图1. 乔治·阿格里科拉著作封面，该书论述了石油的起源。 [来源：公共领域，通过维基共享]（Georgius Aagricola：乔治·阿格里科拉；De ortu & caufis fubterraneorum：从地底升起；De natura eorum qux effluent ex terra：关于流出地球的事物的性质；De natura fofsilium：关于矿物的性质；De ueteribus & nouis metallis：新旧金属；Bermannus,fiue Dere metallica Dialogus：伯曼努斯，金属对话；Interpretatio Germanica uocum rei metallicae,addito Indice foeeundifsimo：金属的德语翻译，附加索引）
尽管原油或其衍生物自新石器时代以来就已被人类使用[1]，但其起源长期以来却一直是个谜。罗杰·培根[2]在1268年出版的专著《第三著作》（Opus Tertium）中写道，亚里士多德[3]和其他古代历史哲学家均未讨论过石油和沥青的起源。在文艺复兴时期，出现了关于石油起源的两种相互矛盾的假设。1546年，乔治·阿格里科拉[4]出版了《在地球的自然环境中》（De Natura eorum quae Efflunt ex Terra）（图1），在这部作品中首次提到了石油这个词，它来自拉丁语petra oleum （石油）。阿格里科拉拓展了亚里士多德关于石油从地下深处喷出的概念，提出沥青是由含硫蒸汽冷凝而来。安德烈亚斯·利巴维乌斯[5]在1597年出版的《炼金术》一书中提出，沥青是由古树的树脂转化形成。此后，一场漫长的科学辩论延续开来，一方观点支持石油的非生物起源，另一方认为石油是由石化有机物转化而来的。
图5. 手性化合物的左旋和右旋异构体的表征。本例中是氨基酸。 [来源：文章（πϵρήλιο）℗（手征性（Chirality with hands）.jpg)[公共领域]，通过维基共享。]许多生物化合物具有旋光活性，即当光束穿过它们时，会向右或向左偏转。这些化合物分别称为右旋化合物（如蔗糖）或左旋化合物（如果糖）。生物体内几乎所有的氨基酸都是左旋的。非生物化合物不具备这种旋光性。它们在光学上不活跃，因为它们具有相同比例的手性化合物左旋和右旋异构体（图5）。生命倾向于优先选择一种异构体而不是另一种，因此生物化合物具有光学特性。然而，石油具有旋光性，这表明其起源于生物。不过，这种特性往往会随着石油的热成熟度而消失，但当微生物降解石油时，这种特性也会被放大。
[11] Trumbore S.E. & Druffel E.R.M. （1995） Carbon isotopes for characterizing sources and turnover of nonliving organic matter. In R. G. Zepp & C. Sonntag (Eds.), Role of Nonliving Organic Matter in the Earth’s Carbon Cycle (pp. 7-22). Chichester: John Wiley & Sons Ltd.
[14] Grantham P.J. & Wakefield L.L. （1988） Variations in the sterane carbon number distributions of marine source rock derived crude oils through geological time. Organic Geochemistry,12-1,61-73.
“Will the defendant please rise! Please state your first and last name, age and origin?” Petra Oleum, Old and Biological would answer all the crude oil in the world if they had to appear before such an imaginary court. Subjected to the question of scientists, oil has been seen by some as a fluid of mineral origin, therefore having an abiotic origin… However, there are many indications of its biological origin, that is, its filiation with what was alive, even if it was millions of years ago. Oil itself confesses by leaving traces everywhere that connect it to life. This article reviews the various proofs of the biological origin of oil by situating them in a historical context.
1.1. From ancient philosophers to Renaissance scholars
Figure 1. Cover page of Georgius Agricola’s book, in which he discusses the origin of oil. [Source: Public domain, via Wikimedia Commons]If crude oil or its derivatives have been used by man since the Neolithic [1], its origin has long remained mysterious. In his treatise Opus Tertium, published in 1268, Roger Bacon [2] comments on the absence of discussion on the origin of oils and bitumen by Aristotle [3] and other philosophers of Ancient History. It was during the Renaissance that two contradictory hypotheses about the origin of oil emerged. In 1546, Georgius Agricola [4] published De Natura eorum quae Efflunt ex Terra (Figure 1), a work in which the first mention of the word oil appears, from the Latin petra oleum (stone oil). Agricola extends Aristotle’s concept of exhalations from the depths of the Earth and proposes that bitumen be the condensation of sulphurous vapours. Andreas Libavius [5] theorized in 1597 in his book Alchemia that bitumen is formed from the resin of old trees. Since then, a long scientific debate has continued between those who support the abiotic origin of oil and those who see it as a product derived from the transformation of fossilized organic matter.
Methane is formed during the alteration of volcanic rocks in the Earth’s mantle where dihydrogen (H2) produced during the weathering of some iron-rich minerals reacts with carbon dioxide (CO2) during a so-called Sabatier reaction. This reaction takes place at high temperatures and pressures in the presence of a nickel catalyst to produce methane and water. Other Fischer-Tropsch reactions -which combine carbon monoxide (CO) and dihydrogen to convert them into hydrocarbons- could occur when cooling magma and produce hydrocarbons by assembling CH2 units. Finally, the thermal decomposition of iron carbonate (FeCO3) in the presence of water can also produce simple hydrocarbons.
While it cannot be denied that abiotic processes lead to the formation of hydrocarbons, these mechanisms can hardly account for the quantity, diversity and location of oil occurences on Earth. The idea that oil originates from sedimentary organic matter, i.e. organisms that have been living, is widely demonstrated by natural observations, laboratory analyses and experiments.
2. Evidence through natural observations
2.1. Oil accumulations in the heart of sedimentary basins
Figure 5. Representation of levorotatory and dextrorotatory isomers of a chiral compound, in this case an amino acid. Source: πϵρήλιο ℗ (Chirality with hands.jpg) [Public domain], via Wikimedia Commons.Manybiological compounds are optically active, i.e. they deflect a light beam passing through them to the right or left when facing light. These compounds are respectively referred to as dextrorotatory (e.g. sucrose) or levorotatory (e.g. fructose). Virtually all amino acids in living organisms are levogyrous. Abiotic compounds do not have this rotational power of light. They are optically inactive because they have an equal proportion of the levorotatory and dextrorotatory isomers of a chiral compound (Figure 5). Life tends to preferentially select one isomer over the other, hence the optical property of biological compounds. However, oil is most often optically active, suggesting its biological origin. However, this property tends to disappear with the degree of thermal maturation of the oils, but it can also be amplified when microbes degrade the oil.
3. Geochemical evidence
3.1. Carbon isotopes
Figure 6. Range of variation in the isotopic ratio 12C/13C of different terrestrial materials. The CO2 of seawater is taken as a reference; negative values correspond to a 13C depletion which is expressed here at ‰ Corg: Organic carbon. [Source: Adapted from Trumbore & Druffel (ref. [10])]Carbon has two naturally stable isotopes: 12C and 13C [10]. The organic matter formed during photosynthesis from atmospheric CO2 or HCO3– dissolved in water is much depleted in 13C because plants preferentially fix 12C. In most plants this depletion is in the order of 0.02% (which is also noted at δ13C = – 20 ‰) compared to the inorganic carbon source. Oil and fossil organic matter have the same 13C depletion as organic matter in living tissues (Figure 6) [11]. This isotopic deficit is systematic, whether we consider individual molecules or total organic matter. No known inorganic processes lead to the formation of high molecular weight molecules with such a 13C deficiency. For gases, the proportion of 12C decreases from methane to propane and in the different forms of butane; this is consistent with the kinetics of isotopic fractionation during thermal cracking of sedimentary organic matter (also called kerogen) or petroleum. The opposite trend, i.e. an increase in 12C from methane to butane, characterizes the polymerization products of the Fischer-Tropsch reaction. This singular isotopic signature is not found in any gas field in the world.
3.2. The discovery of porphyrins in oil
The German chemist Hans Fischer [12] was awarded the Nobel Prize for Chemistry (1930) for his research on organic pigments, particularly in blood (hemin) and plants (chlorophyll).
After plants death, the tetrapyrrole nucleus and the phytol side chain separate and evolve differently. The tetrapyrrole nucleus reorganizes very slightly – in particular magnesium is replaced by nickel or vanadium oxide – thus giving rise to a family of biomarkers: porphyrins.
Alfred Treibs [13], a student of Hans Fischer, highlighted in 1936 the ubiquity of porphyrins in crude oils and kerogen-rich clays and the ways in which they are transformed from chlorophyll, thus providing clear evidence of their plant origin. Alfred Treibs is therefore considered to be the father of organic geochemistry.
3.3. More and more biomarkers
Since this discovery, we no longer count the number of molecules detected in oils that are derived from or even strictly identical to a molecule known in living organisms (Figure 8). These molecules, called biomarkers, are true geochemical fossils because they have a structure very close to the biomolecules of living organisms.
When kerogen undergoes natural thermal cracking to form oil as a result of increasing temperature at depth and over time, some of these biomarkers remain unchanged and even form part of the oil. It is therefore difficult to imagine so many complex molecules present in all oils that would only be produced by abiotic processes.
3.4. A chemical signature of marine oils that co-evolves with phytoplankton diversification
Figure 9. Ternary diagram of the relative proportion of steranes in 414 crude oils as a function of their age. With the exception of a very old oil (600 million years old), there has been an increase in the proportion of C28 sterane at the expense of C29 over geological time. [Source: Adapted from Grantham & Wakefield; ref. [13]]Among biomarkers, steranes are an important class. These molecules are derived fromsterols that play a role in plants and animals in maintaining the structural and functional integrity of cell membranes. A large number of steranes with 27, 28, 29, 29 or even 30 carbon atoms are identified in crude oils.
In the late 1980s, Shell geochemists analyzed more than 400 crude oils generated by 650 to 45 million year-old marine rocks and extracted and identified the different types of steranes and grouped them by increasing number of carbon atoms. It appears that the proportion of steranes with 28 carbon atoms increases over geological time while the proportion of steranes with 29 carbon atoms decreases; that of steranes with 27 carbon atoms remains almost stable (Figure 9) [14].
Figure 10. Comparison of the distribution of the most important phytoplankton groups over the past 600 million years and the ratio of steranes with 28 to 29 carbon atoms found in oils from marine source rocks. [Source: Adapted from Grantham & Wakefield; ref. [13]]For these geochemists, these changes do not reflect the chemical evolution of sterols of a particular variety of a marine organism that would become more abundant over time, but is related to the diversification of phytoplankton organisms, recognized by paleontologists from their mineralized remains. This phytoplankton diversification accelerated significantly during the Jurassic and Cretaceous periods, between 180 and 65 million years ago. The sources of steranes with 28 carbon atoms should thus be linked to the appearance and diversification in the oceans of these periods of microalgae such as dinoflagellates, coccolithophorids (the origin of chalk) and diatoms (Figure 10).
This evolution of the chemical signature of marine oils in parallel with that of phytoplankton obviously suggests a cause-and-effect relationship and provides an element in favour of the biological origin of the oils.
With the development of analytical techniques in the second half of the 20th century, it was possible to reconstruct in the laboratory the temperature and pressure conditions existing at depth in sedimentary basins. It is even possible to perform these experiments in the presence of water, a molecule so abundant in terrestrial rocks. The products obtained during these experiments are physically and chemically very comparable to natural oils. When analysed in terms of their molecular composition, there is a striking similarity between the molecules present in natural oil and those produced by heating kerogen, both in terms of the nature and abundance of the molecules (Figure 11). They are so similar that geochemists consider these molecular distributions as a bit like a fingerprint or DNA and use them to genetically link a natural oil to its source rock.
5. Messages to remember
There is a wealth of evidence for the biological origin of oils, including the presence of molecules that derive directly from molecules synthesized by living organisms, whether prokaryotes or eukaryotes.
Thus, porphyrins, ubiquitous molecules in oils, are derived from different types of chlorophyll.
Similarly, fossil molecules are found in crude oils that can easily be attached to other pigments (carotenoids for example) or molecules that make up the cell walls of prokaryotes or eukaryotes.
The greater proportion of 12C compared to 13C in petroleum molecules, the so-called isotopic signature, is also in favour of their biological origin since life selects the light isotope in the same proportions.
Finally, more than 99% of the oil fields are located in sedimentary basins, i.e. sediments deposited on the bottom of ancient seas or lakes where life was present, as shown by mineralized fossils.
Generally speaking, oil source rocks contain mainly organic matter derived from marine or lacustrine phytoplankton biomass, more or less modified by bacteria.
[2] Roger Bacon (1214 – 1294), an English philosopher, scientist and alchemist, is considered one of the fathers of the scientific method.
[3] Aristotle (384 BC – 322 BC), Greek philosopher of antiquity. He is one of the few to have covered almost all the fields of knowledge of his time: biology, physics, metaphysics, logic, poetics, politics, rhetoric and, occasionally, economics.
[4] Georgius Agricola, known as Agricola, by his real name Georg Pawer (1494 – 1555), a 16th century German scientist, considered the father of mineralogy and metallurgy.
[5] Andreas Libavius (1555 – 1616), real name Andreas Libau, German chemist and doctor. Alchemia, published in 1597, is the first book of systematic chemistry.
[6] The formation of coal deposits began in the Carboniferous with the accumulation of very large quantities of so-called higher plant debris (trees, ferns…) in shallow water layers poor in oxygen (peat-type environment). These conditions allowed some of the organic matter to escape the action of the decomposers. For several million years, the accumulation and sedimentation of these plant debris caused a gradual change in temperature, pressure and oxidation-reduction conditions in the coal layer, leading to the formation of compounds that are increasingly rich in carbon: peat (50 to 55%), lignite (55 to 75%), coal (75 to 90%) and anthracite (> 90%). Most coals have a low oil potential. On the other hand, they produce gas as they mature, in particular methane, which is the cause of firedamping in coal mines.
[7] Mikhail Vasilievich Lomonossov (1711 – 1765) is a chemist, physicist, astronomer, historian, philosopher, poet, playwright, linguist, Slavicist, teacher and Russian mosaicist.
[8] Alexander von Humboldt (1769 – 1859), is a German naturalist, geographer and explorer. Associate member of the French Academy of Sciences and President of the Société de Géographie de Paris. Through the quality of the surveys he conducted during his expeditions, he laid the foundations for scientific exploration.
[9] Louis Joseph Gay-Lussac (1778 – 1850), is a French chemist and physicist, known for his studies on the properties of gases.
[10] Radioactive carbon 14C was initially present in the petroleum-originating compounds (formed from photosynthesis) and in the same proportions as in photosynthetic organisms living at the time. However, since the period of this element is relatively short on a geological scale (5700 years), the current oil no longer contains 14C and therefore cannot be dated by this technique. This property is currently used to distinguish between particles responsible for air pollution from those derived from petroleum products (gasoline, diesel) and those derived from wood combustion, for example.
[11] Trumbore S.E. & Druffel E.R.M. (1995) Carbon isotopes for characterizing sources and turnover of nonliving organic matter. In R. G. Zepp & C. Sonntag (Eds.), Role of Nonliving Organic Matter in the Earth’s Carbon Cycle (pp. 7-22). Chichester: John Wiley & Sons Ltd.
[12] Hans Fischer (1881 – 1945), a German chemist specialising in organic chemistry.
[13] Alfred E. Treibs (1899 – 1983), a German organic chemist and pioneer in organic geochemistry.
[14] Grantham P.J. & Wakefield L.L. (1988) Variations in the sterane carbon number distributions of marine source rock derived crude oils through geological time. Organic Geochemistry, 12-1, 61-73.
