Characteristics of gas geochemistry in Yangbajing geothermal field, Tibet

CO₂ is a main component of gases in the Yangbajing geothermal field, usually higher than 85% in volume; others are N₂, H₀₂S, H₂, CH₄, etc. Helium R/R_a ratios, ranging from 0.14 to 0.46, indicate a crustal component to be dominant in the gases. δ¹³C-CO₂ values and δ³⁴ S-H₂S values are in the range −7.72‰–−11.33‰ and 0. 2‰–8. 3‰, respectively. The gases are inferred to mostly derive from the partial melting of the Nyainqentanglha core complex. The gases of shallow and deep reservoirs are distinctive in chemical compositions and δ³⁴S-H₂S values, which shows their different flowing paths and physical-chemical processes. Sulphur does not approach isotopic equilibrium between H₂S and SO_{-2 4} species in both reservoirs. Significance of steam chemical monitoring is also discussed.