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 δ13CI-δ13C2-δ13C3, δ13C1-C1/C2＋3 and δ13C1-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., δ13C1〈δ13C2〈δ13C3〈δ13C4 and δ2HI〈δ2Hz〈δ2H3; （3） The isotopic differences of δ13C2-δ13C1, δ13C3-δ13C1, δ2H2-δ22Hl and cTZH3-62H1 decrease with in- creasing Ro （%） 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.

Water Role and Its Influence on Hydrogen Isotopic Composition of Natural Gas during Gas Generation

In order to discuss the role and influence of water during the generation of natural gas,the participation mechanism of water during the evolution of organic matter and its influences were summarized.In addition,we carried out an anhydrous cracking experiment of oil extracted from the Feixianguan Formation source rock in a closed system,which led to the establishment of the kinetic models for describing carbon and hydrogen isotopic fractionation during gas generation from organic matter.The models were calibrated and then applied to the northeastern Sichuan Basin.By combining a series of gas generation experiments from octadecane pyrolysis without water or with distilled water in varying mass proportions,several results were proved:(1) the hydrogen isotopic composition of natural gas becomes lighter with the participation of formation water;(2) we can quantitatively study the hydrogen isotopic fractionation with the kinetic model for describing carbon isotopic fractionation; (3) more abundant and reliable geological information can be obtained through the combined application of carbon and hydrogen isotopic indices.