Natural gas refers to all gases naturally present in nature, including gases formed by various natural processes in the atmosphere, hydrosphere, and lithosphere (including oilfield gas, gas field gas, mud volcanic gas, coalbed gas, and biogenic gas).The commonly used definition of "natural gas" for a long time is a narrow definition from an energy perspective, referring to a mixture of hydrocarbons and non hydrocarbon gases naturally stored in the formation. In petroleum geology, it usually refers to oilfield gas and field gas. Its composition is mainly composed of hydrocarbons and contains non hydrocarbon gases.