Major hydrogeochemical processes in the two reservoirs of the Yangbajing geothermal field, Tibet, China Export

@article{citeulike:1859842, abstract = {The Yangbajing geothermal field with the highest reservoir temperature in China is located about 90 km northwest to Lhasa City, capital of Tibet, where high temperature geothermal fluids occur both in shallow and deep reservoirs. The geophysical survey by the INDEPTH (International Deep Profiling of Tibet and the Himalayas) project group proved the existence of magmatic heat source at Yangbajing. In the study area, the hydrochemistry of cold surface waters and groundwaters and that of thermal groundwaters from both reservoirs are distinctively different. However, analysis of the relationship between enthalpy values and Cl concentrations of cold groundwaters and geothermal fluids indicates that the geothermal fluids from the shallow reservoir were formed as a result of mixing of cold groundwaters with geothermal fluids from the deep reservoir. In other words, the geothermal fluids from the deep reservoir flowed upwards into the shallow reservoir where it was diluted by the shallow cold groundwaters to form the shallow geothermal fluids with much lower temperature. A binary mixing model with two endmembers (the cold groundwaters and the deep geothermal fluids) was proposed and the mixing ratios for the geothermal fluid from each shallow well were estimated. Using the mixing ratios, the concentrations of some constituents in shallow geothermal fluids, such as As, B, SiO2, SO42- and F, were calculated and their differences with the actual concentrations were estimated. The results show that the differences between estimated and actual concentrations of As and B are small (the average absolute values being only 1.9{\%} and 7.9{\%}, respectively), whereas those of SiO2, SO42- and F are much bigger, indicating that other hydrogeochemical processes are responsible for the concentrations of these constituents. It is postulated that SiO2 precipitation due to water temperature decrease, H2S oxidation and ion exchange between OH- in geothermal waters and exchangeable F- in fluoride bearing silicate minerals during the geothermal fluid upflow might be the causes for the observed concentration differences.}, author = {Guo, Qinghai and Wang, Yanxin and Liu, Wei}, citeulike-article-id = {1870249}, citeulike-linkout-0 = {doi:10.1016/j.jvolgeores.2007.08.004, http://www.sciencedirect.com/science/article/B6VCS-4PF6B4H-1/2/8bfb5ceea60a592d563e98ffd0416986}, journal = {Journal of Volcanology and Geothermal Research}, keywords = {bibtex-import, geothermal\_field, hydrogeochemical\_process, lhasa, lib\_brahmatwinn, lib\_ecohydrology, reservoir, tibet, yangbajing}, number = {3-4}, pages = {255--268}, posted-at = {2007-11-05 21:22:47}, priority = {2}, title = {Major hydrogeochemical processes in the two reservoirs of the Yangbajing geothermal field, Tibet, China}, url = {doi:10.1016/j.jvolgeores.2007.08.004, http://www.sciencedirect.com/science/article/B6VCS-4PF6B4H-1/2/8bfb5ceea60a592d563e98ffd0416986}, volume = {166}, year = {2007} }

TY - JOUR ID - citeulike:1859842 L3 - citeulike-article-id:1870249 N2 - The Yangbajing geothermal field with the highest reservoir temperature in China is located about 90 km northwest to Lhasa City, capital of Tibet, where high temperature geothermal fluids occur both in shallow and deep reservoirs. The geophysical survey by the INDEPTH (International Deep Profiling of Tibet and the Himalayas) project group proved the existence of magmatic heat source at Yangbajing. In the study area, the hydrochemistry of cold surface waters and groundwaters and that of thermal groundwaters from both reservoirs are distinctively different. However, analysis of the relationship between enthalpy values and Cl concentrations of cold groundwaters and geothermal fluids indicates that the geothermal fluids from the shallow reservoir were formed as a result of mixing of cold groundwaters with geothermal fluids from the deep reservoir. In other words, the geothermal fluids from the deep reservoir flowed upwards into the shallow reservoir where it was diluted by the shallow cold groundwaters to form the shallow geothermal fluids with much lower temperature. A binary mixing model with two endmembers (the cold groundwaters and the deep geothermal fluids) was proposed and the mixing ratios for the geothermal fluid from each shallow well were estimated. Using the mixing ratios, the concentrations of some constituents in shallow geothermal fluids, such as As, B, SiO2, SO42- and F, were calculated and their differences with the actual concentrations were estimated. The results show that the differences between estimated and actual concentrations of As and B are small (the average absolute values being only 1.9{\%} and 7.9{\%}, respectively), whereas those of SiO2, SO42- and F are much bigger, indicating that other hydrogeochemical processes are responsible for the concentrations of these constituents. It is postulated that SiO2 precipitation due to water temperature decrease, H2S oxidation and ion exchange between OH- in geothermal waters and exchangeable F- in fluoride bearing silicate minerals during the geothermal fluid upflow might be the causes for the observed concentration differences. IS - 3-4 JF - Journal of Volcanology and Geothermal Research EP - 268 TI - Major hydrogeochemical processes in the two reservoirs of the Yangbajing geothermal field, Tibet, China VL - 166 SP - 255 KW - bibtex-import KW - geothermal_field KW - hydrogeochemical_process KW - lhasa KW - lib_brahmatwinn KW - lib_ecohydrology KW - reservoir KW - tibet KW - yangbajing AU - Guo, Qinghai AU - Wang, Yanxin AU - Liu, Wei PY - 2007/// UR - doi:10.1016/j.jvolgeores.2007.08.004, http://www.sciencedirect.com/science/article/B6VCS-4PF6B4H-1/2/8bfb5ceea60a592d563e98ffd0416986 ER -