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| Fig. 1: Petroleum in a jar with a beaker. (Source: Wikimedia Commons) |
Oil is used everywhere: to power cars, planes, and busses; to generate electricity for industries and residences; to heat homes; and so on. In 2021, we used approximately 96,908,000 barrels of oil to meet our various energy needs as a society. [1] However, despite our deep reliance on this liquid (pictured in Fig. 1), there are surprising gaps in our knowledge of oil itself.
The question driving this article is simple: to what extent is the current theory behind the origins of oil legitimate? What hard evidence can we find to justify this theory?
The story of oil is a bit complicated. The leading theory, known as the biogenic model of oil formation, is that oil is formed from the remains of organic material. However, some scientists believe that oil has inorganic origins. Since a comparatively greater amount of evidence has been found to support the model that most of the oil we use has biogenic origins, this is the framework under which I will operate. [2] For example, minor constituents of petroleum - like pristane - are also found in the waves of living plants, suggesting an organic origin. [3]
The generally accepted view of oil formation is as follows:
The first phase of fossil fuel formations consists of the anaerobic decay of organic matter. This happens at about 1 meter below the surface of the ground where there is little oxygen (around 0.1mg/l). This chemical stew continues to change; at depths greater than 10 meters, anaerobic bacteria no longer act, and the biological material has transformed into "kerogen". There are three types of kerogen based on the primary source of the biological material that composes it: algal (from algae), liptinic (from plankton), and humic (from woody plants). [3]
The second phase is known as catagenesis, where kerogen transforms into fossil fuel and happens over a range of thousands to millions of years. This only occurs if the kerogen is not exposed to oxygen and in the presence of increased temperature (60 - 170°C for oil) deep inside the earth's crust. As temperatures increase, the oil is cracked into lighter and lighter fuels. [3]
Further detail may be found in Schobert. [3]
Fundamentally, the process by which biological remains transforms into fossil fuels is through deoxygenation. [4] This is because the energy density of organic materials generally increases as we remove oxygen, turning oxygen-rich biomass into energy-dense hydrocarbons.
The evidence for the biological theory of oil is somewhat circumstantial. We do not fully understand all of the mechanisms by which biological material turns into petroleum. Instead, it is common practice to use "truth by the borehole;" that is, does drilling based on that theory lead to finding commercially viable reserves of oil? The biological theory based on oceanic life being the progenitor of oil has seen significant returns in that regard. [2]
Chemical analysis can give us an insight into the origins of oil. The presence of certain compounds known as porphyrins in crude oil have been linked to biological precursors in plants and algae. [5] These compounds are found in most oil samples. [6] For example, in a study of porphyrins in shale oil and oil shale from Colorado and petroleum from California, these disparate samples contained similar structures of porphyrin, which implies some sort of comparable biological origin. [7]
The use of mass spectroscopy in identifying isotopes of carbon found in oil also substantiates the biogenic theory. There are two stable isotopes of carbon found in nature - C-12 and C-13 in a ratio of 99:1. The amount of C-13 in a sample can point to the organic precursors of the source carbon, as organic material has slightly different isotope ratios than the general environment. [8] Let δ13C represent the deviation in parts per thousand:
Here Rs represents the ratio of [C-13]/[C-12] in the sample and Rr the ratio in the reference. [9] Using an Rr = 0.0112372 from the PDB Standard from 1951, Yeh and Epstein found in a study conducted on 114 petroleum samples that δ13C ranged from -29.9 to -31.5‰ for oils from non-marine sediments and from -23.1 to -32‰ for oils from marine sediments. [8,9] (The location of the sediment does not point to the biological precursor of the oil due to climatic and geologic changes that have occurred in the millions of years it takes for oil to form and be discovered.) The range for the δ13C of marine plankton has also been found experimentally and, based on the PDB reference, is between -15 to -33‰, with the δ13C of the lipid profile having a narrower range between -25 to -33‰ (using data from Degen's study of 5000 carbon samples). [9] These ranges contain the ranges found for oil from marine and non-marine sediments, with the δ13C of the lipid profile being quite a close "match". The samples themselves covered a wide spectrum of locations and times, dating from the Cambrian (550 million years ago) to the Pleistocene (2.5 million years ago). [8,9] Similar studies have also been conducted that agree in general with the previous result. [8]
In contrast, the δ13C of land plants is around -22 to -27‰ (for non-tropical land plants) and -25 to -30‰ (for tropical land plants) from Degen's study. [10] Neither of these fully contain the ranges of δ13C of oil from either sediment. Furthermore, neither range extends to -31.5‰ or -32‰, which is the extent of the range of δ 13C of the oil samples.
While we do not have direct evidence to fully outline the complete journey organic material takes to become petroleum, there is at least a consensus among most modern geochemists regarding the biological origins of oil. It may be difficult to even find such evidence as the heat and pressure needed for oil production destroy much of the source rocks we could directly use to verify our theories.
© Hannah Ashai. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "BP Statistical Review of World Energy 2022," British Petroleum, June 2022.
[2] M. Höök et al, "Development of Oil Formation Theories and Their Importance For Peak Oil," Mar. Pet. Geol. 27, 1995 (2010).
[3] H. H. Schobert, Chemistry of Fossil Fuels and Biofuels (Cambridge University Press, 2013), Ch. 8.
[4] M. Sato, "Thermochemistry of the Formation of Fossil Fuels," in Fluid-Mineral Interactions: A Tribute to H. P. Eugster, ed. by R. J. Spencer and I. M. Chou (Geochem. Soc., 1990).
[5] H. J. Callot, R. Ocampo, and P. Albrecht, "Sedimentary Porphyrins: Correlations with Biological Precursors," Energy Fuels 4, 635 (1990).
[6] B. P Tissot, and D. H Welte. Petroleum Formation and Occurrence, 2nd Ed. (Springer, 1989).
[7] J. R. Morandi and H. B. Jensen, "Comparison of Porphyrins from Shale Oil, Oil Shale, and Petroleum by Absorption and Mass Spectroscopy," J. Chem. Eng. Data 11, 81 (1966).
[8] H.-W. Yeh, and S. Epstein, "Hydrogen and Carbon Isotopes of Petroleum and Related Organic Matter," Geochim. Cosmochim. Acta 45, 753 (1981).
[9] A. N Fuex, "The Use of Stable Carbon Isotopes in Hydrocarbon Exploration," J. Geochem. Explor. 7, 155 (1977).
[10] E. Degens, "Biochemistry of Stable Carbon Isotopes," in Organic Geochemistry ed. by G. Eglinton and M. T. J. Murphy (Springer, 1969).
