Crude oil and natural gas are collectively referred to as oil. Crude oil is only a relatively heavy hydrocarbon component formed naturally in liquid form, while natural gas is a relatively light hydrocarbon component existing in gaseous form. The natural gas from gas well is called gas well gas, and the natural gas separated from crude oil from oil well is called associated gas. Natural gas is a mixture of hydrocarbon gases, which also contains water and other impurities. It is mainly composed of carbon, hydrogen, sulfur, nitrogen, oxygen and trace elements, mainly carbon and hydrogen, with carbon accounting for 65% - 80% and hydrogen accounting for 12% - 20%. The composition of natural gas produced in various regions is different, and even the composition of natural gas produced by two different wells in the same reservoir is different,With the different degree of oilfield exploitation, the composition of natural gas produced by the same well will also change. The main hydrocarbon in natural gas is methane, and it also contains ethane, propane, butane, pentane, a small amount of hexane, heptane and other heavier gases. Classification of natural gas There are three classification methods for natural gas (1) According to the characteristics of mineral deposits, it is mainly divided into gas well gas and associated gas. Associated gas: refers to the natural gas separated from crude oil by oil wells. Gas well: refers to the natural gas from the gas well. (2) According to the hydrocarbon composition of natural gas (that is, according to the content of liquid hydrocarbon in natural gas), it can be divided into dry gas, wet gas, lean gas and rich gas. C5 definition method -- division of dry and wet gas Dry gas: refers to the natural gas with heavy hydrocarbon liquid content above C5 (pentane) and composition classification and properties lower than 13.5 cubic centimeter in 1 standard cubic meter of natural gas. Wet gas: refers to the natural gas with the content of hydrocarbon liquid above C5 higher than 13.5 cubic centimeter in 1 standard cubic meter of natural gas. C3 definition method -- division of poor and rich gas Lean gas: refers to the natural gas with hydrocarbon liquid content above C3 less than 94 cubic centimeter in 1 standard cubic meter of natural gas. Rich gas: refers to the natural gas with hydrocarbon liquid content above C3 higher than 94 cubic centimeter in 1 standard cubic meter of natural gas. (3) According to the content of acid gas, natural gas can be divided into acid gas and clean gas. Sour natural gas: refers to the natural gas containing a significant amount of sulfide, carbon dioxide and other sour gas, which must be treated before it can reach the pipeline transportation standard or commodity gas quality index. Clean gas: refers to the gas with little or no sulfide content, which can be exported and utilized without purification.