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Natural gas is all gas existing in nature, including the atmosphere, hydrosphere, lithosphere and all kinds of nature gas (including oil field gas, gas in gas field, mud volcano gas, coal-bed methane and bio gas). And for a long time “natural gas" defined, is a narrow definition starting from the point of view of energy, refers to the natural deposits in the formation of hydrocarbon and non hydrocarbon gas mixture. In petroleum geology, usually refers to the oil and gas field gas which are mainly composed of hydrocarbon, and contains non hydrocarbon gases.