Characteristics of gas geochemistry in Yangbajing geothermal field, Tibet

CO2 is a main component of gases in the Yangbajing geothermal field, usuall y higher than 85% in volume; others are N-2, H2S, H-2, CH4, etc. Helium R/R -a ratios, ranging from 0.14 to 0.46, indicate a crustal component to be do minant in the gases. delta C-13-CO2 values and delta S-34-H2S values are in the range - 7.72 parts per thousand- - 11.33 parts per thousand and 0.2 pa rts per thousand-8.3 parts per thousand, respectively. The gases are inferr ed to mostly derive from the partial melting of the Nyainqentanglha core co mplex. The gases of shallow and deep reservoirs are distinctive in chemical compositions and delta S-34-H2S values, which shows their different flowin g paths and physical-chemical processes. Sulphur does not approach isotopic equilibrium between H2S and SO42- species in both reservoirs. Significance of steam chemical monitoring is also discussed.