Where does oil come from: theories of the origin of oil, its composition and basic properties
Oil fields are a unique storage of energy formed and accumulated over millions of years in the depths of our planet. In this material – about what path the oil traveled before getting there, what it consists of and what properties it has..
Scientists still disagree on how the oil was formed. There are two fundamentally different theories of the origin of oil. According to the first – organic, or biogenic, – from the remains of ancient organisms and plants, which for millions of years were deposited on the seabed or buried in continental conditions. Then they were processed by communities of microorganisms and transformed under the influence of temperature and pressure as a result of tectonic subsidence deep into the depths, forming oil source rocks rich in organic matter.
The necessary conditions for the transformation of organic matter into oil arise at a depth of 1.5–6 km in the so-called oil window – at temperatures ranging from 70 to 190 ° C. In its upper part, the temperature is not high enough – and the oil turns out to be “heavy”: viscous, thick, with a high content of resins and asphaltenes. At the bottom, the temperature of the layers rises so much that the molecules of organic matter are crushed into the simplest hydrocarbons – natural gas is formed. Then, under the influence of various forces, including gradient pressure, hydrocarbons migrate from the source formation to the higher or lower rocks.
The natural process of the formation of oil from organic remains takes on average from 10 to 60 million years, but if an appropriate temperature regime is artificially created for organic matter, then an hour is enough for its transition to a soluble state with the formation of all main classes of hydrocarbons. Supporters of the organic hypothesis interpret such experiments in their favor: the transformation of organic matter into oil is obvious. There are other arguments in favor of the biogenic origin of oil. Thus, most of the industrial accumulations of oil are associated with sedimentary rocks. Moreover, living matter and oil are similar in elemental and isotopic composition. In particular, biomarkers are found in most oil fields, such as porphyrins, chlorophyll pigments that are widespread in nature.
Most scientists today explain the origin of oil by biogenic theory. However, inorganics also give a number of arguments in favor of their point of view. There are various versions of the possible inorganic origin of oil in the bowels of the earth and other cosmic bodies, but they all rely on the same facts. First, many, though not all, deposits are associated with fault zones. Through these faults, according to the proponents of the inorganic concept, oil rises from great depths closer to the surface of the Earth. Secondly, deposits are found not only in sedimentary, but also in igneous and metamorphic rocks (however, they could be there as a result of migration). In addition, hydrocarbons are found in the material erupting from volcanoes. Finally, the third and most powerful argument in favor of the inorganic theory is that that hydrocarbons are present not only on Earth, but also in meteorites, comet tails, in the atmosphere of other planets and in scattered cosmic matter. So, the presence of methane is noted on Jupiter, Saturn, Uranus and Neptune. On Titan, the moon of Saturn, rivers and lakes have been discovered consisting of a mixture of methane, ethane, propane, ethylene and acetylene. If on other planets of the solar system these substances can be formed without the participation of biological objects, why is it impossible on Earth?
From the point of view of modern supporters of the inorganic, or mineral, hypothesis, hydrocarbons are formed from the water and carbon dioxide contained in the Earth’s mantle in the presence of ferrous metal compounds at depths of 100-200 km. High pressure in the bowels of the earth prevents thermal destruction of complex hydrocarbon molecules. In turn, proponents of organic matter do not deny that simple hydrocarbons, such as methane, can also be of inorganic origin. Experiments aimed at confirming the abiogenic theory have shown that the resulting hydrocarbons can contain no more than five carbon atoms, and oil is a mixture of heavier compounds. This contradiction has not yet been explained.
In addition to purely scientific interest, the hypotheses explaining the origin of oil and gas also have political implications. Indeed, since oil can be obtained from inorganic substances and the rate of its formation is not tens of millions of years, as the biogenic concept suggests, but many thousands of times higher, then the problem of the imminent depletion of reserves becomes at least not so unambiguous. However, for oilmen, the question of where the oil comes from is rather fundamental from the point of view of whether theory can predict where exactly to look for deposits. Organics cope with this task better.
From a purely pragmatic point of view, it is important for production to know not even where the oil originated, but where it is now and where it can be extracted from. The fact is that in the earth’s crust, most of the oil does not remain in the parent rock, but moves and accumulates in special geological objects called traps. Even if we assume that oil is of inorganic origin, traps for it are still, with rare exceptions, in sedimentary basins.
Under the influence of various factors, hydrocarbons are squeezed out of the source rocks into reservoir rocks capable of containing fluids (oil, natural gas, water). Thus, the oil field is not at all an underground “lake” filled with liquid, but rather a dense structure. Reservoirs are characterized by porosity (the proportion of voids contained in them) and permeability (the ability to pass fluid through them). For efficient recovery of oil from the reservoir, a favorable combination of both of these parameters is important.
Moving along the reservoir, the fluid at some point can rest against an impenetrable screen for it – a fluid seal. The layers of this rock are called seals, and together with the reservoir, they form traps that trap oil and gas in the field. In the classic version, gas may be present at the top of the trap (it is lighter). From below, the reservoir is underlain by water that is denser than oil.
Trap classifications are extremely diverse ( see some of them in the figure ). The simplest, both from the point of view of geological exploration and for further production, is an anticlinal trap (arched uplift), covered from above by a seal layer. Such traps are formed as a result of bending of the sedimentary cover layers. However, in addition to bending, the inner layers also undergo many other deformations. As a result of tectonic movements, for example, the reservoir may deform and lose its uniformity. In this case, the processes of exploration and production are much more complicated. Another trouble that awaits oil workers from the side of traps is the replacement of permeable rocks with good reservoir properties, such as sandstones, by impermeable ones. Such traps are called lithological traps.
The same age as the dinosaurs
When did the structures in which oil is found today formed? Its main resources are concentrated in relatively young Mesozoic and Cenozoic deposits, formed from several tens of million to 250 million years ago. However, oil production is also carried out from Paleozoic deposits (up to 500 million years ago), and in Eastern Siberia – even from deposits of the Upper Proterozoic, which are more than half a billion years old.
Numerous oil fields are found in the Devonian sediments (420-360 million years ago). During this period, insects and amphibians appeared on Earth, fish and corals reached a wide variety in the seas. During the Permian period (300–250 million years ago), the climate became more arid, as a result of which the seas dried up and thick salt strata were formed, which later became ideal seals.
The era of the domination of dinosaurs – the Jurassic (200-145 million years ago) and Cretaceous (145-66 million years ago) periods of the Mesozoic – is characterized by the maximum flourishing of life and is associated with high sedimentation. Some giant and large deposits (Iran, Iraq) of oil are found in deposits of the Paleogene (66-23 million years ago). There are known oil fields in Quaternary rocks less than 2 million years old (Azerbaijan).
However, the relationship between the age of reservoir rocks and the time of oil formation is not straightforward. This process can be sequential: in the Jurassic or Cretaceous period, organic sediment began to sink down and transform into oil, which, after several tens of millions of years, migrated into reservoirs belonging to younger rock complexes. On the other hand, ancient oil source rocks formed in the Paleozoic could sink to a depth sufficient for oil maturation much later. Thus, in the same reservoirs, you can find both younger and older oil, significantly differing in their properties.
Meanwhile, the moment when dead plankton sinks to the bottom of the sea basin, and the moment when the accumulated layer of organic matter, plunging several kilometers down, gives up oil, millions of years and a whole series of chemical and physical transformations. Therefore, there is nothing surprising in the fact that the composition of oil is extremely diverse and heterogeneous. That is why the oilmen themselves are used to using this word in the plural – when talking about oil exploration or production and implying that each time the extracted fluid will be unique, different from everything that has been previously produced.
At its core, oil is a complex mixture of hydrocarbons of various molecular weights. It is dominated by alkanes, naphthenes and arenas. The simplest of them are alkanes (paraffinic hydrocarbons), in which the maximum number of hydrogen atoms is attached to the carbon atoms. Alkanes include methane, ethane, propane, butane, pentane, etc. They can be represented by gases, liquids and crystalline solids. The amount of alkanes in oil ranges from a quarter to seventy percent of the volume. With a large percentage of alkanes, the oil is considered to be paraffinic. From the point of view of production, such a property is considered problematic – when oil rises from the well and the corresponding decrease in temperature, paraffins can crystallize and fall out on the walls of the wells.
Naphthenes are compounds in which carbon atoms are combined into a cyclic ring (cyclopropane, cyclobutane, cyclopentane, etc.). All bonds of carbon and hydrogen are saturated here, so naphthenic oils have stable properties. Naphthenes can have from 2 to 5 cycles per molecule; chemists try to determine the maturity and other properties of oil by their composition.
Arenes, or aromatic hydrocarbons, also contain cyclic structures – benzene nuclei. They are characterized by greater solubility, higher density and boiling point. Usually oil contains 10–20% arenes, while in aromatic oils their content reaches 35%. The richest arenas are young oil. Arenas are a valuable raw material in the production of synthetic rubbers, plastics, synthetic fibers, aniline dyes and explosives, pharmaceuticals.
They like to call oil black gold, but pure hydrocarbons are colorless. Various impurities, mainly resins, impart color to oils. The asphalt-resinous part of oils is a dark-colored substance. Asphaltenes included in its composition dissolve in gasoline.
Petroleum resins, on the other hand, do not dissolve. They are viscous or hard, but fusible mass. The largest amount of resins is found in heavy dark oils rich in aromatic hydrocarbons. Such oils are highly viscous, which makes it difficult to extract them from the formation.