Stable carbon and hydrogen isotopes of gases from the large tight gas fields in China

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 222.5×108 and 28657×108 m3, 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 δ 13C1-δ 13C2-δ 13C3, δ 13C1-C1/C2+3 and δ 13C1-δ 13C2 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., δ 13C1<δ 13C2<δ 13C3<δ 13C4 and δ 2H1<δ 2H2<δ 2H3; (3) The isotopic differences of δ 13C2-δ 13C1, δ 13C3-δ 13C1, δ 2H2-δ 2H1 and δ 2H3-δ 2H1 decrease with increasing R o (%) and C1/C1−4; (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.