The abiogenic petroleum origin hypothesis suggests that most of Earth's petroleum and natural gas deposits formed without the involvement of living organisms, often called abiotic oil. Most scientific evidence supports the idea that petroleum deposits mainly come from the breakdown of ancient living organisms. However, the presence of hydrocarbons on space objects, such as Saturn's moon Titan, shows that hydrocarbons can sometimes form naturally without living things. A history of theories about abiotic hydrocarbon origins has been recorded.

Thomas Gold's "deep gas hypothesis" suggests that some natural gas deposits may have formed from hydrocarbons deep within Earth's mantle. Earlier studies of rocks from Earth's mantle in many locations have found evidence of hydrocarbons from the mantle. These studies show that hydrocarbons from the mantle can be found worldwide. However, these hydrocarbons are present in very small amounts. While there may be some abiotic hydrocarbon deposits, large amounts of abiotic hydrocarbons globally are considered unlikely.

Overview hypotheses

Some theories suggest that oil and gas did not come from the remains of ancient plants and animals. Instead, they may have come from deep carbon deposits that have been present since Earth was formed.

In 2009, scientists from the KTH Royal Institute of Technology in Stockholm said they had found evidence showing that fossils from animals and plants are not needed to create crude oil and natural gas.

History

In the 16th century, Georgius Agricola first proposed an abiogenic hypothesis. Additional abiogenic hypotheses were suggested in the 19th century, most notably by Alexander von Humboldt in 1804, Dmitri Mendeleev in 1877, and Marcellin Berthelot. In the second half of the 20th century, Soviet scientists revived the hypothesis, but their work had limited influence outside the Soviet Union because most research was published in Russian. The hypothesis was later redefined and popularized in the West by astronomer Thomas Gold, who developed his theories from 1979 to 1998 and published his findings in English.

In the 18th century, Abraham Gottlob Werner and supporters of neptunism believed that basaltic sills were solidified oils or bitumen. Though these ideas were incorrect, the idea that petroleum might be linked to volcanic activity remained. In 1804, Alexander von Humboldt proposed an inorganic abiogenic hypothesis after observing petroleum springs near the Bay of Cumaux in Venezuela. He stated that petroleum forms through distillation from deep within the Earth and originates from ancient rocks beneath volcanic forces. Other early supporters of the abiogenic hypothesis included Dmitri Mendeleev and Marcellin Berthelot.

In 1951, Soviet geologist Nikolai Kudryavtsev introduced the modern abiotic hypothesis of petroleum. Based on his study of the Athabasca Oil Sands in Alberta, Canada, he argued that the vast amounts of hydrocarbons could not be explained by "source rocks," leading him to propose abiotic deep petroleum as the most likely explanation. Later, some researchers suggested humic coals as potential 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.

After Thomas Gold’s death in 2004, Jack Kenney of Gas Resources Corporation became a prominent advocate for the hypothesis. His work has been supported by studies conducted by researchers at the Royal Institute of Technology (KTH) in Stockholm, Sweden.

Foundations of abiogenic hypotheses

Inside the Earth's mantle, carbon can be found in different forms, including hydrocarbons like methane, as well as elemental carbon, carbon dioxide, and carbonates. The abiotic theory suggests that all the hydrocarbons found in petroleum could form either directly in the mantle through non-living processes or through biological activity acting on those non-living hydrocarbons. According to this theory, hydrocarbons that originate from non-living sources in the mantle can move upward into the Earth's crust. These hydrocarbons may eventually reach the surface or become trapped in solid rock layers that prevent their movement, forming petroleum reservoirs.

Abiotic theories often argue against the idea that certain molecules in petroleum, called biomarkers, prove petroleum has a biological origin. Instead, they claim these molecules may come from microorganisms that feed on petroleum as it moves upward through the crust. They also note that some of these molecules are found in meteorites, which have never interacted with living organisms, and that some can form through chemical reactions in petroleum without requiring life.

Some evidence supporting abiogenic theories includes:

Recent investigation of abiogenic hypotheses

As of 2009, most research does not focus on proving that petroleum or methane forms without living organisms. However, the Carnegie Institution for Science found that ethane and heavier hydrocarbons can form under high pressure and temperature in the Earth's upper mantle. Research related to life in space, deep underground life, and chemical reactions in certain rocks continues to help scientists understand how non-living processes might contribute to petroleum formation.

Key research areas include:
• How rocks allow oil to move through them and the paths oil might take.
• Chemical reactions in mantle rocks and other natural processes similar to laboratory experiments that create hydrocarbons.
• Hydrocarbons found in space objects like meteorites, comets, and asteroids, as well as ancient sources of carbon on Earth. These hydrocarbons may form from reactions involving materials like chromium, iron, nickel, vanadium, manganese, and cobalt.
• Studies of chemical clues in groundwater, rocks, gases, and the makeup of noble gases and nitrogen in oil fields.

Common criticisms include:
• If oil formed in the mantle, it would likely be found near fault zones, as these areas allow oil to move from the mantle to the Earth's crust. Mantle areas near subduction zones are also more oxidizing. However, oil deposits are not typically found in fault zones, with some exceptions.

Proposed mechanisms of abiogenic petroleum

Thomas Gold studied hydrocarbon deposits that may have formed a long time ago. Meteorites are thought to show what materials were present when Earth formed. Some meteorites, like carbonaceous chondrites, contain carbon-based materials. If a large amount of this material is still inside Earth, it might have been moving upward for billions of years. The high pressure and temperature in Earth’s mantle could allow many hydrocarbon molecules to remain stable. Even if molecules break apart, the pressure might cause them to reform. The balance of molecules depends on conditions and the ratio of carbon to hydrogen in the material.

Russian scientists found that hydrocarbons could form inside Earth’s mantle. Experiments at very high temperatures and pressures showed that hydrocarbons, such as n-alkanes up to C₁₀H₂₂, can form from iron oxide, calcium carbonate, and water. Since these materials are found in the mantle and in subducted crust, it is not necessary for all hydrocarbons to come from ancient deposits.

Hydrogen gas and water have been found more than 6,000 meters deep in the upper crust in places like the Siljan Ring boreholes and the Kola Superdeep Borehole. Studies in the western United States suggest that water sources from near Earth’s surface may extend to depths of 10,000 to 20,000 meters. Hydrogen gas can form when water reacts with minerals like silicates, quartz, and feldspar at temperatures between 25°C and 270°C. These minerals are common in rocks like granite. Hydrogen may react with carbon compounds in water to create methane and other carbon-based compounds.

One reaction that creates hydrogen without silicates involves iron(II) oxide. This reaction is similar to the Schikorr reaction, which involves iron(II) hydroxide, magnetite, hydrogen, and water. This process works best at low pressures. At pressures greater than 5 gigapascals, very little hydrogen is formed.

Thomas Gold reported finding hydrocarbons in the Siljan Ring borehole, with amounts increasing with depth. However, the project was not successful commercially. Some geologists later said no hydrocarbons were found in the samples.

In 1967, Soviet scientist Emmanuil B. Chekaliuk suggested that petroleum could form from inorganic carbon, such as carbon dioxide, hydrogen, or methane, under high temperatures and pressures. This idea is supported by scientific evidence, including the synthesis of oil within Earth’s crust using chemically reductive rocks. A process called the serpentinite mechanism is proposed to create inorganic hydrocarbons.

Serpentinites are rocks that form from peridotites and dunites, which contain a lot of olivine and some Fe-Ti spinel minerals. These rocks may also contain nickel, chromium, or chromite, which are needed for the process. However, creating serpentinites requires water and is linked to metamorphism. Serpentinites are unstable at mantle temperatures and can break down into other minerals. This suggests that methane production with serpentinites is limited to areas like mid-ocean ridges and subduction zones. Water has been found as deep as 12,000 meters, so reactions depend on local conditions. Oil formed by this process in certain regions is limited by available materials and temperatures.

A chemical process for creating oil without life is called serpentinization, which begins with methane forming from olivine reacting with water and carbon dioxide. Olivine, made of forsterite and fayalite, changes into serpentine, magnetite, and silica through reactions. When this happens with carbon dioxide at temperatures above 500°C, methane is produced. Another reaction may also form magnesite and silica.

Methane can be converted into longer hydrocarbon chains using catalysts like iron or nickel, a process called spinel hydrolysis. Minerals like magnetite, chromite, and ilmenite, found in some rocks, may help create higher hydrocarbons during hydrothermal events. Chemically reduced rocks and high temperatures are needed for methane to form longer chains.

Calcium carbonate can break down at around 500°C through a reaction that produces methane, calcium oxide, and water. However, calcium oxide is not a common mineral in natural rocks, so this reaction is unlikely to occur.

Theoretical models suggest methane under mantle conditions may be unstable compared to larger hydrocarbons, but high-temperature reactions often produce amorphous carbon and hydrogen instead. Experiments using diamond anvil cells showed that methane and inorganic carbonates can partially form light hydrocarbons under high pressure.

Biotic (microbial) hydrocarbons

The "deep biotic petroleum hypothesis" suggests that not all petroleum found in Earth's rocks can be fully explained by the traditional view of petroleum geology. Thomas Gold used the term "the deep hot biosphere" to describe microbes that live underground.

This hypothesis differs from biogenic oil because it states that deep-dwelling microbes produce oil that is not from sedimentary sources or surface carbon. Instead, these microbes are only a contaminant in primordial hydrocarbons. Some parts of microbes create molecules called biomarkers.

Deep biotic oil forms as a byproduct of the life cycle of deep microbes, while shallow biotic oil forms as a byproduct of the life cycle of shallow microbes.

In a 1999 book, Thomas Gold cited the discovery of thermophile bacteria in Earth's crust as evidence supporting the idea that these bacteria could explain certain biomarkers in extracted petroleum. A rebuttal to biogenic origins based on biomarkers was presented by Kenney, et al. (2001).

Methane is common in crustal fluids and gases. Scientists continue to study how to distinguish biogenic and abiogenic sources of methane by analyzing carbon isotope patterns in observed gases (Lollar & Sherwood 2006). Few examples of abiogenic methane-ethane-butane exist because chemical reactions, whether organic or inorganic, tend to enrich light isotopes. The carbon isotope pattern of methane overlaps with that of inorganic carbonate and graphite in the crust, which are heavily depleted in carbon. This depletion occurs through isotopic fractionation during metamorphic reactions.

One argument for abiogenic oil suggests that the high carbon depletion in methane comes from observed carbon isotope depletion with depth in the crust. However, diamonds, which are from the mantle, are not as depleted as methane, indicating that methane's carbon isotope patterns are not controlled by mantle values.

Commercially extractable concentrations of helium (greater than 0.3%) are found in natural gas from the Panhandle-Hugoton fields in the U.S., as well as in some Algerian and Russian gas fields.

Helium found in most petroleum, such as in Texas, has a distinct crustal character with an Ra ratio less than 0.0001 of the atmosphere.

Certain chemicals in naturally occurring petroleum share similarities with compounds found in living organisms. These include terpenoids, terpenes, pristane, phytane, cholestane, chlorins, and porphyrins, which are large, chelating molecules related to heme and chlorophyll. Materials that suggest biological processes include tetracyclic diterpane and oleanane.

The presence of these chemicals in crude oil results from the inclusion of biological material in the oil. These chemicals are released by kerogen during the production of hydrocarbon oils, as they are resistant to degradation. Some argue that biomarkers enter oil during its rise through contact with ancient fossils. However, a more likely explanation is that biomarkers are traces of biological molecules from bacteria (archaea) that consume primordial hydrocarbons and die in that environment. For example, hopanoids are parts of bacterial cell walls found in oil as contaminants.

Nickel (Ni), vanadium (V), lead (Pb), arsenic (As), cadmium (Cd), mercury (Hg), and other metals are frequently found in oils. Some heavy crude oils, like Venezuelan heavy crude, contain up to 45% vanadium pentoxide in their ash, making it a commercial source for vanadium. Supporters of the abiotic hypothesis argue that these metals are common in Earth's mantle, but high levels of nickel, vanadium, lead, and arsenic are typically found in marine sediments.

Analysis of 22 trace elements in oils correlates more closely with chondrite, serpentinized fertile mantle peridotite, and the primitive mantle than with oceanic or continental crust. These elements show no correlation with seawater.

Sir Robert Robinson studied the chemical composition of natural petroleum oils and concluded that they were too hydrogen-rich to likely come from the decay of plant debris, suggesting a dual origin for Earth's hydrocarbons. However, several processes that generate hydrogen could supply kerogen hydrogenation, aligning with the conventional explanation. Olefins, unsaturated hydrocarbons, would be expected to dominate in materials derived from such processes. He also wrote: "Petroleum … [seems to be] a primordial hydrocarbon mixture into which bio-products have been added."

This hypothesis was later corrected by Robinson, as he had only short-duration experiments available. Olefins are thermally unstable (which is why natural petroleum rarely contains them), and in laboratory experiments lasting longer than a few hours, 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 and animal matter, such as fats in phytoplankton and zooplankton. These oils and waxes occur in small amounts and do not significantly affect the hydrogen/carbon ratio of biological materials. However, after the discovery of highly aliphatic biopolymers in algae and the fact that oil-generating kerogen represents concentrated forms of such materials, no theoretical issues remain. Additionally, millions of source rock samples analyzed by the petroleum industry confirm the large quantities of petroleum found in sedimentary basins.

Empirical evidence

Occurrences of non-living petroleum in commercial amounts have been reported in offshore Vietnam, the Eugene Island block 330 oil field, and the Dnieper-Donets Basin. However, scientists believe these findings can also be explained by the theory that petroleum comes from living organisms. Modern geologists suggest that non-living petroleum might exist in commercially valuable amounts, but no known deposit has strong evidence proving it originated from non-living sources.

Some scientists, particularly those from the Soviet school of thought, pointed to oil found in non-sedimentary rocks like granite, metamorphic, or volcanic rocks as evidence for non-living origins. Others argued that these rocks can hold oil from living sources that migrated from nearby sedimentary rocks through natural processes.

Some observations have been used to support the idea that petroleum forms without life, but each can also be fully explained by living sources. For example, the Lost City hydrothermal field was found to produce non-living hydrocarbons. Scientists noted that carbon in the field likely came from rocks deep in Earth’s mantle, not from seawater. This suggests that non-living hydrocarbons may form naturally in areas with certain types of rocks, water, and heat.

The Siljan Ring meteorite crater in Sweden was proposed as a place to test the non-living theory because the impact cracked granite rocks, potentially allowing oil to rise from Earth’s mantle. The thin layer of sediment in the crater might have trapped oil without exposing it to the high heat and pressure needed to create oil from living sources. However, scientists later found that the oil in the area came from organic-rich rocks that were heated by the meteorite impact.

In 1986–1990, the Gravberg-1 well was drilled deep into the Siljan Ring to search for oil. The drilling stopped at 6,800 meters due to technical issues, and $40 million was spent. Some hydrocarbons were found, but scientists determined they came from the diesel fuel used during drilling, not from deep within Earth. Similar results were found in the Stenberg-1 well drilled nearby in 1991–1992.

In Australia, scientists found bacterial mats and other materials from living organisms in deep wells, which some argue support the non-living theory. However, these findings are still debated.

The Panhandle-Hugoton gas field in the U.S. contains commercial helium, and some scientists suggest this supports the idea that gas and helium come from Earth’s mantle. The Bạch Hổ oil field in Vietnam is located in fractured granite at great depth, and some claim it supports the non-living theory. Others argue the oil came from living sources in nearby rock layers.

Gas reservoirs in the Pannonian and Vienna basins in Hungary and Austria show signs of carbon from Earth’s mantle. In northeastern China, the Shengli Field and Songliao Basin are thought to contain gas from non-living sources.

The Chimaera gas seep in Turkey has been active for thousands of years and was the source of the first Olympic fire. Studies show the gas is about half from living sources and half from non-living sources. Scientists believe deep, pressurized gas from non-living processes may sustain the flow. The area has contact between certain rocks, which could allow reactions like the Fischer–Tropsch process to form hydrocarbon gases.

Geological arguments

Various theories about the origin of oil that do not involve living organisms consider the following observations as important evidence:

• Models such as the serpentinite synthesis, graphite synthesis, and spinel catalysis show that the process of forming hydrocarbons can occur.
• Oil from deep within the Earth’s mantle may rise through cracks and become trapped by layers of sediment that cover these cracks.
• Old calculations about the amount of oil in very large oilfields suggested that the oil could not have come from nearby rock layers, which implies oil might come from deep underground.
• Hydrocarbons have been found inside diamonds, which form deep in the Earth.

Supporters of the idea that oil can form without living organisms use the following points:

• Some models suggest the Earth formed at a low temperature, which might have preserved carbon deposits deep in the Earth’s mantle that could produce hydrocarbons.
• Methane has been found in gases and fluids near mid-ocean ridges, where underwater volcanoes and hot springs are common.
• Diamonds found in certain types of rocks, such as kimberlites and lamproites, may come from deep in the Earth where methane is believed to exist.

Arguments against the idea that chemical reactions like the serpentinite mechanism can create oil in the Earth’s crust include:

• As rocks get deeper, there is less space for fluids to move. However, many studies show that water and other fluids can flow through rocks at all depths.
• No oil has been found in areas with hard, crystalline rocks in large landmasses, even in places where oil was expected to be found. For example, Siljan Lake is one such area.
• There is no clear evidence that the way carbon is distributed in methane found in the Earth’s crust comes only from non-living processes.
• A drilling project near Siljan Lake did not find large amounts of oil, which contradicts a theory that predicted oil would be found there.
• The helium found in the Siljan Gravberg-1 well had less of a type of helium that comes from the Earth’s mantle. This well only produced a small amount of oil, which later was found to come from materials used during drilling.
• A rule about oil and gas (not coal) explains that gas can form from oil and its source rocks. Natural gas is lighter than oil, so it rises to the top of rock layers, pushing oil downward. If gas fills a rock layer completely, oil may escape upward.
• Diamondoids in oil, gas, and condensates come from carbon found in living organisms, unlike the carbon in normal diamonds.

Both theories about where oil comes from have a low success rate in predicting where large oil or gas fields might be found. Finding a large oil or gas field usually requires drilling more than 500 exploration wells. A team of scientists from the United States and Russia created an artificial intelligence program and technology to help predict where large oil and gas deposits might be.

In 1986, the team made a map predicting where large oil and gas fields might be found in the Andes region of South America, based on the idea that oil comes from deep within the Earth. The model proposed by Prof. Yury Pikovsky from Moscow State University suggests that oil moves from the Earth’s mantle to the surface through channels formed where deep cracks in the Earth intersect.

The technology used maps that show areas where different land structures meet and a program that identifies these areas as likely places for large oil or gas fields. The map predicted that 11 such areas, which had not been explored at the time, might contain large oil or gas fields. These areas covered only 8% of the total area of the Andes region. In 2018, results showed that six large oil or gas fields had been found in the predicted areas.

In the 1960s, Donald Hings received patents for methods to locate deep structures in the Earth that might indicate the presence of oil formed without living organisms. These methods are still used today by geophysicists to find deep oil and gas deposits.

Extraterrestrial argument

Methane is found on Saturn's moon Titan and in the atmospheres of Jupiter, Saturn, Uranus, and Neptune. Scientists, such as Thomas Gold, use this as evidence that hydrocarbons can form without the involvement of living organisms. On Earth, natural gas is mostly made of methane. Some comets contain large amounts of organic compounds, enough to fill cubic kilometers of space. For example, during a probe's flight through the tail of Halley's Comet in 1986, scientists found hydrocarbons. In 2015, the Curiosity Rover's Mars Science Laboratory collected drill samples from Mars' Gale Crater. These samples, which are about 3 billion years old, contained benzene and propane.