Fossil fuels.
Extraction of oil

Explain the Process of Formation of Petroleum

Explain the process of formation of petroleum

Petroleum is a fossil fuel used in many uses due to its calorific value as a source of energy. However, since it is a limited resource, it is considered a non-renewable energy source. This naturally occurring fuel is also called crude oil.

Special facilities refine crude oil in order to obtain derived petroleum products such as diesel fuel,  heating oil, gasoline, asphalt, jet fuel, waxes, lubricating oils, and petrochemical feedstocks.

Petroleum is a derivative of old fossilized dead organisms, such as zooplankton and algae. Large quantities of these organic material remains were deposited at the bottom of the sea or lake. Therefore, they are covered with layers of sediments like sand, clay, and stagnant water (water without dissolved oxygen) and silt faster than they could decompose in an aerobic way.

About 1 m below this sediment or the oxygen concentration in the water was low, below 0.1 mg / l, and there were anoxic conditions. At this point, the temperatures also remained constant.

As other layers settle in the seabed or lake, there is intense heat and pressure. Initially, it was transformed into a waxy material known as kerogen. The kerogen is found in several oil shales around the world. As a result, it was converted, with more heat, into liquid and gaseous hydrocarbons through a process known as catagenesis.

How Is Petroleum Formed Inside the Earth?

Petroleum formation occurs through several reactions at high temperatures and pressure. These phases of this formation of petroleum are detailed below.

The First Phase of Diagenesis: Anaerobic Decomposition

Without abundant oxygen, aerobic bacteria were prevented from rotting organic matter after being buried under a layer of sediment or water.

Due to such anaerobic bacteria, in 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 combine amino acids, monosaccharides, phenols, and aldehydes with fulvic acids. Fats and waxes did not hydrolyze widely under these mild conditions.

The Second Phase of the Diagenesis: Formation of Kerogen

Some phenolic compounds produced from previous reactions functioned as bactericides, and the order of actinomycetal bacteria had antibiotic compounds. 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.

Consequently, the compounds in this mixture began to combine in a little-known way to form kerogen. The combination occurred similarly to how phenol and formaldehyde molecules react to urea-formaldehyde resins. Still, kerogen formation happened in a more complex anaerobic manner due to a greater variety of reagents. The whole process of the formation of kerogen from the beginning of anaerobic decomposition is called diagenesis.

Catagenesis: the Transformation of Kerogen into Fossil Fuels

Kerogen formation keeps going to a depth of around 1 kilometer from the Earth's surface, where temperatures can reach about 50 degrees Celsius. Kerogen formation is at the midpoint between organic matter and fossil fuels. The kerogen 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 converts into fossil fuels like oil and natural gas.

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

Because the catagenesis was closed from external reagents, the fuel mixture's resulting composition depended on the kerogen's composition through reaction stoichiometry. There are three main types of kerogen: type I (algae), II (lipid), and III (humic), which were formed mainly from algae, plankton, and woody plants.

The catagenesis was pyrolytic even though it occurred at relatively low temperatures, from 60 to several hundred degrees Kelvin. Pyrolysis was possible due to the long reaction times involved. The heat came from the decay of radioactive materials from the crust, especially 40-K, 232-Th, 235-U, and 238-U.

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 kerogen. Above the maximum, 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.

What Is Kerogen?

Chemistry is a mixture of organic chemical compounds forming part of rock formations' organic matter.

It is insoluble in ordinary organic solvents due to its molecular weight (more than 1,000 daltons) of constituent compounds. The soluble portion is known as bitumen. When heated to the correct temperatures in the earth's crust, some types of kerogens release petroleum or natural gas. When such kerogens are present at high rates in rocks such as shales, they form possible mother rocks.

Kerogen-rich shales that have not been heated to high temperatures to release their hydrocarbons can form bituminous shale deposits.

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


Published: January 14, 2020
Last review: November 28, 2022