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The identification of the origin and source of natural gas is always a difficult and hot issue. Hereinto, the maturity identification is one of the most important scientific problems. Many empirical equations have been established to decipher the relationship between the maturity of gas source rocks and the carbon isotopic composition of natural gas. However, these equations proposed often fail to identify the maturity of the source rocks correctly, which in turn prevents the identification of genetic types and source rocks of the natural gas because the petroliferous sedimentary basins in China are complex and diverse, with multiple sets of source rocks and different thermal history. In this paper, the oil-associated gas from the Permian lacustrine source rocks and the coal-derived gas from the Jurassic source rocks in Junggar and Turpan-Hami basins have been investigated to decipher the relationship between the maturity (vitrinite reflectance) of gas source rocks and the carbon isotopic composition of methane. The equations established are δ13C1=25lgRo−42.5 for oil-associated gas, and δ13C1=25lgRo−37.5 for coal-derived gas. These new equations are suitable for the maturity identification of source rocks in most petroliferous basins, and favorable for the identification of the genetic type and source of natural gas, which is very important to improve the geological theory of natural gas. [ABSTRACT FROM AUTHOR]

By the end of the year 2010, a total of 15 large tight gas fields have been found in China, located in the Ordos, Sichuan, and Tarim basins. The annual production and total reserves of these fields in 2010 were 222.5×10 and 28657×10 m, respectively, accounting for 23.5% and 37.3%, respectively, of the total annual production and reserves of natural gases in China. They took a major part of all natural gas production and reserves in China. According to the analyses of 81 gas samples, the stable carbon and hydrogen isotopic compositions of tight gases in China have following characteristics: (1) Plots of δC- δC- δC, δC-C/C and δC- δC demonstrate the coal-derived origin of tight gases in China; (2) For the primary alkane gases, both carbon and hydrogen isotopic values increase with increasing molecular mass, i.e., δC< δC< δC< δC and δH< δH< δH; (3) The isotopic differences of δC- δC, δC- δC, δH- δH and δH- δH decrease with increasing R (%) and C/C; (4) There are seven causes for the carbon and hydrogen isotopic reversal, however, the carbon and hydrogen isotopic reversal of tight gases in China is caused mainly by multiple stages of gas charge and accumulation. [ABSTRACT FROM AUTHOR]