The main part of natural gas is methane (table. 3.1).

Table 3.1

The composition of natural gas

Components	Formula	The content, %
Methane	CH4	88-95
Ethan	C2H6	3-8
Propane	C3H8	0,7-2,0
Bhutan	C4H10	0,2-0,7
Pentane	C5H12	0,03-0,5
Carbon dioxide	CO2	0,6-2,0
Nitrogen	N2	0,3-3,0
Helium	He	0,01-0,5

Methane is widely distributed in nature. He always enters in the composition of the reservoir oil. A lot of methane dissolved in formation waters at depth 1,5-5 km. Gaseous methane forms deposits in porous and fractured sedimentary rocks. In small concentrations it is present in the waters of the rivers, lakes and oceans, in soil air and even in the atmosphere. The main mass of methane dispersed in sedimentary and igneous rocks.

The wide distribution of methane in nature suggests, he was formed in different ways. There are several processes, leading to the formation of methane: biochemical, catalytic thermal, radiation-chemical, mechanochemical, metamorphic, cosmogenic.

Biochemical processthe formation of methane occurs in the silts, the soil, sedimentary rocks and hydrosphere. Know more than a dozen bacteria, as a result of the activity of organic compounds (proteins, fiber, fatty acids) methane is generated. Even oil at great depths under the action of bacteria, contained in the reservoir water, breaks down to methane, nitrogen and carbon dioxide.

Catalytic thermal processthe formation of methane is to convert the gas into organic matter of sedimentary rocks under the influence of elevated temperature and pressure in the presence of clay minerals, playing the role of catalyst. This process is similar to the formation of oil. Initially, organic matter, accumulating on the bottom of water bodies and on land, exposed-biodegradable. Bacteria while destroying the simplest connection. As you descend organic matter deep into the Earth and a corresponding increase of temperature the activity of the bacteria is attenuated and completely stops at temperatures 100 ⁰With. Then turn on another mechanism – the destruction of complex organic compounds (the remains of living matter) in simpler hydrocarbons and, in particular, in methane, under the influence of increasing temperature and pressure. An important role in this process is played by natural catalysts - the aluminosilicates, involved in the various, especially clay rocks, and trace elements and their compounds.

If oil is formed from organic matter of sapropelic type – precipitation of shelf seas and oceans, resulting from phyto- and zooplankton, enriched with fatty substances, the source for the formation of methane is organic matter of humic type, comprising the remains of plant organisms. This substance forms when thermocatalytic mainly methane. The main zone of petroleum formation corresponds to the temperatures of rocks from 60 to 150 ⁰With, encountered at the depth 1,5-6 km. In the main zone of petroleum formation along with the oil is formed and methane (in relatively small quantities), and its heavier homologues. Powerful zone of intense gas formation corresponds to the temperature 150-200 ⁰C and above. It is located below the main zone of petroleum formation. In this area in the hard temperature conditions, there is a deep thermal destruction not only of dispersed organic matter, but hydrocarbons of oil shale and oil. This creates a large amount of methane.

Radiation-chemical process methane formation takes place under the influence of radioactive radiation on various carbon compounds. Seen, black fine clay sediments with high concentrations of organic matter, as a rule, and enriched uranium. This is due to the fact, the accumulation of organic matter in the sediments favor deposition of the salts of uranium. Under the influence of radioactive radiation the organic matter decomposes with the formation of methane, hydrogen and carbon monoxide. The latter itself breaks down into carbon and oxygen, then the carbon combines with hydrogen, also forming methane.

Mechanochemical processthe formation of methane is the formation of hydrocarbons from organic matter (coal) under the influence of constant and variable mechanical loads. In this case, the contacts of the mineral grains of rocks are formed by high voltage, the energy which is involved in the transformation of organic matter.

Metamorphic processthe formation of methane is associated with coal conversion under high temperatures into carbon. This process is part of the overall process of transformation of substances at temperatures above 500 ⁰With. In such circumstances, the clay transformed into crystalline schists and granite, limestone – marble and t. p.

Cosmogenic process the formation of methane describes the "space" hypothesis for the formation of oil E. Sokolova.

The bulk of most methane gas fields in the world has a thermocatalytic origin. It is formed at a depth of 1 to 10 km. A large proportion of methane has a biochemical origin. The main quantity of it is formed at depths of up to 1-2 km.

Natural gases are divided into three groups:

- gases, produced from purely gas deposits;

- gases, produced from gas condensate fields;

- gases, produced together with oil from oil fields.

All gases are a mixture of paraffin hydrocarbons with nitrogen, hydrogen sulfide, carbon dioxide and other components, but in different proportions. Gases pure gas fields in the light most, they 90 % and more consist of methane. Gases of oil deposits (they are also called associated petroleum gas) the most severe, methane in them from 30 to 70 %. Gases of gas-condensate deposits are a little more serious, gases pure gas fields, but easier, than petroleum gas; methane in them from 80 to 90 %.

Natural gas is colorless, and in the absence of hydrogen sulfide has no odor.

In our country there are numerous fields of combustible gases, especially in oil-rich areas, for example, near Saratov, Stavropol, Wow and others. Natural combustible gases are recycled to lesopererabatyvajushchih plants, building close to major oil and gas fields.

Natural gas is a valuable fuel, it burns completely leaving no ash, without forming carbon monoxide (WITH). Its calorific value is very high (40-50 kJ/kg). In addition, natural gas – the raw material for the chemical industry.

Associated gas is a gaseous hydrocarbon, exist in the oil in dissolved state. During the extraction gases are extracted to the surface. The amount of gas in cubic meters, dissolved in 1 tons of oil in reservoir conditions, call gas factor. Used associated gas as feedstock for the chemical industry. Can also be used as fuel.