Fossil fuels.
Extraction of oil

Petroleum Formation

Petroleum formation

Petroleum is a fossil fuel used in many applications due to its calorific value. As it is a limited resource, it is considered a non-renewable energy source.

Petroleum is a derivative of old fossilized organic materials, such as zooplankton and algae. Large quantities of these remains were deposited at the bottom of the sea or lake where they were covered with stagnant water (water without dissolved oxygen) or sediments such as mud and silt faster than they could decompose aerobically. Approximately 1 m below this sediment or the concentration of oxygen in the water was low, below 0.1 mg / l and there were anoxic conditions. The temperatures also remained constant.

How Are Plant Fossils Formed?
How Are Plant Fossils Formed?

As other layers settled in the seabed or lake, intense heat and pressure accumulated in the lower regions. This process changed the organic matter generated through processes such as photosynthesis and solar energy. Initially it was transformed into a waxy material known as querogen. The querogen is found in several oil shales around the world. Subsequently, it was transformed, with more heat, into liquid and gaseous hydrocarbons through a process known as catagenesis.

Petroleum formation occurs from the pyrolysis of hydrocarbons in a variety of thermodynamic reactions, mainly endothermic, at high temperature or pressure, or both. These phases of oil formation are described in detail below.

First Phase of Diagenesis: Anaerobic Decomposition

In the absence of abundant oxygen, aerobic bacteria were prevented from rotting organic matter after being buried under a layer of sediment or water. However, some anaerobic bacteria were able to reduce sulfates and nitrates between the matter to H 2 S and N 2, respectively, by the use of matter as a source for other reagents.

Due to such anaerobic bacteria, at the beginning this issue began to be separated mainly by hydrolysis: the polysaccharides and proteins were hydrolyzed into simple sugars and amino acids respectively. These were anaerobically oxidized at an accelerated rate by the enzymes of the bacteria: for example, the amino acids underwent oxidative deamination of amino acids, which in turn reacted even more to the ammonia and keto acids.

The monosaccharides in turn eventually decompose into CO 2 and methane. Anaerobic decomposition products of amino acids, monosaccharides, phenols and aldehydes combined with fulvic acids. Fats and waxes did not hydrolyse widely under these mild conditions.

Second Phase of the Diagenesis: Formation of Querogen

Some phenolic compounds produced from previous reactions functioned as bactericides and the order of actinomycetal bacteria produced antibiotic compounds (for example, streptomycin). Thus, the action of anaerobic bacteria ceased about 10 m below water or sediment. The mixture at this depth contained unreacted and partially reacted fulvic acids, fats and waxes, slightly modified lignin, resins and other hydrocarbons. As more layers of organic matter settled in the seabed or lake, intense heat and pressure accumulated in the lower regions.

As a consequence, the compounds in this mixture began to combine in a little known way to form querogen. The combination occurred in a similar way to how phenol and formaldehyde molecules react to urea-formaldehyde resins, but the formation of querogen occurred in a more complex anaerobic manner due to a greater variety of reagents. The total process of formation of querogen from the beginning of anaerobic decomposition is called diagenesis, a word that means a transformation of materials by dissolution and recombination of its constituents.

Catagenesis: Transformation of Querogen into Fossil Fuels

Querogen formation continued to the depth of approximately 1 kilometer from the Earth's surface, where temperatures can reach around 50 degrees Celsius. Querogen formation represents an intermediate point between organic matter and fossil fuels: the querogen can be exposed to oxygen, oxidized and therefore lost or could be buried deeper inside the earth's crust and undergo conditions that allow it to slowly transform into fossil fuels like oil.

The latter occurred through catagenesis in which the reactions were mostly radical rearrangements of querogen. These reactions took thousands to millions of years and there were no external reagents involved. Due to the radical nature of these reactions, the querogen reacted to two kinds of products: those with a low H / C ratio (anthracene or similar products) and those with a high H / C ratio (methane or similar products); that is, products rich in carbon or hydrogen.

Because the catagenesis was closed from external reagents, the resulting composition of the fuel mixture depended on the composition of the querogen through reaction stoichiometry. There are 3 main types of querogen: type I (algae), II (lipid) and III (humic), which were formed mainly from algae, plankton and woody plants. (This term includes trees, shrubs and lianas) respectively.

The catagenesis was pyrolytic despite the fact that it occurred at relatively low temperatures (compared to commercial pyrolysis plants) from 60 to several hundred degrees Kelvin. Pyrolysis was possible due to the long reaction times involved. The heat for the catagenesis came from the decomposition of radioactive materials from the crust, especially 40 K, 232 Th, 235 U and 238 U. The heat varied with the geothermal gradient and was typically 10-30 ° C per km deep from Earth's surface. However, unusual magma intrusions could have created more localized heating.

Geologists often refer to the temperature range in which oil is formed as an "oil window." Below the minimum temperature, the oil remains trapped in the form of querogen. Above the maximum temperature, the oil is converted into natural gas through the thermal cracking process. Sometimes, oil formed at extreme depths can migrate and get trapped at a much more superficial level. The oil sands of Athabasca are an example of this.

Abiogenic Petroleum

Russian scientists proposed an alternative mechanism to the one described above in the mid-1850s, the hypothesis of the abiogenic origin of oil (oil formed by inorganic means), but this is in contradiction with geological and geochemical evidence. Abiogenic sources of oil have been found, but never in commercially profitable quantities. "The controversy is not about whether there are abiogenic oil reserves," said Larry Nation of the American Association of Petroleum Geologists. "The controversy is about how much they contribute to the Earth's general reserves and how much time and effort geologists should devote to looking for them."

What Is the Querogen?

Chemistry is a mixture of organic chemical compounds that constitute a part of the organic matter in sedimentary rocks. It is insoluble in normal organic solvents due to the enormous molecular weight (more than 1,000 daltons) of the constituent compounds. The soluble portion is known as bitumen. When heated to the correct temperatures in the earth's crust, some types of cherogens release petroleum or natural gas, which are particular fossil fuels used, among other things, in the generation of non- renewable energy. When such kerogens are present in high concentration in rocks such as shales, they form possible mother rocks. Querogen-rich shales that have not been heated to high temperatures to release their hydrocarbons can form bituminous deposits of shale.

The name "kerogen" was introduced by Scottish organic chemist Alexander Crum Brown in 1912.


Published: January 14, 2020
Last review: January 14, 2020