What is Petroleum, and How is it Formed?

Petroleum is a yellowish-black liquid that is found in various geological formations beneath the Earth’s surface. Petroleum is a fossil fuel that is formed when dead organisms – mostly zooplankton (organisms that float around in seas, oceans, and other freshwater bodies), are buried underneath sedimentary rocks. They are unable to completely decay because of the lack of oxygen. The material that does not decay is subjected to intense pressure and high temperatures and is the raw material for petroleum.

The name petroleum refers to both the naturally occurring, unprocessed crude oil, as well as the petroleum products that are formed by refined crude oil.

When extracted by humans, petroleum is refined into different types of fuels. The various components of petroleum are separated by fractional distillation – the method of separating a liquid mixture into fractions that differ in boiling point using a fractionating column.

Natural petroleum springs are very rare, and petroleum is mainly extracted by oil drilling that is carried out after extensive studies of structural geology, reservoir characterization, and sedimentary basin analysis.

Most nations with the largest oil reserves in the world belong to the Organization of Petroleum Exporting Countries (OPEC). The top three oil-producing countries are Saudi Arabia, Russia, and the United States. The developments in hydraulic fracturing and horizontal drilling in the United States in 2018 made the country the world’s largest producer of petroleum. 80 percent of the world’s readily available petroleum reserves are located in the Middle East, with a whopping 62.5 percent coming from Kuwait and Qatar alone.



Petroleum consists of crude oil and all gaseous, solid, and liquid hydrocarbons. The hydrocarbons are mainly cycloalkanes, alkanes, and different aromatic hydrocarbons. While the lighter hydrocarbons (butane, ethane, methane, and propane) exist in the gaseous state, heavier hydrocarbons and pentane exist in liquid or solid states.

The ratio of liquid, solid, and gas depends entirely on the subsurface conditions and on the phase diagram (the conditions at which distinct thermodynamic phases occur and coexist at equilibrium) of the petroleum mixture. The proportion of light hydrocarbons in a petroleum mixture differs with regards to each oil field and can range from as much as 97% by weight in lighter oils to 50% in heavier oils.

Other organic compounds contain oxygen, sulfur, nitrogen, and traces of metals like vanadium, copper, iron, and nickel. Some oil reserves even contain live bacteria.

The molecular composition of crude oil can vary greatly from one formation to the next, but the proportion of chemical elements is more or less the same.

Composition by Weight

Element Percent Range
Carbon 83 to 85%
Hydrogen 10 to 14%
Nitrogen 0.1 to 2%
Oxygen 0.05 to 1.5%
Sulfur 0.05 to 6%
Metals <0.1%

Four Main Types of Petroleum (Crude Oil)

Crude oil is a term used to describe different types of raw oil that are extracted from the ground. This crude oil can vary considerably in density and consistency. It can be found as very thin and volatile to very thick and semi-solid oil. The color of crude oil varies from a light golden yellow to a dark black. Within the petrochemical industry, crude oil is divided into four main categories based on three factors:

Viscosity – This is the ability to flow. Higher viscosity oils are thick and take more energy and a longer time to pump from the ground.

Volatility – The ability to evaporate quickly and easily into the atmosphere is volatility. Higher volatility oils evaporate more easily. During the extraction process, the surrounding environment is controlled to ensure very little oil is lost during extraction.

Toxicity – The poisonous nature of the oil to the environment, humans, and wildlife is its toxicity. The extraction of this oil requires utmost care due to its toxic nature.

The four main types of crude oils based on viscosity, volatility, and toxicity are as follows:

Very light oils – Includes jet, gasoline, petroleum ether, petroleum spirit, petroleum naphtha, and kerosene. These oils are very volatile, and they evaporate very fast. The evaporation process evaporates the toxicity levels of these oils.

Light oils – Includes Grade 1 and Grade 2 fuel oils, most domestic fuel oils, and diesel fuel oils. They are all moderately volatile and moderately toxic.

Medium oils – These are the most common type of crude oil. They have high viscosity, low volatility, and high toxicity.

Heavy fuel oils – Includes heaviest Grade 3, 4, 5, and 6 fuel oils and marine fuels. These are very viscous, the least volatile, and the most toxic.

Classification of Petroleum According to Chemical Composition

Class of petroleum Composition of 250-300 degrees Celsius fraction, wt. %
  Par. Napth. Arom. Wax Asph.
Paraffinic 46-61 22-32 12-25 1.5-10 0-6
Paraffinic-naphthenic 42-45 38-39 16-20 1-6 0-6
Naphthenic 15-26 61-76 8-13 Trace 0-6
Paraffinic-naphthenic-aromatic 27-35 36-47 26-33 0.5-1 0-10
Aromatic 0-8 57-78 20-25 0-0.5 0-20


Uses of Petroleum

In simple terms, petroleum occurs when several hydrocarbons combine with particular minerals (for example, sulphur) under extreme pressure.

The use of petroleum can be dated as far back as 2000 years ago in Ancient China. Today, it is estimated that approximately 95 million barrels of petroleum are consumed universally every single day.

Once extracted, petroleum undergoes the refining process and is separated into a variety of consumer products like gasoline (petrol), asphalt, kerosene, and chemical reagents that are used to manufacture plastics, pharmaceuticals, and pesticides.

Petroleum is mainly refined into fuel oil and gasoline that are both ‘primary energy’ (a form of energy found naturally, and that has not been subjected to any type of human-engineered conversion process). 84 percent (by volume) of hydrocarbons that are present in petroleum are converted into petroleum-based, energy-rich fuels (like gasoline, jet, diesel, heating, and other fuel oils) and liquid petroleum gas (propane or butane).

Where is Petroleum Found?

Petroleum is mainly found in porous rock formations in the upper strata of some areas of the Earth’s crust. Some petroleum is also found in oil sands or bituminous sands.

How is Petroleum Formed?

Petroleum is derived from fossilized organic matter such as algae and zooplankton. Oil and gas are formed from all the vast amounts of organic matter (animal and plant life) that was deposited as sediment on the seabed. This matter was compressed on the sea bed by stagnant water and several billions of tons of sand and silt over the course of millions of years – faster than the matter could decompose aerobically.

Around one meter below all this sediment, water/oxygen concentrations were low and anoxic conditions existed. Temperatures also remained constant. But as the layers of sediment built up over time, there was a buildup of intense pressure and heat in the lower areas. This caused the organic matter to change – first into kerogen (found in various oil shales), and then over time, with the increase in pressure and temperature, into liquid and gaseous hydrocarbons via catagenesis (the cracking process that results in the conversion of organic kerogens into hydrocarbons).

Formation of petroleum occurred from hydrocarbon pyrolysis (the thermal decomposition of matter at elevated temperatures in an inert atmosphere. The process involves irreversible chemical changes). This hydrocarbon pyrolysis occurred in mostly endothermic reaction either at high pressure, high temperature, or both. The following phases describe the process in detail.

  1. Diagenesis: The transformation of materials by dissolution and recombination of their constituents. The complete process of kerogen formation – right from the time of anaerobic decay, is called diagenesis.

First Phase of Diagenesis: Anaerobic Decay

Aerobic bacteria could not decay all the organic matter buried under layers of sediment or water because of the lack of rich oxygen. However, the anaerobic bacteria could reduce all the sulfates present in the matter into H2S and reduce the nitrates present in the matter into N2. It did this by using the matter as a source for other reactants.

First, the matter was broken apart by the anaerobic bacteria, for the most part by hydrolysis (any chemical reaction where a molecule of water breaks one or more chemical bonds). Proteins and polysaccharides were hydrolyzed by the process of hydrolysis into amino acids and simple sugars, respectively.

The amino acids and simple sugars were then anaerobically oxidized at a very fast rate by enzymes present in bacteria. Amino acids underwent oxidative deamination to imino acids. These imino acids further reacted to ammonia and alpha-keto acids. Monosaccharides decayed into CO2 and methane.

All the anaerobic decay products (monosaccharides, amino acids, aldehydes, and phenols) combined to fulvic acids. Under this mild condition, fats and waxes could not be deeply hydrolyzed.

Second Phase of Diagenesis: Kerogen Formation

During this second phase, some of the phenolic compounds that were produced from earlier reactions worked as actinomycetal (an order of actinobacteria) and bactericides (a substance that kills bacteria) and produced antibiotic compounds (like streptomycin). With this, the action of anaerobic bacteria stopped at around ten meters below the sediment or water level. At this depth, the mixture contained partially reacted and unreacted fats and waxes, fulvic acids, mildly modified lignin, resins, and a few other hydrocarbons.

As more layers of organic matter settled on the seabed or lakebed, there was a buildup of intense pressure and heat in the lower regions. As a result, compounds of this mixture began to combine to kerogen. The combination was similar to the way molecules of phenol and formaldehyde react to urea-formaldehyde resins. However, the formation of kerogen was more complex than this because of the bigger variety of reactants.

  1. Catagenesis: The transformation of kerogen into fossil fuels.

The formation of kerogen continued to about half a mile from the Earth’s surface, where temperatures could reach approximately 50 degrees Celsius. The formation of kerogen represents a midpoint between the conversion of organic matter to fossil fuels.

Two things can happen to kerogen. It can either be exposed to oxygen, get oxidized, and be lost in the process, or it could be buried deeper in the Earth’s crust and be subjected to conditions that let it slowly convert/transform into fossil fuels like petroleum. Kerogen that was buried deeper in the Earth’s crust transformed into fossil fuels through catagenesis – the reactions were mainly radical rearrangements of the kerogen.

All the reactions that occurred during catagenesis took place over thousands to millions of years. No external reactions were involved during this phase. Due to the radical nature of the reactions, the kerogen reacted with two classes of products – those with a low H/C ratio (anthracene and similar products) and those with a high H/C ratio (methane and similar products).

As catagenesis was blocked off from all external reactants, the resulting fuel mixture composition was solely dependent on the composition of the kerogen through reaction stoichiometry. Three main types of kerogen existed:

Type I kerogen: Algal – that was produced mainly from algae.

Type II kerogen: Liptinic – that was produced mainly from plankton.

Type III kerogen: Humic – that was produced mainly from woody plants (trees, shrubs, and lianas).

The process of catagenesis was pyrolytic in nature, despite the fact that it happened at relatively low temperatures (when compared with the commercial pyrolysis of plants) that ranged from around 60 degrees Celsius to several hundred degrees Celsius. The reason pyrolysis was possible was because of the long reaction times involved. The heat for catagenesis came from the decomposition of radioactive substances of the crust (Potassium-40, Thorium, Uranium-235, and Uranium-238). The heat ranged with a geothermal gradient and was around 10-30 degrees Celsius/miles of depth from the Earth’s surface. It is believed that unusual magma intrusions also could have created higher localized heating.

The temperature range in which oil forms is referred to as “oil window.” Below the minimum temperature, oil is trapped in the form of kerogen. Above the maximum temperature, the oil is converted, by thermal cracking, to natural gas.

Invariably, oil that is formed at very low depths migrate and gets trapped at shallower levels – like the Athabasca Oil Sands.


Petrol Formation at Different Temperatures

During the formation of petrol, the actual temperature that is applied to the organic matter is crucial in determining the properties of the resulting petroleum. Lower temperatures during the formation stage will result in thicker and darker raw petroleum deposits – the most solid of which is bitumen substance.

If the heat fluctuates during the formation process of petroleum, gas is produced. This gas often separates itself from the petroleum, sometimes remaining mixed up with the raw oil. If the temperatures are extremely high (over 450 degrees Fahrenheit), the original biomass is destroyed, and no gas or petroleum is formed.

As the mud and silt above the organic deposits become heavier and the forces placed on the mud and silt begin to change the bottom layers of the compressing layer above the petroleum, shale is formed. As shale forms, the oil is forced out of the space it was originally formed. The raw petroleum moves to another rock formation (usually known as a reservoir rock) and lays trapped until it is accessed.

The Migration of Petroleum

We know now how petroleum is formed. In short, microscopic phytoplankton die and sink to the bottom of the sea in the oxygen-free sediments. They are buried deeper and deeper over time and are subjected to a lengthy process of chemical conversion into a thickening layer of sediment by bacterial decomposition and maturing. This created the formation of both liquid and gas hydrocarbons within the source rock.

As the oil and gas formed, they seeped out of their source rock. Hydrocarbons are lighter than water. As a result, the gas and the oil migrated upward in porous, water-bearing rocks. This migration of oil and gas takes place over thousands of years and can extend across several miles. The migration continues until the gas and oil are stopped by impermeable rock (reservoir rock), or until the gas or oil leak into the sea.

Reservoir rocks are porous rock and are constantly saturated with water, oil, and gas. The ratio of these differs in each rock.

How is Petroleum Found?

Improved seismic techniques have now increased the odds of identifying precise locations of smaller and more-difficult-to-find reservoirs of petroleum/crude oil. Gravimeters and magnetometers are used to search for petroleum.

Oil Extraction and Recovery

The extraction of petroleum begins with drilling wells into an underground reservoir. Once the oil has been tapped, a geologist notes its presence. New drilling techniques are used to intersect a long but thin reservoir horizontally first, and then by taking a turn vertically into an “L” shape. This ensures that the oil and/or gas can be extracted from the reservoirs without having to create too many wells.

Oil extraction and recovery undergoes three stages:

Primary stage: Primarily driven by natural mechanisms, thanks to the abundance of petroleum found, natural water displaces oil downward into a well. Expansion of natural gas occurs at the top of the reservoir. The natural gas initially dissolves in the crude oil and gravity drainage that results in the oil moving into the reservoir from the upper and lower parts of the well. The recovery of petroleum during this stage is 5 to 15 percent.

Secondary recovery stage: Pressure in the well decreases over time. Secondary recovery is used to extract petroleum from the reservoirs. Secondary recovery uses techniques that increase the reservoir’s pressure. 35 to 40 percent of petroleum is extracted during this stage.

Enhanced recovery stage: Here, the mobility of the oil is increased by thermally enhanced oil recovery methods (TEOR) to increase extraction.

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