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Petroleum | Petroleum oil | Crude oil

The chemical composition of petroleum
Crude oil characterization 
Sulfur content
Water content
The content of mechanical impurities

Petroleum (Crude oil | Petroleum oil ) — combustible oily liquid, belonging to the group of mountain sedimentary rocks along with sand, clay, and limestone; It has an exceptionally high calorific value: during combustion, it emits significantly more thermal energy than other combustible mixtures.

Petroleum is mined and used from the 6th millennium BC. The most ancient industries are known on the banks of the Euphrates, in Kerch, in the Chinese province of Sichuan.

The chemical composition of petroleum


In the composition of oil emit hydrocarbon, asphalt-resin and ash components, as well as porphyrins and sulfur.

Hydrocarbons contained in oil are divided into three main groups: methane, naphthenic and aromatic.

Methane (paraffin) hydrocarbons are chemically most stable, and aromatic — the least stable (they have a minimum hydrogen content). At the same time, aromatic hydrocarbons are the most toxic components of petroleum oil.

The oil-asphalt-resin compound is partially soluble in gasoline: the soluble part is asphaltenes, and the insoluble part is tar.

Interestingly, in the resin the oxygen content reaches 93% of its total amount in the composition of the oil.

Porphyrins are nitrogenous compounds of organic origin, they are destroyed at a temperature of 200-250 ° C.

Sulfur is present in the composition of the oil either in the free state or in the form of compounds of hydrogen sulfide and mercaptans.

The ash part of the oil is the residue obtained when it is burned, consisting of various mineral compounds.

Crude oil characterization

Oil produced directly from wells is called crude. When leaving the oil reservoir, the oil contains particles of rocks, water, and also dissolved salts and gases in it.

These impurities cause equipment corrosion and serious difficulties in the transportation and processing of crude oil. Thus, for the export or delivery to remote refineries from mining sites, its industrial processing is necessary: water, mechanical impurities, salts, and solid hydrocarbons are removed from it, and gas is released.

Gas and most light hydrocarbons must be separated from the oil because they are valuable products and may be lost during storage. In addition, the presence of light gases during the transportation of oil through the pipeline can lead to the formation of gas bags on elevated sections of the route.

Petroleum oil refined from impurities, water and gases are delivered to refineries, where various types of petroleum products are produced from it during processing.
The quality of both crude oil and petroleum products derived from it is determined by its composition: it is he who determines the direction of oil refining and affects the final products.

The most important characteristics of crude oil are density, sulfur content, fractional composition, as well as viscosity and water content, chloride salts and mechanical impurities.


One of the main properties of unprocessed oil is its density, which depends on the content of heavy hydrocarbons, such as paraffin and resins. For its expression is used as the relative density, expressed in g / cm3, and the density, expressed in units of the American Petroleum Institute — API, measured in degrees.
Relative density = compound weight / water mass API = (141.5 / relative density) — 131.5

Petroleum Relative density, г/см3 DensityAPI, °API
light 0,800-0,839 36°-45,4°
average 0,840-0,879 29,5°-36°
heavy 0,880-0,920 22,3°-29,3°
very heavy oil more 0,920 less 22,3°
The lower the density of oil, the easier the process of refining and the higher the quality of the oil produced from it.

Sulfur content

According to the sulfur content, oil in Europe and Russia is divided into low-sulfur (up to 0.5%), sulfur (0.51-2%) and high-sulfur (more than 2%), in the USA — sweet (up to 0.5%), medium-sweet / medium acid (0.51-2%) and acidic (more than 2%).

The classification adopted in the USA, which seems at first glance unusual, has, however, a direct relationship to taste. In the early days of Pennsylvania oil production, kerosene derived from it was used as lamp oil to illuminate rooms.

Kerosene with a high content of sulfur gave a disgusting smell when burned, so it was more appreciated by kerosene with a low content of sulfur, sweet in taste. Hence the terminology.

Sulfur compounds in the composition of oil, as a rule, are harmful impurities. They are toxic, have an unpleasant smell, contribute to the deposition of tar, in compounds with water they cause intense corrosion of the metal. Especially in this respect, hydrogen sulfide and mercaptans are dangerous. They are highly corrosive, destroy non-ferrous metals and iron. Therefore, their presence in the marketable oil is unacceptable.


Oil is a mixture of several thousand chemical compounds, most of which — a combination of carbon and hydrogen atoms — hydrocarbons; Each of these compounds is characterized by its own boiling point, which is the most important physical property of oil, widely used in the refining industry.

At each stage of the boiling oil, certain compounds evaporate. Compounds evaporating in a given interval of temperature are called fractions, and the temperatures at the beginning and end of boiling are referred to as boiling boundaries of the fraction or boiling limits.

Thus, fractionation is the separation of a complex mixture of components into simpler mixtures or individual components.

The fractions boiling up to 350 ° C are called light distillates. The fraction boiling above 350 ° C is the residue after the selection of light distillates and is called fuel oil.

The fuel oil and the fractions derived from it are dark. Names of the fractions are assigned depending on the direction of their further use.

As a rule, crude oil contains the following fractions:

Boiling temperature Fractions
above 430°C Mazut
230-430°С Gas oil
160-230°С Kerosene
105-160°С Naphtha
32-105°С Petrol
less 32°С Hydrocarbon Gases
Different oils differ greatly in composition. In light oil, usually more gasoline, naphtha and kerosene, in heavy - gas oil and fuel oil. The most common oil with a gasoline content of 20-30%.

Water content

During extraction and processing, oil is mixed twice with water: when it comes out at high speed from the well, together with its associated water and in the process of desalting, i.e. rinsing with fresh water to remove chloride salts.

In the oil and petroleum products, water can be contained as a simple suspension, then it is easily settled during storage, and in the form of a stable emulsion, then you have to resort to special dehydration methods.

Part of the emulsion is captured by traps, collected and accumulated in earthen barns and oil ponds, where light fractions evaporate from it. These oils are called «granary». They are highly water and resinous, with a high content of mechanical impurities, it is difficult to dehydrate.

Being present in oil, especially with chloride salts dissolved in it, water complicates its processing. In the presence of water in the carburetor and diesel fuel, their calorific value is reduced, clogging and blockage of the engine parts of the car or airliner occur. In addition, the water content in the oil increases its tendency to oxidation, accelerates the corrosion of metal parts in contact with oil.

The content of mechanical impurities

The presence of mechanical impurities in the oil is explained by the conditions of its occurrence and production methods.

Mechanical impurities consist of particles of sand, clay, and other hard rocks, which, settling on the surface of the water, contribute to the formation of the oil emulsion. In the septic tanks, tanks and pipes during the heating of oil, some mechanical impurities settle on the bottom and walls, forming a layer of dirt and solid sediment.

At the same time, the productivity of equipment decreases, and when a deposit is deposited on the walls of pipes, their thermal conductivity decreases. Mass fraction of mechanical impurities up to 0.005% inclusive is estimated as their absence.


Viscosity is determined by the structure of hydrocarbons that make up oil, i.e. their nature and ratio, it characterizes the possibility of spraying and pumping oil and oil products: the lower the viscosity of the liquid, the easier it is to transport it through pipelines, to produce its processing.

This characteristic is especially important for determining the quality of oil fractions obtained from oil refining and the quality of standard lubricating oils. The higher the viscosity of the oil fractions, the greater the temperature of boiling.

Light oil with a low content of sulfur, water, salts, and mechanical impurities, as well as low viscosity, has the greatest value.

Oil exploration includes geological exploration, the purpose of which is to identify and prepare for the development of oil deposits, as well as their geological and economic assessment.

Exploration work can be divided into 2 stages: prospecting and exploration. At the exploratory stage, an initial assessment and study of potential oil and gas fields take place, followed by test drilling.

The first exploratory wells are drilled to the maximum depth. Usually, the upper layer (floor) of deposits is investigated first, and then deeper layers. At the exploration stage, the field is directly prepared for development and production.