Geochemical characteristics of ultra-deep natural gas in the Sichuan Basin, SW China

The natural gas components and geochemistry of 38 ultra-deep gas wells（burial depth greater than 6 000 m） in the Sichuan Basin were analyzed to determine the genesis of ultra-deep natural gas in the basin. The ultra-deep natural gas components of the basin have the following characteristics: Methane has an absolute advantage, which can be up to 99.56% with an average of 86.6%; ethane is low, with an average of 0.13%; there is nearly no propane and butane. So it is dry gas at over-mature thermal stage. The content of H2 S can be up to 25.21%, with an average of 5.45%. The alkane gas isotopes are: the carbon isotope varies from-32.3‰ to-26.7‰ for methane and from-32.9‰ to-22.1‰ for ethane. There is nearly no carbon isotopic reversal among methane and its homologues. Hydrogen isotope varies from-156‰ to-114‰ for methane, and from-103‰ to-89‰ for some ethane. The carbon isotope of CO2 varies from-17.2‰ to 1.9‰ and most of them fall within the range of 0±3‰. According to the δ13C1-δ13C2-δ13C3 plot, except some wells, all other ultra-deep gas wells are dominated by coal-derived gas. Based on the CO2 origin distinguishing plot and δ13CCO2, except some individual wells, most of the ultra-deep CO2 are of carbonate metamorphic origin. H2 S in the ultra-deep layer of Longgang and Yuanba gas fields belongs to thermochemical sulfate reduction（TSR）, while H2 S from Well Shuangtan belongs to thermal decomposition of sulfides（TDS）.