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/C_1?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.