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Abiogenic origins of petroleum
Thomas Gold first became interested in the origins of petroleum in the 1950s, postulating a theory on the abiogenic formation of fossil fuels. Gold engaged in thorough discussion on the matter with Fred Hoyle, who even included a chapter on "Gold's Pore Theory" in his 1955 book Frontiers in Astronomy.

While Russian scientists had long been at work explicating possible abiogenic origins of petroleum, the Cold War blocked knowledge of their publications until the 1990s. Thus, Thomas Gold was credited with the idea in the United States when current events prompted him to submit an opinion piece to the Wall Street Journal in June 1977 titled, "Rethinking the origins of oil and gas." Concern about gasoline shortages that began in 1973 were still troubling the economy. A striking discovery in the deep-sea just four months earlier (February 1977) was another impetus: Exploration and photography of a deep-sea hydrothermal vent showed a dense amount of life living on chemical energy. Stationary organisms depending on vent outflows included albino clams and tube worms larger than ever seen in surface marine ecosystems. Most astonishing was that such ecosystems were based on microbial life living entirely on chemosynthetic rather than photosynthetic ways of capturing energy and building living cells.

Science communicator Paul Davies explained Gold's theory in this way: "Conventional wisdom has it that oil and coal are remnants of ancient surface life that became buried and subjected to extremes of temperature and pressure. Gold maintains that these deposits are not fossil fuels in the normal sense, but the products of primordial hydrocarbons dating from the time of the Earth's formation. He claims that over the aeons the volatile gases migrate towards the surface through cracks in the crust, and either leak into the atmosphere as methane, become trapped in sub-surface gas fields, or are robbed of their hydrogen to become oil, tar or carbonaceous material like coal."

As to the ubiquity of abiotic hydrocarbons in the solar system, a 1999 profile of Gold in the Washington Post quoted him as saying, "it always seemed absurd to me to see petroleum hydrocarbons on other planets, where there was obviously never any vegetation, even as we insist that on Earth they must be biological in origin."

Earthquakes from rising methane
Having established the theoretical foundations of his abiogenic petroleum hypothesis, Gold began in-depth thinking and research on the kinds of empirical evidence that might land in its favor. First was collaborating with a former graduate student of his at Cornell University: Steven Soter. Soter had received his PhD in astronomy in 1971 and had recently concluded another faculty collaboration at Cornell: working with Carl Sagan in the writing of the television series, Cosmos: A Personal Voyage. Gold and Soter teamed up to investigate the knowns and unknowns about earthquakes from the standpoint of plausible causation by or regular co-occurrence with sudden escape of large volumes of methane gas. The result was a series of papers, including two with "earthquakes" in the title: "North Sea-quakes" (New Scientist 1979) and "Fluid Ascent through the Solid Lithosphere and its Relation to Earthquakes" (Pure and Applied Geophysics 1985). Their 1980 article in Scientific American was titled "The Deep-Earth-Gas Hypothesis" and the explanatory value of the idea was presented as, providing "a unified basis for explaining a number of otherwise rather puzzling phenomena that either give warning of earthquakes or accompany them." Even so, they cautioned, "The sampling of such gas­es is just beginning, and the data will not yet support confident conclusions."

The puzzling phenomena associated with earthquakes include "flames that shoot from the ground, earthquake lightss, fierce bubbling in bodies of wa­ter, sulfurous air and the strange behavior of animals, loud explosive and hissing noises, and visible waves rolling slowly along alluvial ground." They constructed a map of the world depicting major oil-producing regions and areas with historical seismic activity. Several oil-rich regions, such as Alaska, Texas, the Caribbean, Mexico, Venezuela, the Persian Gulf, the Urals, Siberia, and Southeast Asia, were shown to be lying on major earthquake belts. Gold and Soter suggested that these belts may explain the upward migration of gases originating at depth. "The fact is that oil and gas fields show a distinct association with such earthquake-prone regions. The association suggests to us that the deep faults may provide a con­duit for the continuous input of nonbio­logical methane streaming up from be­low. Moreover, the upward migration of methane and other gases in fault zones may contribute to the triggering of earthquakes."

He also pointed to the abundance of helium in oil and gas reserves as evidence for "a deep source of the hydrocarbons". Moreover, a few oil reserves thought to have been exhausted were suddenly generating vast amounts of crude oil. From this, Gold proposed that the Earth may possess a virtually endless supply – suggesting as much as "at least 500 million years' worth of gas" – of fossil fuels.

The helium anomaly
In later publications, Gold emphasized that the immense amounts of helium gas that surges upward during commercial petroleum production at some sites is proof in itself that substantial lightweight gases have indeed persisted at depth since the amalgamation of cosmic debris into planet Earth during the birth of this solar system. In his 1998 book, Gold closed his fourth chapter, "Evidence for Deep-Earth Gas," with a section titled "The Association of Helium with Hydrocarbons."

A test: Drilling deep into granite
Gold began testing his abiogenic petroleum theory in 1986. With the backing of a group of investors, Vattenfall and the Gas Research Institute, drilling of a deep borehole – named Gravberg-1 – commenced into the bedrock near Lake Siljan in Sweden. This was the site of a large meteor crater, which would have "opened channels deep enough for the methane to migrate upward" and formed deposits in caprock just a few miles beneath the surface. He estimated that the fractures near Lake Siljan reached down nearly 40 kilometers into the earth.

In 1987, approximately 900 oilbbl of drilling lubricant disappeared nearly 20000 ft into the ground, leading Gold to believe that the lubricant had fallen into a methane reservoir. Soon after, the team brought up nearly 100 liters of black oily sludge to the surface. Gold claimed that the sludge contained both oil and remnants of archaebacteria. He argued that "it suggests there is an enormous sphere of life, of biology, at deeper levels in the ground than we have had any knowledge of previously" and that this evidence would "destroy the orthodox argument that since oil contains biological molecules, oil reserves must have derived from biological material".

The announcement of Gold's findings was met with mixed reactions, ranging from "furious incredulity" to "deep skepticism". Geochemist Geoffrey P. Glasby speculated that the sludge could have been formed from the Fischer–Tropsch process, a catalyzed chemical reaction in which synthesis gas, a mixture of carbon monoxide and hydrogen, is converted into liquid hydrocarbons. Critics also dismissed Gold's archaebacteria finding, stating that "since micro-organisms cannot survive at such depth, the bacteria prove that the well has been contaminated from the surface". Geochemist Paul Philp analyzed the sludge and concluded that he could not differentiate between the samples of sludge and oil seep found in sedimentary shale rocks near the surface. He reasoned that oil had migrated from the shale down to the granite deep in the ground. Gold disputed Philp's finding, believing that the oil and gas could have just as easily migrated up to the surface: "They would have it that the oil and gas we found down there was from the five feet of sediments on the top – had seeped all the way down six kilometres down into the granite. I mean, such complete absurdity: you can imagine sitting there with five feet of soil and six kilometres underneath of dense granitic rock, and that methane produced up there has crawled all the way down in preference to water. Absolute nonsense."

In light of the controversy surrounding the sludge and possible drill contamination, Gold abandoned the project at Gravberg-1, calling it a "complete fiasco", and redesigned the experiment by replacing his oil-based drilling lubricant with a water-based one.

The drill hit oil in the spring of 1989, but only collected about 80 oilbbl. Gold stated, "It was not coming up at a rate at which you could sell it, but it showed there was oil down there." The drill then ran into technical problems and was stopped at a depth of 6.8 kilometers. The hole was closed, but a second hole was opened for drilling closer to the "center of the impact ring where there was even less sedimentary rock". By October 1991, the drill hit oil at a depth of 3.8 kilometers, but many skeptics remained unconvinced of the site's prospects. Geologist John R. Castaño concluded that there was insufficient evidence of the mantle as the hydrocarbon source and that it was unlikely that the Siljan site could be used as a commercial gas field. In 2019, a study of gases and secondary carbonate minerals revealed that long-term microbial methanogenesis has occurred in situ deep within the fracture system of the crater (for at least 80 million years) and with an obvious spatial link to seep oils of surficial sedimentary origin, at odds with Gold's theories of deep abiotic gas migration.

Gold's later views on the drilling results can be found in chapter 6, "The Siljan Experiment," of his 1998 book. Another section of the book titled "The Upwelling Theory of Coal Formation" presents another argument in favor of the abiogenic model that he had not presented in an earlier paper. Similarly, he also presents arguments pertaining to the origin of diamonds and that microbial processes are the cause of mineral concentrations at depth.

Dispute nearly forgotten
In 1996 a paper published in the journal Social Studies of Science was titled, "Which Came First, the Fossil or the Fuel?" The author concluded:

Beginning in the late 1970s, Gold revived the 'abiogenic' theory, which holds that hydrocarbons are primordial, not remnants of decayed biology. By contesting the central tenet of petroleum geology, Gold precipitated a bitter scientific controversy. Both sides employed novel rhetorical strategies in order to impute interests, to contest expertise, to recruit allies from peripheral disciplines, and to claim the mantle of scientific method; and both managed to construct plausible interpretations of the available data.

The author reported that even the Siljan drilling results had not been sufficient to fully resolve the long-standing dispute about origins, although Gold's hypothesis is favored by only a very slim minority. As of 2024, "fossil fuel" is still the prevailing term widely in use in reference to petroleum resources, both within academia and without. Such terminology includes 21st century communications about the causes of anthropogenic climate change and proffered solutions to the crisis, such as the 2023 IPCC report, "Climate Change 2023 Synthesis Report: Summary for Policymakers." Overall, within the academic disciplines of geobiology and petroleum geology, criticism of Gold's abiogenic theory has been severe — but not entire. The dispute is more set aside and forgotten than resolved.

As to whether Gold's framing of distinct chemosynthetic microbes are active and ubiquitous at depth, the established authorities have moved in his direction. (See next section.)

"Deep Hot Biosphere" theory
In a 1992 paper, "The Deep, Hot Biosphere", Gold first suggested that microbial life is widespread in the porosity of the crust of the Earth, down to depths of several kilometers, where rising temperatures finally set a limit. The subsurface life obtains its energy not from photosynthesis but from chemical sources in fluids migrating upwards through the crust. The mass of the deep biosphere may be comparable to that of the surface biosphere. Subsurface life may be widespread on other bodies in the solar system and throughout the universe, even on worlds unaccompanied by other stars.

A 1993 article by journalist William Broad, published in The New York Times and titled "Strange New Microbes Hint at a Vast Subterranean World," carried Gold's thesis to public attention. The article began, "New forms of microbial life are being discovered in such abundance deep inside the Earth that some scientists are beginning to suspect that the planet has a hidden biosphere extending miles down whose total mass may rival or exceed that of all surface life. If a deep biosphere does exist, scientists say, its discovery will rewrite textbooks while shedding new light on the mystery of life's origins. Even skeptics say the thesis is intriguing enough to warrant new studies of the subterranean realm."

The 1993 article also features how Gold's thesis expands possibilities for astrobiology research: "Dr. Thomas Gold, an astrophysicist at Cornell University known for bold theorizing, has speculated that subterranean life may dot the cosmos, secluded beneath the surfaces of planets and moons and energized by geological processes, with no need for the warming radiation of nearby stars. He wrote in The Proceedings of the National Academy of Sciences last year that the solar system might harbor at least 10 deep biospheres. 'Such life may be widely disseminated in the universe,' he said, 'since planetary type bodies with similar subsurface conditions may be common as solitary objects in space, as well as in other solar-type systems.'"

Gold also published a book of the same title, The Deep Hot Biosphere, in 1999, which expanded on the arguments in his 1992 paper and included speculations on the origin of life and on horizontal gene transfer. According to Gold, bacteria feeding on the oil accounts for the presence of biological debris in hydrocarbon fuels, obviating the need to resort to a biogenic theory for the origin of the latter. The flows of underground hydrocarbons may also explain oddities in the concentration of other mineral deposits. In short, Gold said about the origin of natural hydrocarbons (petroleum and natural gas): Hydrocarbons are not biology reworked by geology (as the traditional view would hold), but rather geology reworked by biology.

Freeman Dyson wrote the foreword to Gold's 1999 book, where he concluded, "Gold's theories are always original, always important, usually controversial — and usually right. It is my belief, based on fifty years of observation of Gold as a friend and colleague, that the deep hot biosphere is all of the above: original, important, controversial — and right." (Dyson also delivered a eulogy at Gold's memorial service, a segment of which pertaining to the deep hot biosphere theory is posted on youtube.)

Following Gold's death, scientific discoveries amplified and also shifted understanding of the deep hot biosphere into what is now generally called deep biosphere. However, it is only at great depth where naturally occurring geochemical processes induced by intense heat and pressure produce elemental hydrogen and carbon dioxide upon which novel metabolisms of life (especially among the primitive Archaea) could have evolved. A retrospective paper published in the same journal as Gold's 1992 paper featured the metabolic and genetic discoveries of life forms at depth that Gold's paper inspired. Titled "The Deep, Hot Biosphere: Twenty-five years of retrospection," the authors conclude:

The pioneering ideas proposed by Thomas Gold inspired a generation of researchers in the field of geobiology to dive deeper into the possibilities of subsurface life, spawning hundreds of relevant publications.... Deep hydrocarbon deposits on Mars, Titan, and worlds beyond could play host to life similar to that in Earth’s own crust. The techniques used to better study and understand deep, hot biospheres on Earth could then be applied to robotically probe targets in deep space as we move into the next century of scientific discovery. Technology is advancing at a rate wherein we may find that Gold’s deep, hot biosphere is not only true, but common across the universe.

A term Gold coined in his 1999 book carries forward, too, and is a reminder of the worldview shift he advocated. The term is "surface chauvinism". Gold wrote, "In retrospect, it is not hard to understand why the scientific community has typically sought only surface life in the heavens. Scientists have been hindered by a sort of 'surface chauvinism.'"