# Abiogenic petroleum origin

Abiogenic petroleum origin is a hypothesis that was proposed as an alternative mechanism of petroleum origin. According to the abiogenic hypothesis, petroleum was formed from deep carbon deposits, perhaps dating to the formation of the Earth. Supporters of the abiogenic hypothesis suggest that a great deal more petroleum exists on Earth than commonly thought, and that petroleum may originate from carbon-bearing fluids that migrate upward from the mantle. The presence of huge amounts of methane on Saturn's moon Titan and in the atmospheres of Jupiter, Saturn, Uranus and Neptune is cited[1] as evidence of the formation of hydrocarbons without biology.[2]

The hypothesis was first proposed by Georg Agricola in the 16th century and various abiogenic hypotheses were proposed in the 19th century, most notably by Prussian geographer Alexander von Humboldt, the Russian chemist Dmitri Mendeleev and the French chemist Marcellin Berthelot. Abiogenic hypotheses were revived in the last half of the 20th century by Soviet scientists who had little influence outside the Soviet Union because most of their research was published in Russian. The hypothesis was re-defined and made popular in the West by Thomas Gold who published all his research in English.[1]

Although the abiogenic hypothesis was accepted by many geologists in the former Soviet Union, it fell out of favor at the end of the 20th century because it never made any useful prediction for the discovery of oil deposits.[1] 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).[1] Geologists now consider the abiogenic formation of petroleum scientifically unsupported, and they agree that petroleum is formed from organic material.[1] However, some argue that the abiogenic theory cannot be dismissed yet because the mainstream theory still has to be established conclusively.[3]

It has been recently discovered that thermophilic bacteria, in the sea bottom and in cooling magma, produce methane and hydrocarbon gases,[4][5] but studies indicate they are not produced in commercially significant quantities (i.e. in extracted hydrocarbon gases, the median abiogenic hydrocarbon content is 0.02%, or 1 part in 5,000).[6]

## History of the abiogenic hypothesis

The abiogenic hypothesis is usually traced to the early part of the 19th century. At the time, the chemical nature of petroleum was not known.

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 (Cumaná) on the northeast coast of Venezuela.[7] In 1804 he is quoted as saying, "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 18th century 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. Other prominent proponents of what would become the abiogenic hypothesis included Mendeleev[8] and Berthelot.

Russian geologist Nikolai Alexandrovitch Kudryavtsev proposed the modern abiotic hypothesis of petroleum in 1951. On the basis of his analysis of the Athabasca Oil Sands in Alberta, Canada, he 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.[9] 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.[1] More recently, Jack Kenney of Gas Resources Corporation has come to prominence.[10][11][12]

## State of current research

The weight of evidence currently shows that petroleum is derived from ancient biomass.[13] However, it still has to be established conclusively, which means that alternative theories like abiogenic petroleum cannot be dismissed for now.[3] The initial evidence was based on the isolation of molecules from petroleum that closely resemble known biomolecules (Figure).

Structure of a biomarker extracted from petroleum and simplified structure of chlorophyll a.

Petroleum geologists agree that oil originates from vast quantities of dead marine plankton or plant material that sank into the mud of shallow seas. Under the resulting anaerobic conditions, organic compounds remained in a reduced state where 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 80 and 180 °C. Most of the organic compounds degraded into the straight-chain hydrocarbons that comprise most of petroleum. This process is called the generation kitchen.[citation needed] 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.

Oil buried deeper entered the "gas window" of more than 160 °C and was converted into natural gas by thermal cracking. This gives the prediction that only unassociated gas and not oil will be found below a certain depth. At greater depths, even natural gas would be pyrolyzed.

A 2006 review article by Glasby presented arguments against the abiogenic origin of petroleum on a number of counts.[1]

## Foundations of the hypotheses

Within the mantle, carbon may exist as hydrocarbons—chiefly methane—and as elemental carbon, carbon dioxide, and carbonates.[12] The abiotic hypothesis is that the full suite of hydrocarbons found in petroleum can either be generated in the mantle by abiogenic processes,[12] or by biological processing of those abiogenic hydrocarbons, and that the source-hydrocarbons of abiogenic origin 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 hypotheses reject the supposition that certain molecules found within petroleum, known as biomarkers, are indicative of the biological origin of petroleum. They contend that these molecules mostly come from microbes feeding on petroleum 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.[11]

The hypothesis is founded primarily upon:

Proponents Item
Gold The presence of methane on other planets, meteors, moons and comets[14][15]
Gold, Kenney Proposed mechanisms of abiotically chemically synthesizing hydrocarbons within the mantle[10][11][12]
Kudryavtsev, Gold Hydrocarbon-rich areas tend to be hydrocarbon-rich at many different levels[2]
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[2]
Gold Interpretations of the chemical and isotopic composition of natural petroleum[2]
Kudryavtsev, Gold The presence of oil and methane within non-sedimentary rocks upon the Earth[16]
Gold The existence of methane hydrate deposits[2]
Gold Perceived ambiguity in some assumptions and key evidence used in the conventional understanding of petroleum origin.[2][10]
Gold Bituminous coal creation is based upon deep hydrocarbon seeps[2]
Gold Surface carbon budget and oxygen levels stable over geologic time scales[2]
Kudryavtsev, Gold The biogenic explanation does not explain some hydrocarbon deposit characteristics[2]
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[17]
Gold The association of hydrocarbons with helium, a noble gas[2]

## Investigation of the hypotheses

Little research is directed on establishing abiogenic petroleum or methane, although the Carnegie Institution for Science have found that ethane and heavier hydrocarbons can be synthesized under conditions of the upper mantle.[18] Research mostly related to astrobiology and the deep microbial biosphere and serpentinite reactions, however, continue to provide insight into the contribution of abiogenic hydrocarbons into petroleum accumulations.

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[23]
• Possible energy sources and metabolic pathways which may be used in a deep biosphere[24][5]
• Investigations into the reworking primordial hydrocarbons by bacteria and their effects on carbon isotope fractionation

## 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.[25]

### Creation within the mantle

Russian researchers 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.[12] 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 6,000 metres (20,000 ft) deep in the upper crust 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 10,000 metres (33,000 ft) to 20,000 metres (66,000 ft). Hydrogen gas can be created by water reacting with silicates, quartz, and feldspar at temperatures in the range of 25 °C (77 °F) to 270 °C (518 °F). 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.[26]

One reaction not involving silicates which can create hydrogen is:

Ferrous oxide + water → magnetite + hydrogen

$\mathrm{3FeO + H_2O \rarr Fe_3O_4 + H_2}$

The above reaction operates best at low pressures. At pressures greater than 5 gigapascals (49,000 atm) almost no hydrogen is created.[20]

However, the Siljan Ring borehole found no hydrocarbon, despite Thomas Gold's hypothesis predicting that they should be found. (see Siljan section in this article)

### Serpentinite mechanism

In 1967, the Ukrainian scientist Emmanuil B. Chekaliuk 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[27] is via natural analogs of the Fischer-Tropsch process known as the serpentinite mechanism or the serpentinite process.[17][28]

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

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, talcschist 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 12,000 metres (39,000 ft),[29] 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.[28] 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

$\mathrm{3Fe_2SiO_4 + 2H_2O \rarr 2Fe_3O_4 + 3SiO_2 + 2H_2 }$

Reaction 1b:
Forsterite + aqueous silica → serpentinite

$\mathrm{3Mg_2SiO_4 + SiO_2 + 4H_2O \rarr 2Mg_3Si_2O_5(OH)_4}$

When this reaction occurs in the presence of dissolved carbon dioxide (carbonic acid) at temperatures above 500 °C (932 °F) Reaction 2a takes place.

Reaction 2a:
Olivine + water + carbonic acid → serpentine + magnetite + methane

$\mathrm{(Fe,Mg)_2SiO_4 + nH_2O + CO_2 \rarr Mg_3Si_2O_5(OH)_4 + Fe_3O_4 + CH_4}$

or, in balanced form: $\mathrm{18 Mg_2SiO_4 + 6 Fe_2SiO_4 + 26 H_2O + CO_2}$$\mathrm{12 Mg_3Si_2O_5(OH)_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

$\mathrm{(Fe,Mg)_2SiO_4 + nH_2O + CO_2 \rarr Mg_3Si_2O_5(OH)_4 + Fe_3O_4 + MgCO_3 + SiO_2}$

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

$\mathrm{nCH_4 + nFe_3O_4 + nH_2O \rarr C_2H_6 + Fe_2O_3 + HCO_3 + H^+}$

Reaction 3 results in n-alkane hydrocarbons, including linear saturated hydrocarbons, alcohols, aldehydes, ketones, aromatics, and cyclic compounds.[28]

### Carbonate decomposition

Calcium carbonate may decompose at around 500 °C (932 °F) through the following reaction:[20]

Reaction 5:
Hydrogen + calcium carbonate → methane + calcium oxide + water

$\mathrm{4H_2 + CaCO_3 \rarr CH_4 + CaO + 2H_2O}$

Note that CaO (lime) is not a mineral species found within natural rocks. Whilst this reaction is possible, it is not plausible.

### Evidence of abiogenic mechanisms

• Theoretical calculations by J.F. Kenney using scaled particle theory (a statistical mechanical model) for a simplified perturbed hard-chain predict that methane compressed to 30,000 bars (3.0 GPa) or 40,000 bars (4.0 GPa) kbar at 1,000 °C (1,830 °F) (conditions in the mantle) is relatively unstable in relation to higher hydrocarbons. However, these calculations do not include methane pyrolysis yielding amorphous carbon and hydrogen, which is recognized as the prevalent reaction at high temperatures.[11][12]
• Experiments in diamond anvil high pressure cells have resulted in partial conversion of methane and inorganic carbonates into light hydrocarbons.,[30][31]

## 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.[2][32][33]

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 a 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.

### Microbial biomarkers

Thomas Gold, in a 1999 book, cited the discovery of thermophile bacteria in the Earth's crust as new support for the postulate that these bacteria could explain the existence of certain biomarkers in extracted petroleum.[2] A rebuttal of biogenic origins based on biomarkers has been offered by Kenney, et al. (2001).[1][11]

#### Isotopic evidence

Methane is ubiquitous in crustal fluid and gas.[5] 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.[34]

Commercially extractable concentrations of helium (greater than 0.3%) are present in natural gas from the Panhandle-Hugoton fields in the USA, as well as from some Algerian and Russian gas fields.[35][36]

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.[37][38]

The Chimaera gas seep, near Antalya (SW Turkey), new and thorough molecular and isotopic analyses including methane (~87% v/v; D13C1 from -7.9 to -12.3 ‰; D13D1 from -119 to -124 ‰), light alkanes (C2+C3+C4+C5 = 0.5%; C6+: 0.07%; D13C2 from -24.2 to -26.5 ‰; D13C3 from -25.5 to -27 ‰), hydrogen (7.5 to 11%), carbon dioxide (0.01-0.07%; D13CCO2: -15 ‰), helium (~80 ppmv; R/Ra: 0.41) and nitrogen (2-4.9%; D15N from -2 to -2.8 ‰) converge to indicate that the seep releases a mixture of organic thermogenic gas, related to mature Type III kerogen occurring in Paleozoic and Mesozoic organic rich sedimentary rocks, and abiogenic gas produced by low temperature serpentinization in the Tekirova ophiolitic unit.[39]

### 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 a result of the inclusion of biological material in the oil; these chemicals are released by kerogen during the production of hydrocarbon oils, as these are chemicals highly resistant to degradation and plausible chemical paths have been studied. Abiotic defenders state that biomarkers get into oil during its way up as it gets in touch with ancient fossils. However a more plausible explanation is that biomarkers are traces of biological molecules from bacteria (archaea) that feed on primordial hydrocarbons and die in that environment. For example, hopanoids are just parts of the bacterial cell wall present in oil as contaminant.[2]

### 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. Abiotic supporters argue that these metals are common in Earth's mantle, but relatively high contents of nickel, vanadium, lead and arsenic can be usually found in almost all marine sediments.

Analysis of 22 trace elements in 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.[17]

### Reduced carbon

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, assuming a dual origin for Earth hydrocarbons.[25] However, several processes which generate hydrogen could supply kerogen hydrogenation which is compatible with the conventional explanation.[40]

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."

This has however been demonstrated later to be a misunderstanding by Robinson, related to the fact that only short duration experiments were available to him. Olefins are thermally very unstable (that is why natural petroleum normally does not contain such compounds) and in laboratory experiments that last more than a few hours, the olefins are no longer present.[citation needed]

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. However, after the discovery of highly aliphatic biopolymers in algae, and that oil generating kerogen essentially represent concentrates of such materials, no theoretical problem exists anymore.[citation needed] Also, the millions of source rock samples that have been analyzed for petroleum yield by the petroleum industry have confirmed the large quantities of petroleum found in sedimentary basins.

## Field observations

Proponents sometimes cite occurrences of abiotic petroleum in commercial amounts, in the oil wells in offshore Vietnam, Eugene Island block 330 oil field, and the Dnieper-Donets Basin, but the origins of all these wells can be explained with the biotic theory.[1][41] Modern geologists think that commercially profitable deposits of abiotic petroleum could be found, but no current deposit has convincing evidence that it originated from abiotic sources.[41]

The Soviet school saw evidence of their hypothesis in the fact that some oil reservoirs exist in non-sedimentary rocks such as granite, metamorphic or porous volcanic rocks. However, critics note that non-sedimentary rocks served as reservoirs for biologically originated oil expelled from nearby sedimentary source rock through common migration or re-migration mechanisms.[41]

The following observations have been commonly used to argue for the abiogenic hypothesis, however all the observations of actual petroleum can also be fully explained by biotic origin.[41]

### Lost City Hydrothermal Vent Field

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."[42]

### Siljan Ring, Sweden

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 not having been subjected to the heat and pressure conditions (known as the "oil window") normally required to create biogenic oil. However, some geochemists concluded by geochemical analysis that the oil in the seeps came from the organic-rich Ordovician Tretaspis shale, where it was heated by the meteorite impact.[43]

In 1986–1990 The Gravberg-1 borehole was drilled through the deepest rock in the Siljan Ring in which proponents had hoped to find hydrocarbon reservoirs. It stopped at the depth of 6,800 metres (22,300 ft) due to drilling problems, after private investors spent \$40 million.[44] Some eighty barrels of magnetite paste and hydrocarbon-bearing sludge were recovered from the well; Gold maintained that the hydrocarbons were chemically different from, and not derived from, those added to the borehole, but analyses showed that the hydrocarbons were derived from the diesel fuel-based drilling fluid used in the drilling.[44][45][46][47] This well also sampled over 13,000 feet (4,000 m) of methane-bearing inclusions.[48]

In 1991–1992, a second borehole, Stenberg-1, was drilled a few miles away to a depth of 6,500 metres (21,300 ft), finding similar results. Again, no abiotic hydrocarbons were found.[1][34]

### Bacterial mats

Direct observation of bacterial mats and fracture-fill carbonate and humin of bacterial origin in deep boreholes in Iran, Australia,[49]

### Example proposed abiogenic methane deposits

Panhandle-Hugoton field (Anadarko Basin) in the south-central United States is the most important gas field with commercial helium content. Some abiogenic proponents interpret this as evidence that both the helium and the natural gas came from the mantle.[37][38][50][51]

The Bạch Hổ 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.[52] However, others argue that it contains biogenic oil which leaked into the basement horst from conventional source rocks within the Cuu Long basin.[16][53]

A major component of mantle-derived carbon is indicated in commercial gas reservoirs in the Pannonian and Vienna basins of Hungary and Austria.[54]

Natural gas pools interpreted as being mantle-derived are the Shengli Field[55] and Songliao Basin, northeastern China.[56][57]

The Chimaera gas seep, near Çıralı, Antalya (southwest Turkey), has been continuously active for millennia and it is known to be the source of the first Olympic fire in the Hellenistic period. On the basis of chemical composition and isotopic analysis, the Chimaera gas is said to be about half biogenic and half abiogenic gas, the largest emission of biogenic methane discovered; deep and pressurized gas accumulations necessary to sustain the gas flow for millennia, posited to be from an inorganic source, may be present.[39] Local geology of Chimaera flames, at exact position of flames, reveals contact between serpentinized ophiolite and carbonate rocks. Fischer-Tropsch process can be suitable reaction to form hydrocarbons gases.

## 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[17][28]
• The likelihood that abiogenic oil seeping up from the mantle is trapped beneath sediments which effectively seal mantle-tapping faults[27]
• 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, Geological proof of the deep origin of Petroleum, 1951)
• The presence of hydrocarbons encapsulated in diamonds [58]

### 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 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[59]
• The presence of methane within the gases and fluids of mid-ocean ridge spreading centre hydrothermal fields[60]

## Geological argument against

Oil deposits are not directly associated with tectonic structures.

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.[61]
• The lack of any hydrocarbon 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.[34] See Siljan Lake.
• 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)[5]
• 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 oil,[34] thus providing a counter example to Kudryavtsev's Rule[44] and failing to locate the predicted abiogenic oil.
• Helium in the Siljan Gravberg-1 well was depleted in 3He and not consistent with a mantle origin[62]
• The Gravberg-1 well only produced 84 barrels (13.4 m3) of oil, which later was shown to derive from organic additives, lubricants and mud used in the drilling process.[44][45][46]
• 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.[63]
• Ubiquitous diamondoids in natural hydrocarbons such as oil, gas and condensates are composed of carbon from biological sources, unlike the carbon found in normal diamonds.[64]

### 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.).[25]

## Extraterrestrial argument

The presence of methane on Saturn's moon Titan and in the atmospheres of Jupiter, Saturn, Uranus and Neptune is cited as evidence of the formation of hydrocarbons without biology,[1][41] for example by Thomas Gold.[2] (Terrestrial natural gas is composed primarily of methane). Some comets contain "massive amounts of an organic material almost identical to high grade oil shale (kerogen)," the equivalent of cubic kilometers of such mixed with other material;[65] for instance, corresponding hydrocarbons were detected during a probe fly-by through the tail of Comet Halley in 1986.[66]

## References

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