Natural gas is the general term of the mixed gas mainly hydrocarbon existing in the underground reservoir rocks, including oil field gas, gas field gas, coal bed gas, mud volcano gas and gas produced by biology and so on. The main ingredient of natural gas is methane, normally accounting for 85-95%, the next is hexane, propane and butane etc.. The natural gas is excellent fuel and chemical industrial material. The associated gas is the volatility part of crude oil existing in the top of oil-bearing layers in the form of gas. The associated gas exists in all the layers where there is crude oil only differ with various proportion of oil and gas. Even in the same oil field the resource of oil and gas are not necessary the same. Through different approaches and different courses they gathered in the same reservoir rocks. The non-associated gas has nothing to do with liquid gathering and maybe come from plant substance. The outputs of globe natural gas are mainly gas field gas and oil field gas. The exploitation on coal bed gas has attracted world's attention day by day. 

The widely distributed sedimentary rocks and numerious land facies basins provide advantageous geological condition for the storage of various natural gases. According to the 1993 prediction of the long-term resources reserves, the total reserves of natural gas is up to 38,000 billion cubic meters, onshore natural gas mainly distribute in central and western regions, respectively accounting for 43.2% and 39.0% of the onshore reserves. The layers of natural gas in China mainly are the third system of Cenozoic Erathem and Palaeozoic Erathem, the Cenozoic Erathem respectively accounting for 43.2% and 39.0% of the total reserves, the Mesozoic Erathem 11.1%, the Previous Palaeozoic Erathem 25.5% and Latter Palaeozoic Erathem26.1%. The formation of natural gas resources are as follows: highly ripened pyrolysis gas and coalbed gas dominate, accounting for 28.3% and 20.6% of all natural gas resources respectively; oil field associated gas accounts for 18.8%; coalbed gas accounts for 27.6%; and gas produced by biology accounts for 4.7%. Verified deposits of China's natural gas resources lie in 10 large basins: Bohai Bay, Sichuan, Songliao, Junggar, Yingge Sea-Southeast Qiongzhou, Qaidam, Tu-Ha, Tarim, Bohai, and Erdos. China's gas fields are mainly medium and small ones, most of which are of complex geological structures and hard to explore and develop. During the period from 1991 to 1995, the outputs of China's natural gas increase from 16.073 billion m3 to 17.947 billion m3, with an average growth rate of 2.33% annually.