Geochemical characteristics of noble gases in natural gas and their application in tracing natural gas migration in the middle part of the western Sichuan Depression, China

Noble gases in natural gas, from Xiaoquan, Xinchang, Hexingchang and Fenggu gas reservoirs in the middle part of the western Sichuan Depression, China, were analysed. Results show that the volume content of crustal noble gases accounts for 97.9% to 99.7% of the total noble gas content, indicating that the noble gases in the study area are very largely derived from the crust. Moreover, the ⁴⁰Ar time-accumulating effect of source rocks is used to determine the complex relationship between gases and source rocks in this area, and the results agree well with that from analysis of source rock light hydrocarbons. Due to the short migration distance, the separation of ⁴He and ⁴⁰Ar is not significant in Xujiahe natural gas and Lower and Middle Jurassic natural gas, so it is difficult to trace natural gas migration. However, this separation characteristic of ⁴He and ⁴⁰Ar in Middle and Upper Jurassic natural gas is significant, which indicates that natural gas migration was from the Middle Jurassic to Upper Jurassic formations. In addition, the variation trends of ³He/⁴He ratio and δ¹³C₁ value indicates that natural gas migration is from the Xujiahe formation to the Jurassic layer in the study area.