificial catalytic materials often use rare materials, but some catalysts use somewhat more common materials such as silicon dioxide, aluminum oxide, iron or nickel. Methane production is most common although more complex products such as ethane, propene, propane, and butane have also appeared. The high temperatures needed for direct reactions are reduced to lower temperatures when a catalyst is present.

Although reactions similar to the Fischer-Tropsch process can create hydrocarbons, laboratory and commercial experience has found that catalytic surfaces fail due to carbide formation, catalyst oxidation, sulfur poisoning or being covered with carbon deposits (such as through the Boudouard reaction). Natural formations where such reactions take place continuously would require conditions which avoid such problems. Spreading centers are a special case where new material is being added, so additional catalytic surfaces may (or may not) be created.

Evidence of abiogenic mechanisms

• Scaled particle theory for a simplified perturbed hard-chain, statistical mechanical model predicts that methane compressed to 30 or 40 kbar at 1000 °C (conditions in the mantle) yields hydrocarbons having properties similar to petroleum
• Experiments in diamond anvil high pressure cells have confirmed this theory

Biotic (microbial) hydrocarbons

The "deep biotic petroleum hypothesis", similar to the abiogenic petroleum origin hypothesis, holds that not all petroleum deposits within the Earth's rocks can be explained purely according to the orthodox view of petroleum geology. Thomas Gold used the term the deep hot biosphere to describe the microbes which live underground.

This hypothesis is different from biogenic oil in that the role of deep-dwelling microbes is a biological source for oil which is not of a sedimentary origin and is not sourced from surface carbon. Deep microbial life is only contaminant of primordial hydrocarbons. Parts of microbes yield molecules as biomarkers.

Deep biotic oil is considered to be formed as a byproduct of the life cycle of deep microbes. Shallow biotic oil is considered to be formed as a byproduct of the life cycles of shallow microbes.

The 2nd Law of thermodynamics prohibits petroleum formation at low pressure and temperature. Petroleum is stable within earth's mantle at depths around 150-200 km. At low pressure levels (for instance sedimentary basins) may occur bacterial contamination that leave their fingerprints in oil. It's impossible to form petroleum from biogenic detritus.

Deep microbes

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. Proponents of abiogenic petroleum origin contend that deep microbial life is responsible for the biomarkers (see below) that are generally cited as evidence of biogenic origin. 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 bacteria 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.

Deep microbial sources for petroleum and hydrocarbon chemicals within some sedimentary basins and within some crystalline rocks may explain some contradictory evidence as to the source of these oils.

Specifically, the presence of biomarkers in the extremely rare examples of Proterozoic oils and within oils found in Mesozoic and younger crystalline reservoirs, could be explained as coming from deep-dwelling bacteria.

The abiogenic hypothesis of oil sees the role of deep microbes as providing these biomarkers as contaminants of abiogenic petroleum accumulations, not as products of plant and plankton detritus which have been converted to petroleum via orthodox biogenic processes.

Microbial biomarkers

Extremophile organisms living within the crust (deep heat-loving bacteria thermophiles) are considered a plausible source of biomarkers which are not sourced from kerogen.

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.

The case for shallow bacterial life creating petroleum is apparent from circumstantial evidence at "tar seeps" in sandstone outcrops where live oil is encountered down-dip (e.g. Midway-Sunset field, San Joaquin Valley, California). Bacteria are considered to have "degraded" higher gravity oil to bitumens.

Extrapolation of bacterial degradation to still higher gravity oils and finally to methane leads to the suggestion that all petroleum up to tar and most of the carbon in coal are derivatives of methane, which is progressively stripped of its hydrogen by bacteria and archaea. The resultant partial methane molecules, CH3, CH2, CH, may be called "an-hydrides". Anhydride hypothesis, a New Theory of Petroleum and Coal Generation, is offered by C. Warren Hunt (1999).{

Due to the difficulty in culturing and sampling thermophilic bacteria little was known of their chemistry. As more is learned of bacterial chemistry, more biomarker chemicals can be attributed to bacterial sources. Although extremophile micro-organisms exist deep underground and some metabolize carbon, some of these biomarkers are so far only known from surface plants and remain the most reliable chemical evidence of biogenic genesis of petroleum.

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.

It must also be acknowledged that, if extremophilic bacteria prove to be the source of some parts of known oils, that this remains a biological process.

Thorough rebuttal of biogenic origins based on biomarkers has been offered by Kenney, et al. (2001).

Microbial evidence from petroleum geochemistry

If the above mechanism for microbial petroleum genesis is active and prevalent within the Earth crust and the hypothesis holds true, the geochemistry of petroleum deposits within the Earth’s crust should reflect this mechanism of formation.

The geochemistry of petroleum deposits has been widely and deeply studied by oil companies and academia for more than a century in order to elucidate the origin of petroleum and develop predictive scientific models. Certain findings of this research can be used to interpret petroleum as being either of biogenic or abiogenic origin. These include biomarker chemicals, the optical activity of oils, chirality and the trace metal abundances of oils.

Isotopic evidence

Methane is ubiquitous in crustal fluid and gas . Research continues to attempt to characterise crustal sources of methane as biogenic or abiogenic using carbon isotope fractionation of observed gases (Lollar & Sherwood 2006). There are few clear examples of abiogenic methane-ethane-butane, as the same processes favor enrichment of light isotopes in all chemical reactions, whether organic or inorganic. δ13C of methane overlaps that of inorganic carbonate and graphite in the crust, which are heavily depleted in 12C, and attain this by isotopic fractionation during metamorphic reactions.

One argument for abiogenic oil cites the high carbon depletion of methane as stemming from the observed carbon isotope depletion with depth in the crust. However, diamonds, which are definitively of mantle origin, are not as depleted as methane, which implies that methane carbon isotope fractionation is not controlled by mantle values.

Helium isotope geochemistry is a clear indicator of mantle source within gases. Within the major precambrian shield there is no evidence of mantle helium in gases or groundwaters, which disproves the theory of continued outgassing of primordial methane and helium along structures in the Precambrian basement. Furthermore, there are few examples of primordial helium or mantle helium trapped within oil and gas occurrences. Helium gas has close association with petroleum. Although ³He is primordial, much He gas is from radioactive decay of uranium. Helium gas is associated with light oils, sometimes accompanied by nitrogen that allow petroleum to reach shallow levels in crust. Because helium is a very light gas, commercial accumulations are not (un?)common at Panhandle-Hugoton in USA, Algerian and Russian gas fields.

Panhandle-Hugoton field (Anadarko Basin) in Texas-Oklahoma, USA is the most important gas field with commercial helium content. Helium trapped with hydrocarbons (mainly methane) and nitrogen is possible if there is an efficient seal overlying the reservoir such as salt. Helium trapped within most petroleum occurrences, such as the occurrence in Texas, is of a distinctly crustal character with an Ra ratio of less than 0.0001 that of the atmosphere.

Biomarker chemicals

Certain chemicals found in naturally occurring petroleum contain chemical and structural similarities to compounds found within many living organisms. These include terpenoids, terpenes, pristane, phytane, cholestane, chlorins and porphyrins, which are large, chelating molecules in the same family as heme and chlorophyll. Materials which suggest certain biological processes include tetracyclic diterpane and oleanane.

The presence of these chemicals in crude oil is assumed to be as a result of the inclusion of biological material in the oil. This is predicated upon the theory that these chemicals are released by kerogen during the production of hydrocarbon oils.

However, since the advent of abiogenic hypothesis, the veracity of these assumptions has been called into question and new lines of evidence used to provide alternative explanations. Some are provided by many scientists from around the world including Russia.

Odd-number carbon abundance

Members of the n-alkane series found in petroleum have a slightly greater abundance of odd-numbered carbon chains (propane, pentane, etc.) Likewise, linear carbohydrate molecules in living systems exhibit the same preference for odd carbon numbers.

All mixtures of linear hydrocarbon chains, be they artificial, natural or biological, exhibit this tendency. It arises from the geometry of the covalent bond in linear molecules, so the greater abundances of odd-numbered hydrocarbons need not be of biological origin.

Trace metals

Nickel (Ni), vanadium (V), lead (Pb), arsenic (As), cadmium (Cd), mercury (Hg) and others metals frequently occur in oils. Some heavy crude oils, such as Venezuelan heavy crude have up to 45% vanadium pentoxide content in their ash, high enough that it is a commercial source for vanadium. These metals are common in Earth's mantle, thus their compounds in oils are often called as abiomarkers.

Analysis of 22 trace elements in 77 oils correlate significantly better with chondrite, serpentinized fertile mantle peridotite, and the primitive mantle than with oceanic or continental crust, and shows no correlation with seawater.

Reduced carbon

Petroleum is composed mainly of n-alkanes. Sir Robert Robinson studied the chemical makeup of natural petroleum oils in great detail, and concluded that they were mostly far too hydrogen-rich to be a likely product of the decay of plant debris. However, several processes which generate hydrogen could supply kerogen hydrogenation which is compatible with conventional petroleum generation theories.

Olefins, the unsaturated hydrocarbons, would have been expected to predominate by far in any material that was derived in that way. He also wrote: "Petroleum ... [seems to be] a primordial hydrocarbon mixture into which bio-products have been added."

The presence of low-oxygen and hydroxyl-poor hydrocarbons in natural living media is supported by the presence of natural waxes (n=30+), oils (n=20+) and lipids in both plant matter and animal matter, for instance fats in phytoplankton, zooplankton and so on. These oils and waxes, however, occur in quantities too small to significantly affect the overall hydrogen/carbon ratio of biological materials.

Geological framework

The proposed mechanism for abiogenic petroleum production is robust in hypothesis, leaving aside ambiguous geochemical evidence. The abiogenic hypothesis on the origin of petroleum seeks to explain the origin of commercial accumulations of petrochemicals via chemical mechanisms such as serpentinite catalysis.

The geological observations which are used to support the abiogenic origin of petrochemical deposits should be evaluated on a case-by-case basis for each hydrocarbon deposit, with the presence of no one line of evidence used in isolation to infer genetic conclusions when equivocal or contradictory evidence is available.

The geological observations proposed for the abiogenic hypothesis are presented below, followed by investigation of several key deposits on a case by case basis to evaluate their genesis.

Direct observations

The following are the direct tests of the abiogenic hypothesis of petroleum or impartial evidence generated by observations of the Earth which can be used to argue the hypothesis for or against, and are presented as such.

• The Lost City Hydrothermal Vent Field was determined to have abiogenic hydrocarbon production. Proskurowski et al. wrote, "Radiocarbon evidence rules out seawater bicarbonate as the carbon source for FTT reactions, suggesting that a mantle-derived inorganic carbon source is leached from the host rocks. Our findings illustrate that the abiotic synthesis of hydrocarbons in nature may occur in the presence of ultramafic rocks, water, and moderate amounts of heat.
• The Siljan Ring meteorite crater, Sweden, was proposed by Thomas Gold as the most likely place to test the hypothesis because it was one of the few places in the world where the granite basement was cracked sufficiently (by meteorite impact) to allow oil to seep up from the mantle; furthermore it is infilled with a relatively thin veneer of sediment, which was sufficient to trap any abiogenic oil but was modelled as untenable for a biogenic origin of any oil (it had not developed the 'oil window' and structural traps typical of biogenic plays).

Drilling of the Siljan Ring with the Gravberg-1 7,500 m borehole penetrated the lowest reservoirs. Hydrocarbons were found, though in an economically unviable form of sludge. It was proposed that the eight barrels of oil produced were from the diesel fuel based drilling fluid used to do the drilling, but the diesel was demonstrated to be not of the kind of oil found in the shaft. This well also sampled over of methane-bearing inclusions. To be safe, a second hole was drilled a few miles away with no diesel fuel based drilling fluid and this produced 15 tons of oil.

• Methanogenesis of groundwaters associated with ultramafic dykes and serpentinites, South Island of New Zealand
• Methane outflows are common from drillholes within large Archaean serpentinised olivine adcumulate bodies, such as the Honeymoon Well complex, Yakabindie ultramafic, Mt Clifford dunite, in the Yilgarn Craton, Western Australia.
• Direct observation of bacterial mats and fracture-fill carbonate and humin of bacterial origin in deep boreholes in Iran, Australia, Sweden and Canada
• Presence of deep-dwelling microbes in the Lechuguilla Cave complex, New Mexico

Example proposed abiogenic methane deposits

Panhandle-Hugoton field (Anadarko Basin) in Texas-Oklahoma, USA is the most important gas field with commercial helium content.

The White Tiger oil field in Vietnam has been proposed as an example of abiogenic oil because it is 4,000 m of fractured basement granite, at a depth of 5,000 m. . However, others argue that it contains biogenic oil which leaked into the basement horst from conventional source rocks within the Cuu Long basin .

A major component of mantle-derived carbon is indicated in commercial gas reservoirs in the Pannonian and Vienna basins of Hungary and Austria.

Natural gas pools interpreted as being mantle-derived are the Shengli Field and Songliao Basin, northeastern China.

The geological argument for abiogenic oil

Given the known occurrence of methane and the probable catalysis of methane into higher atomic weight hydrocarbon molecules, the abiogenic hypothesis considers the following to be key observations in support;

• The serpentinite synthesis, graphite synthesis and spinel catalysation models prove the process is viable
• The association of oil deposits with key tectonic structures and plate boundaries, generally in arcs
• The likelihood that abiogenic oil seeping up from the mantle is trapped beneath sediments which effectively seal mantle-tapping faults
• Kudryavtsev's Rule that states petroleum can be found in all layers of a sedimentary basin; subsequently proven to be of limited application; it has also been stated as applying to hydrocarbon deposits, including natural gas, petroleum, and coal. Nikolai Kudryavtsev pointed that the eruptions of mud-volcanoes have liberated such large quantities of methane that even the most prolific gasfield underneath should have been exhausted long ago and also provided several other geological arguments about abiotic and deep origin of petroleum.
• Mass-balance calculations for supergiant oilfields which argue that the calculated source rock could not have supplied the reservoir with the known accumulation of oil, implying deep recharge (Kudryavtsev, 1951)
• Ubiquitous presence of nickel and vanadium (Ni, V) in all oils of the world. Also including other trace elements such as Zn, Pb, Cu, Cd, Cr, Co, As, Sb, Te, Hg, Au, Ag. All these trace-elements settings are related to mantle rocks (dunite/peridotite and serpentinites).
• Common association of helium with hydrocarbons, mainly with methane and nitrogen in gas fields.

Incidental evidence

The proponents of abiogenic oil use several arguments which draw on a variety of natural phenomena in order to support the hypothesis

• The ubiquitous presence of carbon, methane, ammonia and a variety of amino acids within extraterrestrial bodies such as meteorites, comets and on several moons within the Solar System. The Earth acquired much carbon during its creation.
• However, Earth has several anomalies which indicate a complex past which may have affected primordial material. The formation of the Moon was a geologically significant event. Unexplained ratios of elements suggest material has been lost, perhaps through gases being lost to space and through collisional erosion. However this argument is found to be highly speculative by some.
• The modelling of some researchers which shows the Earth was accreted at relatively low temperature, thereby perhaps preserving primordial carbon deposits within the mantle, to drive abiogenic hydrocarbon production
• The presence of natural gas eruptions, flames and explosions during earthquakes and during some volcanic eruptions, mainly in mud volcanoes.
• The presence of vast quantities of methane hydrate (methane clathrate) within deep pelagic oozes within the oceans of the Earth, cited as evidence of abiogenic methane generation from serpentinitisation of the oceanic crust.
• The presence of continuous methane upwelling through gas chimneys (gas vent) in oceans forming pockmark features, cold seeps, methane related diagenetic carbonates, bentonic ecosystems such as cold-water corals (deep-water corals), methane flares from sea bottom, shale diapirs formed by gas interaction, submarine and terrestrial mud-volcanoes. It is important to note that bacterial reworking of primordial methane that come from great depths yield biogenic methane at shallow levels in crust
• The presence of methane within the gases and fluids of mid-ocean ridge spreading centre hydrothermal fields
• The presence of intraplate earthquakes and deep focus earthquakes, apparently caused by movement of vast quantities of mantle methane and hydrocarbons

The geological argument against

Key arguments against chemical reactions, such as the serpentinite mechanism, as being the major source of hydrocarbon deposits within the crust are;

• The lack of available pore space within rocks as depth increases
• This is contradicted by numerous studies which have documented the existence of hydrologic systems operating over a range of scales and at all depths in the continental crust.
• The presence of no commercial hydrocarbon deposits within the crystalline shield areas of the major cratons especially around key deep seated structures which are predicted to host oil by the abiogenic hypothesis
• Limited evidence that major serpentinite belts underlie continental sedimentary basins which host oil
• Lack of conclusive proof that carbon isotope fractionation observed in crustal methane sources is entirely of abiogenic origin (Lollar et al. 2006)
• Mass balance problems of supplying enough carbon dioxide to serpentinite within the metamorphic event before the peridotite is fully reacted to serpentinite
• Drilling of the Siljan Ring failed to find commercial quantities of gas, thus providing a counter example to Kudryavtsev's Rule and failing to locate the predicted abiogenic gas
• Helium in the Siljan Gravberg-1 well was depleted in 3He and not consistent with a mantle origin

• The distribution of sedimentary basins is caused by plate tectonics, with sedimentary basins forming on either side of a volcanic arc, which explains the distribution of oil within these sedimentary basins
• Kudryavtsev's Rule has been explained for oil and gas (not coal): Gas deposits which are below oil deposits can be created from that oil or its source rocks. Because natural gas is less dense than oil, as kerogen and hydrocarbons are generating gas the gas fills the top of the available space. Oil is forced down, and can reach the spill point where oil leaks around the edge(s) of the formation and flows upward. If the original formation becomes completely filled with gas then all the oil will have leaked above the original location.

Arguments against the incidental evidence

• Gas ruptures during earthquakes are more likely to be sourced from biogenic methane generated in unconsolidated sediment from existing organic matter, released by earthquake liquefaction of the reservoir during tremors
• The presence of methane hydrate is arguably produced by bacterial action upon organic detritus falling from the littoral zone and trapped in the depth due to pressure and temperature
• The likelihood of vast concentrations of methane in the mantle is very slim, given mantle xenoliths have negligible methane in their fluid inclusions; conventional plate tectonics explains deep focus quakes better, and the extreme confining pressures invalidate the hypothesis of gas pockets causing quakes
• Further evidence is the presence of diamond within kimberlites and lamproites which sample the mantle depths proposed as being the source region of mantle methane (by Gold et al).

Bibliography

• Kudryavtsev N.A., 1959. Geological proof of the deep origin of Petroleum. Trudy Vsesoyuz. Neftyan. Nauch. Issledovatel Geologoraz Vedoch. Inst. No.132, pp. 242-262