Genesis on Hydrogen Sulfide of Feixianguan Formation in Sichuan Basin 

Key words: 
• H2S

Abstract: Hydrocarbons in oolitic reservoir gases of Feixianguan Formation in northeastern Sichuan Basin mainly consist of methane with 75%～90% of CH4, 0～0.15% of C2, and the nonhydrocarbon mainly consists of H2S and CO2, and their relative contents are 5%～20% and 1%～10%，respectively. The high sulfurbearing gas distribution area is identical to the gypseous distribution area. These is a view that the H2S in natural gases is formed by thermochemical sulfate reduction (TSR). As largemiddle gas reservoirs with high H2S contents have been found in the northeastern of Sichuan basin, the study on the genetic mechanism of H2S has become the hotsport. Based on the analysis and summary of the current research status of H2S genetic mechanism at home and abroad，these experiment techniques of Gold tube pyrolysis, hydrous pyrolysis and quartz tube pyrolysis have been used to simulate the dyhrocarbon generation of marl source rock, and the stable sulfur isotopic composition of pyrolysis H2S, CaSO4 and S have been measured. The results show that it is very easy that the reaction of sulphur and nhexane, and the quantity of generating H2S is very large under the lower temperature. Reverse, it is very difficult that the reaction of CaSO4 and nhexane, and the quantity of generating H2S is very small under the higher temperature. The content of H2S is as high as or even over the methane during the simulation experiment of immature marl with high organic matter content，better type and large amount of FeS2 of Feixianguan formation. The stable sulfur isotopic composition of H2S originated from the reactions of nhexane and sulphur as well as CaSO4 is higher than that of sulphur and CaSO4. The H2S in marine formation is originally sourcing from the SO2－ 4in formation. It is the main reason of H2S concentration in the Lower Triassic Feixianguan Fm. carbonate reservoirs in the East Sichuan Basin that the source rocks with high FeS2 generate H2S directly and the H2S origin from the reactions of hydrocarbon and sulphur in reservoirs.