The term petroleum originally meant “rock oil”. The definition of petroleum in geology is broader: It is defined as a naturally occurring and usually complex mixture of dominantly hydrocarbon substances – liquid, gas, or solid [Dott/Reynolds, 10].
The common definition used for petroleum is narrower: it is synonymous with crude oil and is defined as follows: Crude Oil/Petroleum is a naturally occurring complex mixture of liquid hydrocarbons with trace quantities of oxygen-, sulfur- or nitrogen-containing compounds and of metallic constituents (for example, nickel, vanadium). Some of these hydrocarbons may be very low-boiling, such as methane, in which case the compounds may in part be dissolved in the crude oil and may in part occur as associated gas [Dott/Reynolds, 10].
The term bitumen is commonly only used for the semi-solid and solid hydrocarbons, but has also been used interchangeably with petroleum in its liquid and solid form in the “geological” definition above [Dott/Reynolds, 10].
What creates petroleum and the recognition of its properties has been a recurring theme of speculation and experiments since its discovery. The high migration ability of gaseous and liquid hydrocarbons has been a difficulty in this regard. But also the solid forms show migration (albeit to a lesser extent) [Schmidt/Romey, 18]. Therefore, where the petroleum is found today is not necessarily where it formed. In this regard it is often spoken of as source rock, as well as primary and secondary migration [Dott/Reynolds, 69].
Below, the main theories on the cause and origin of oil are presented, and the resulting controversy that ensued.
Emergence from coal
A fairly early idea was that oil is produced from coal by distillation. Coal would be the “source rock”. Although coal seams often lie above oil layers [Corliss 1989, 171 f], in many places (e.g., in Pennsylvania) oil, but no coal, is available, and a migration path is considered elusive at best. Moreover, it was shown that coal-oils differ chemically from petroleum/crude oil. Nevertheless, this approach has never been discarded completely and still has supporters [Dott/Reynolds, 48]. Thus, for example, a correlation between coal and petroleum deposits with higher wax content was found [ibid, 54].
Also, technically the process of converting coal into artificial petroleum using coal liquefaction has been implemented in many varieties (hydrogenation, extraction, synthesis) on an industrial scale [Schmidt/Romey, 127-148].
Development from land plants
In this theory, although the same hypothetical source material is recognized as with the coal approach, different chemical processes are now directly producing petroleum from the source material instead of coal. The appearance of the hydrocarbon methane in marshes, ponds, and bogs is interpreted as supporting evidence for this idea. It is contradicted, however, by the fact that in many regions there is no relation between carbon-bearing and petroleum-bearing strata. Moreover, petroleum in coral limestone cannot be of terrestrial plant origin [Dott/Reynolds, 52]. A modification of this theory sees terrestrial plant material (maybe already as humus [ibid, 53]) carried by rivers into the sea, where it is sedimented together with marine remains and changed into petroleum [ibid, 52].
Development from marine plants
Algae (plant-like through photosynthesis, but not in the true sense plants) are considered as the most likely source of oil from a marine origin. After dying, they would go down to the seabed and could create petroleum while decaying under pressure. Even seaweed from salt marshes has been considered as a source of petroleum [Dott/Reynolds, 56-57]. Fossilized tracks (attributed to seaweed) often occurring in Paleozoic rocks in association with petroleum-producing strata have made sure that the algae theory has enjoyed great popularity in the 20th century [ibid 57]. Meanwhile, these fossil traces are assigned mostly to marine worms [wiki/Chondrites (genus)].
Development from marine animals
Macroscopic animals (mussels, fish, etc.) as well as microorganisms were also early on considered as a potential source of petroleum because many oil-bearing strata were found including marine fossils. Exactly these fossils are considered as the source of the oil. But this implies that the oil does not migrate and that it remains in the source rock [Dott/Reynolds, 60]. In contrast to this, a distillation theory is proposed, in which the oil is distilled from the source rocks (e.g., black shale with fossil remains) and then migrates to other reservoir rocks [ibid, 57]. A big boost for the thesis was the Engler-Höfer theory. Engler showed through experiments that petroleum can be extracted from the remains of marine animals, which is chemically not very different from naturally found petroleum [ibid, 63].
Formation from organic remains (plants and marine animals)
A modification of the Engler-Höfer theory eventually allowed the two camps of a vegetable and animal origin to come together. The observation that petroleum limestone has a higher sulfate and nitrogen content (suspected animal origin) than, for example, petroleum derived from black shale (widely assumed plant origin) led to the adoption of a dual (animal and vegetable) formation theory [Dott/Reynolds, 65-66]. Also crucial was the discovery that petroleum is in the same way optically active (polarization) as it is known from animal (with cholesterol) and plant material (phytosterols) [ibid, 67].
The idea of a biogenic origin of petroleum is therefore now being used extensively by the geologists. A certain disagreement exists on the question of whether any oil is already formed in the source rock and then migrates into a reservoir or whether an “intermediate” product (called kerogen) is produced in the source rock and then migrates into a reservoir, where during diagenesis (hardening) of the reservoir rock it is transformed into the final product [ibid, 69].
Chemical formation in the Earth’s interior
Less well known in public is the fact that there are also theories of petroleum origin which claim an abiogenic basis. According to these theories, simple hydrocarbons rise from the inner earth along columns and (tectonic) fractures, combine in the crust and upper mantle by means of catalytic processes into complex hydrocarbons, and fill underground reservoirs or flow out of the earth through volcanoes. The required carbon is used directly, i.e., not first consumed by organisms, and released again after their decay. The required carbon is found, according to this theory, in base igneous and metamorphic rocks. The processes involved have been the object of much speculation and chemical experiments in the late 19th century. Many, even very well-known, chemists (including, e.g., Dimitri Mendelejef) dealt with the topic. As a consequence different variants for the abiogenic production of hydrocarbons were created in fast succession [Dott/Reynolds, 27-31]. In the beginning of the 20th Century, one of the most active proponents of an abiogenic formation of petroleum was the Canadian Eugene Coste [Coste]. He gave examples of carbon and hydrocarbons in old plutonic rocks (e.g., granite) as well as in current volcanoes. He pointed out similarities between phenomena and materials in oil/gas deposits and volcanic eruptions. The similarity of symptoms in the field of oil/gas reserves is based, according to today’s views, on the confusion of true magmatic-volcanic activity with simple mud and gas expulsion. In addition, petroleum is ‘cold’ when it is taken out of the earth [ibid, 20]. Coste also attempted to demonstrate the inadequacy of biogenic theories [Dott/Reynolds, 20]. This argument he shares with virtually all representatives of the abiogenic mindset: deposits of petroleum in igneous basement rocks predate the onset of life forms that would have produced it according to the biogenic theory [ibid, 22]. Contrary to this it is asserted that only small amounts of petroleum were found in these rock formations; the rest can be found in sedimentary rock [ibid, 42]. For the small amount of petroleum in basement rocks, there are now new theories on the movement of fluids within the earth and on the permeability of crystalline rocks which would also allow for a migration to such material [Glasby, 92].
After the Second World War, the idea of an abiogenic origin of petroleum received a new boost in the USSR. At the beginning of the cold war, they started a search for alternatives, since access to the largest known oil deposit was then cut off. The theory is based on the production of hydrocarbons from carbon monoxide, carbon dioxide, free hydrogen, and water in the context of Fischer-Tropsch reactions (synthetic production of hydrocarbons) in the mantle. These materials rise vertically up along folds and crevices in the crust. This allows them to take on small metallic traces, which are reflected at times in petroleum.
About the pro-biogenic argument of the optical activity of natural petroleum, the representatives of the Russian theory point out that optically active materials can be synthesized from non-optically active matrices at relatively low temperatures (around 130 °C) [Glasby, 87]. In addition to this, the optical activity decreases with the depth of the oil layer [Corliss 1989, 170]. On the other hand, the opponents of abiogenic approach noted that the upper mantle is much to oxidizing for methane to be the dominant form of carbon down below, as required by the theory [Glasby, 88]. In the biogenic approach, the metallic trace amounts are attributed to organisms that decompose to kerogen (e.g., containing nickel and vanadium porphyrins) [Ibid, 94]. The problem with this approach is that only iron and magnesium porphyrins have been detected in biological material so far [Corliss 1989, 168]. According to the abiogenic-volcanic theory, the largest oil reserves would have to be found on the most active sites of plate tectonics if the idea with the rise of the hydrocarbons from the inner earth holds any merit. But that is not the case [Glasby, 92]. As an argument for the abiogenic theory, it is contemplated that many oil fields have been developed in the Caspian basin based on these ideas. This again is contradicted by others: often, the oil would not, as claimed, be recovered from the basement rock itself, but instead from nearby sediment [ibid, 93].
The Russian-Ukrainian theory, as it is called today, was first published extensively in Russian and has only been observed with greater delay in the West. Most notably the work of JF Kenney must be emphasized, who made this theory known [Kenney].
A similar version on the origin of abiogenic petroleum was presented at the end of the 20th century by Thomas Gold. There is no surprise that he was exposed to plagiarism accusations from Russia [Glasby, 86]. Gold explains the few biological traces in petroleum with a biosphere of thermophilic and hyperthermophilic microbes in the upper layers of the Earth’s crust that are exposed to the rising flow of hydrocarbons. These microbes are, for example, also known from the environment of deep-sea volcanoes. Gold also provides a living area up to a depth of 10 km.
Problematic for Gold’s theory is that, for example, methane can only be converted to higher hydrocarbons when a pressure of more than 30 kbar exists, which roughly corresponds to a depth of 100 km. In such depths, however, Gold’s microbes are not viable [Glasby 90].
Cosmic source of hydrocarbons
In the outer planets and moons of the solar system, carbon, hydrogen, and both simple and complex hydrocarbons have been detected. The same applies also for meteorites and comets, where now is even talk of ‘organic matter’. CO, CO2, CH4, C2H2, C2H6, etc. [Llorca, 9] were found. Particular attention must be paid to Saturn’s moon Titan, whose surface is supposed to be deeply covered with hydrocarbons [Cardona, 55]. To assume that these substances were present in the base material from which the solar system and therefore the Earth was formed is not very far-fetched [Dott/Reynolds, 34]. Of such opinion was, e.g., the astronomer Fred Hoyle, who believed that large amounts of hydrocarbons had accumulated deep within the earth, and so far only a small fraction of them has found its way into the region near the surface [ibid, 35]. The theory of cosmic origin of hydrocarbons provides the basic material for the above-mentioned abiogenic theories.
Usually only as a footnote, a variant of the theory of cosmic origin of hydrocarbons is mentioned, namely the idea that meteorites have supplied the hydrocarbons to Earth. This niche existence is justified, because the amount appropriated to meteors would never be sufficient to explain the already discovered and exploited oil reserves on Earth. But over this argument other theories are disregarded that permit feeding of much bigger quantities of hydrocarbons to Earth. Immanuel Velikovsky has to be named here in the first place. In Worlds in Collision, he had streams of naphtha (= petroleum) falling to Earth from the heavens during a close encounter with the planet Venus [Velikovsky 1950, 53-58]. Dwardu Cardona, however, attributes the naphtha streams instead to Saturn eruptions [Cardona, 62-65].
Investigations of interplanetary dust entering Earth’s atmosphere show that the dust is penetrating it slowly and without melting [Llorca, 7]. Extraterrestrial hydrocarbons could therefore enter Earth’s atmosphere potentially unscathed. This approach can explain subsurface petroleum reserves but has problems with petroleum in igneous basement rocks and metamorphic rocks.
Biogenic and abiogenic origin of petroleum
That some scientists being confronted with all these pro and contra arguments have chosen a dual position on the question of a biological or non-biological origin of petroleum is not surprising. Varying parts of petroleum are respectively seen as biogenic or abiogenic.
A recent biological origin of at least petroleum-like material can be observed in the Gulf of California [FAZ], where an extremely tight time frame of 5,000 years for its origin is indicated. However, the hydrocarbons that occur in young sediments are not really petroleum, just only approximately comparable [Dott/Reynolds, 73]. While the 13C/12C ratio speaks for biological components, the aromatic constituents commonly contained in petroleum either point to another source of the hydrocarbons or another process step, which has not yet occurred in the Gulf of California.
It turns out that the components of older oil compare rather nicely to a mix of ancient hydrocarbons to which some biomass was added than the conditions one would expect from a purely biological origin [ibid, 33]. The argument is based on the dominance of an odd number of carbon atoms in biological hydrocarbon molecules. This dominance disappears at lower layers containing petroleum [Corliss 1989, 169-170]. But even these analysis results are now called into question [Glasby, 91].
The already mentioned Thomas Gold was initially a follower of the duplex theory but later moved to the theory of microorganisms [Cardona, 60]. He became essentially an abiogenic theorist. Many of the different approaches to the origin of petroleum make a provincialistic impression in the way that they are starting from a suitable statement for a special oil deposit and are later extended to all other deposits [Dott/Reynolds, 68-69].
The debate about whether petroleum is of biogenic or abiogenic origin is continuing to run hot [e.g. Bardi/Pfeiffer]. The geologist Jonathan Clarke has prepared the following list of observations in internet-forum discussions, which should be explained in his opinion by the representatives of the abiogenic theory. The list is presented with its incorrect numbering scheme [Bardi/Pfeiffer].
1) The almost universal association of petroleum with sedimentary rocks.
2) The close link between petroleum reservoirs and source rocks as shown by biomarkers (the source rocks contain the same organic markers as the petroleum, essentially chemically fingerprinting the two).
3) The consistent variation of biomarkers in petroleum in accordance with the history of life on earth (biomarkers indicative of land plants are found only in Devonian and younger rocks, that formed by marine plankton only in Neoproterozoic and younger rocks, the oldest oils containing only biomarkers of bacteria).
3) [!] The close link between the biomarkers in source rock and depositional environment (source rocks containing biomarkers of land plants are found only in terrestrial and shallow marine sediments, those indicating marine conditions only in marine sediments, those from hypersaline lakes containing only bacterial biomarkers).
4) Progressive destruction of oil when heated to over 100 degrees (precluding formation and/or migration at high temperatures as implied by the abiogenic postulate).
5) The generation of petroleum from kerogen on heating in the laboratory (complete with biomarkers), as suggested by the biogenic theory.
6) The strong enrichment in 12C of petroleum indicative of biological fractionation (no inorganic process can cause anything like the fractionation of light carbon that is seen in petroleum).
7) The location of petroleum reservoirs down the hydraulic gradient from the source rocks in many cases (those which are not are in areas where there is clear evidence of post migration tectonism).
8) The almost complete absence of significant petroleum occurrences in igneous and metamorphic rocks.
The evidence usually cited in favor of abiogenic petroleum can all be better explained by the biogenic hypothesis, e.g.:
9) Rare traces of cooked pyrobitumens in igneous rocks (better explained by reaction with organic rich country rocks, with which the pyrobitumens can usually be tied).
10) Rare traces of cooked pyrobitumens in metamorphic rocks (better explained by metamorphism of residual hydrocarbons in the protolith).
11) The very rare occurrence of small hydrocarbon accumulations in igneous or metamorphic rocks (in every case these are adjacent to organic rich sedimentary rocks to which the hydrocarbons can betied via biomarkers).
12) The presence of undoubted mantle derived gases (such as He and some CO2) in some natural gas (there is no reason why gas accumulations must be all from one source; given that some petroleum fields are of mixed provenance, it is inevitable that some mantle gas contamination of biogenic hydrocarbons will occur under some circumstances).
13) The presence of traces of hydrocarbons in deep wells in crystalline rock (these can be formed by a range of processes, including metamorphic synthesis by the Fischer-Tropsch reaction, or from residual organic matter as in 10).
14) Traces of hydrocarbon gases in magma volatiles (in most cases magmas ascend through sedimentary succession, any organic matter present will be thermally cracked and some will be incorporated into the volatile phase; some Fischer-Tropsch synthesis can also occur).
15) Traces of hydrocarbon gases at mid ocean ridges (such traces are not surprising given that the upper mantle has been contaminated with biogenic organic matter through several billion years of subduction, the answer to 14 may be applicable also).
16) Traces of hydrocarbons in hydrothermal fluids; these are also all compositionally consistent with derivation from either country rocks or Fischer-Tropsch synthesis.
Given this list, the following question comes to mind: Why is the Middle East so rich in oil and gas resources, an area which is affected by a wide variety of geological conditions? What is the geological similarity [Corliss 1989, 176]? The vertical distribution of hydrocarbons is interesting because according to Kudryavtsev’s rule, usually where petroleum is found at all, it is also found in all the layers below it, albeit in widely varying concentrations. At a depth of more than 5,000 meters petroleum is however rarely encountered anymore [ibid, 177].
The controversy over the question of whether petroleum originates from biologically or abiogenetically created hydrocarbons has unfortunately also a political dimension. If the petroleum is actually filling in from the upper mantle layers of Earth, one has to assume a nearly limitless quantity of petroleum compared to today’s calculations. “Peak Oil”, i.e., the point of maximum oil production, would be far, far away. This, however, may be undesirable for environmental reasons. This discussion therefore draws comparable lines to the current climate debate, which has left the status of a purely scientific discourse a long time ago.
The role of dolomite in connection with petroleum is of great importance. An estimated 80% of the rocks with petroleum reservoirs in North America and the Middle and Far East are affected by dolomitization [Braitwhaite, 1]. The storage capacity of the dolomite is usually attributed to its higher porosity, as the process of dolomitization of the limestone is supposed to reduce the volume by 12%. According to recent conflicting considerations, dolomite inherits the porosity of the original limestone but is better protected against a subsequent compression [Braitwhaite, 141]. That would imply an early diagenetic dolomitization. These considerations, however, all disregard the still existing “dolomite problem “, as well as the general (hydro-)carbon quantity problem. Where did the necessary quantities of carbon and hydrogen for the production of petroleum come from? A possible source is diatomic carbon molecules found in comets [Wiki/Diatomic carbon].
Particularly noteworthy is the observation that in the layers which are regarded as the source of petroleum often animal fossils are found, especially marine fossils. They are seen mainly as the source for the petroleum [Dott/Reynolds, 60]. But is this really the case? If this were so, then why have the discovered fossils not turned into oil, while others seem to have been? Why are these fossils in such a good condition that you can still see the “meat”? Why have in other layers, such as the ‘Old Red Sandstone’ of Scotland, marine animals died in huge numbers under catastrophic conditions and not turned into oil? Could it not be the case that these animals have died directly from the petroleum-sand mixture (or only sand), in which they are found [Cardona, 57-58]? In this version of the theory, the petroleum is to a large extent of an extraterrestrial nature.
In recent years through continued development enormous reserves of petroleum have opened up. It is important to look at these new reserves for a critical consideration. This is about petroleum which is bound in shale. Oil shale is usually defined as an organic material inheriting fine-grained sedimentary rock that contains significant amounts of oil and combustible gas that can be released by destructive distillation. The largest known deposit of this type is the Green River Formation (Fig. 1) in the western U.S. It consists of three basins. The sediments filled into the basins contain oil shale of conservatively estimated 213 billion tons [Dyni, 1]. The deposits range in various grades up to at least a depth of 1,000 m [Dyni, 30]. The oil shales of the formation are known to be impervious [Tisot/Son, 425], which is why the extraction of the oil is so difficult. A late migration of oil into the shale is therefore excluded. To get to the oil or natural gas, the method of hydrofracking usually used. With the aid of chemicals, water and sand under pressure, the impenetrability of the shale is reduced; it is not a very environmentally friendly mining technique. Even usage of atom bombs to break up the rock has been thought of [Tisot/Son, 425]. Fortunately, this approach was not pursued. Newer fracking technologies replace water and chemicals by liquid propane.Another fascinating development is known for more than twenty years [USGS], but comes just now into focus: Methane Hydrate. Gas hydrate is a crystalline solid consisting of gas molecules, usually methane, each surrounded by a cage of mater molecules. It looks much like water ice. The water does not bind the methane chemically. The methane is released from the lattice when warmed or depressurized. Methane Hydrate is stable in ocean floor sediments at more than 300 meters depth as well as in arctic regions [FoxNews]. Conservatively estimated, the amount of carbon bound in gas hydrates is twice that of carbon in known normal fossil fuel reservoirs.
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