Abiogenic petroleum origin is a term used to describe a number of different hypotheses which propose that petroleum and natural gas are formed by inorganic means rather than by the decomposition of organisms. The two principal abiogenic petroleum hypotheses, proposing that all petroleum is abiogenic, the deep gas hypothesis[further explanation needed] of Thomas Gold and the deep abiotic petroleum hypothesis,[further explanation needed] have been scientifically reviewed and largely rejected.[why?]
Earlier studies of mantle-derived rocks from many places have shown that hydrocarbons from the mantle region can be found widely around the globe. However, the content of such hydrocarbons in such rocks are extremely low and seem to lack accumulation in concentrations that would render them feasible for exploitation. Scientific consensus on the origin of oil and gas is that all natural oil and gas deposits on Earth are fossil fuels and are, therefore, biogenic. Globally significant amounts of abiotic oil in the crust can even be ruled out. Yet, abiogenesis of small quantities of oil and gas remains an area of ongoing research in limited, mostly just laboratory, scale.
Some abiogenic hypotheses have proposed that oil and gas did not originate from fossil deposits, but have instead originated from deep carbon deposits, present since the formation of the Earth. Additionally, it has been suggested that hydrocarbons may have arrived on Earth from solid bodies such as comets and asteroids from the late formation of the Solar System, carrying hydrocarbons with them.
Certain abiogenic hypotheses gained some popularity among geologists over the past several centuries. Scientists in the former Soviet Union widely held that significant petroleum deposits could be attributed to abiogenic origin, though this view fell out of favor toward the end of the 20th century because they did not make useful predictions for the discovery of oil deposits. It has now been generally accepted that the theory of abiogenic formation of petroleum has insufficient scientific support and that oil and gas fuels on Earth are formed almost exclusively from organic material.
The abiogenic hypothesis regained some support in 2009 when researchers at the Royal Institute of Technology (KTH) in Stockholm reported they believed they had proven that fossils from animals and plants are not necessary for crude oil and natural gas to be generated. In his 2014 publication Chemistry of the Climate System, German chemist Detlev Moller documents sufficient reliable evidence to show that both processes can be shown to co-exist, that they're not mutually exclusive.
An abiogenic hypothesis was first proposed by Georgius Agricola in the 16th century and various additional abiogenic hypotheses were proposed in the 19th century, most notably by Prussian geographer Alexander von Humboldt,[when?] the Russian chemist Dmitri Mendeleev[when?] and the French chemist Marcellin Berthelot.[when?] 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 developed his theories from 1979 to 1998, and published his research in English.
Abraham Gottlob Werner and the proponents of neptunism in the 18th century regarded basaltic sills as solidified oils or bitumen. While these notions proved unfounded, the basic idea of an association between petroleum and magmatism persisted. Alexander von Humboldt proposed an inorganic abiogenic hypothesis for petroleum formation after he observed petroleum springs in the Bay of Cumaux (Cumaná) on the northeast coast of Venezuela. He is quoted as saying in 1804, "the 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". Other early prominent proponents of what would become the generalized abiogenic hypothesis included Dmitri Mendeleev and Berthelot.
In 1951, the Soviet geologist Nikolai Alexandrovitch Kudryavtsev proposed the modern abiotic hypothesis of petroleum. 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 therefore offered abiotic deep petroleum as the most plausible explanation. (Humic coals have since been proposed for the source rocks.) Others who continued Kudryavtsev's work included 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 a prominent proponent of the abiogenic hypothesis in the West until his death in 2004. More recently, Jack Kenney of Gas Resources Corporation has come to prominence, supported by studies by researchers at the Royal Institute of Technology in Stockholm.
The weight of evidence currently shows that petroleum is derived from ancient biomass. However, it still has to be established conclusively, which means that abiogenic alternative theories of petroleum formation cannot be dismissed.
A 2006 review article by Geoffrey Glasby presented arguments against the abiogenic origin of petroleum[clarification needed] on a number of counts;  on the other hand several recent studies demonstrate that hydrocarbons heavier than methane can be produced by abiogenic processes
Within the mantle, carbon may exist as hydrocarbons—chiefly methane—and as elemental carbon, carbon dioxide, and carbonates. The abiotic hypothesis is that the full suite of hydrocarbons found in petroleum can either be generated in the mantle by abiogenic processes, 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 generally 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.
Some of the evidence used to support abiogenic theories includes:
|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 conventional understanding of petroleum origin.|
|Gold||Bituminous coal creation is based upon deep hydrocarbon seeps|
|Gold||Surface carbon budget and oxygen levels stable over geologic time scales|
|Kudryavtsev, Gold||The biogenic explanation does 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[clarification needed]|
This section's representation of one or more viewpoints about a controversial issue may be unbalanced or inaccurate.(July 2009)
As of 2009[update], little research is directed towards establishing abiogenic petroleum or methane, although the Carnegie Institution for Science has reported that ethane and heavier hydrocarbons can be synthesized under conditions of the upper mantle. 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
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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.
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. Because such materials are in the mantle and in subducted crust, there is no requirement that all hydrocarbons be produced from primordial deposits.
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.
One reaction not involving silicates which can create hydrogen is:
The above reaction operates best at low pressures. At pressures greater than 5 gigapascals (49,000 atm) almost no hydrogen is created.
Thomas Gold reported that hydrocarbons were found in the Siljan Ring borehole and in general increased with depth, although the venture was not a commercial success.
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 is via natural analogs of the Fischer-Tropsch process known as the serpentinite mechanism or the serpentinite process.
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 12,000 metres (39,000 ft), 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.
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).
Fayalite + water → magnetite + aqueous silica + hydrogen
Forsterite + aqueous silica → serpentinite
When this reaction occurs in the presence of dissolved carbon dioxide (carbonic acid) at temperatures above 500 °C (932 °F) Reaction 2a takes place.
Olivine + water + carbonic acid → serpentine + magnetite + methane
or, in balanced form: →
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;
Olivine + water + carbonic acid → serpentine + magnetite + magnesite + silica
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.
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.
Methane + magnetite → ethane + hematite
Calcium carbonate may decompose at around 500 °C (932 °F) through the following reaction:
Hydrogen + calcium carbonate → methane + calcium oxide + water
Note that CaO (lime) is not a mineral species found within natural rocks. Whilst this reaction is possible, it is not plausible.
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 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.
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. A rebuttal of biogenic origins based on biomarkers has been offered by Kenney, et al. (2001).
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.
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.
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.
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 
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.
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.
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. However, several processes which generate hydrogen could supply kerogen hydrogenation which is compatible with the conventional explanation.
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.
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. 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.
Occurrences of abiotic petroleum in commercial amounts in the oil wells in offshore Vietnam are sometimes cited, as well as in the Eugene Island block 330 oil field, and the Dnieper-Donets Basin. However, the origins of all these wells can also be explained with the biotic theory. 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.
The Soviet school saw evidence of their[clarification needed] hypothesis in the fact that some oil reservoirs exist in non-sedimentary rocks such as granite, metamorphic or porous volcanic rocks. However, opponents noted that non-sedimentary rocks served as reservoirs for biologically originated oil expelled from nearby sedimentary source rock through common migration or re-migration mechanisms.
The following observations have been commonly used to argue for the abiogenic hypothesis, however each observation of actual petroleum can also be fully explained by biotic origin:
The Lost City hydrothermal 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 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.
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. 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. This well also sampled over 13,000 feet (4,000 m) of methane-bearing inclusions.
Direct observation of bacterial mats and fracture-fill carbonate and humin of bacterial origin in deep boreholes in Australia are also taken as evidence for the abiogenic origin of petroleum.
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.
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. However, others argue that it contains biogenic oil which leaked into the basement horst from conventional source rocks within the Cuu Long basin.
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. 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 hydrocarbon gases.
Given the known occurrence of methane and the probable catalysis of methane into higher atomic weight hydrocarbon molecules, various abiogenic theories consider the following to be key observations in support of abiogenic hypotheses:
The proponents of abiogenic oil also use several arguments which draw on a variety of natural phenomena in order to support the hypothesis:
Arguments against chemical reactions, such as the serpentinite mechanism, being a source of hydrocarbon deposits within the crust include:
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 biological intermediate forms, for example by Thomas Gold. (Terrestrial natural gas is composed primarily of methane). Some comets contain massive amounts of organic compounds, the equivalent of cubic kilometers of such mixed with other material; for instance, corresponding hydrocarbons were detected during a probe flyby through the tail of Comet Halley in 1986.
The first suggestion of the emanation theory for the origin of petroleum seems to have come from Alexander von Humboldt, who in 1804, in describing the petroleum springs in the Bay of Cumeaux on the Venezuelan coast, throws out the suggestion that 'the petroleum is the product of a distillation from great depths [...]'.
However, it is now generally accepted, but not conclusively proven, that petroleum formation predominantly arises from the decay of organic matter in the earth. (...) Nevertheless, alternative theories should not be dismissed until it can be conclusively established that petroleum formation is due to one particular aspect of geochemistry.
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