Estimation of reservoir temperature using silica and cationic solutes geothermometers:a case study in the Tengchong geothermal area

Reservoir temperature estimation is vitally important for assessing the exploitation potential of a geothermal field.In this study,the concentrations of major chemical constituents in geothermal water sampled from boiling and hot springs in the Tengchong hydrothermal area were measured,and quartz and cationic solutes geothermometers were used to calculate subsurface temperatures.Log(Q/K) diagrams and Na-K-Mg triangular diagrams were applied to evaluating the equilibrium status of geothermal water samples with regard to reservoir minerals,and results were used to select suitable geothermometers.The results show that samples RH01,RH03,RH04,RH05,and LL16 were in or very close to full equilibrium with the selected minerals,and therefore a NaK geothermometer is appropriate.A K/Mg geothermometer,however,is applicable to LP08 and PZH18 whose chemical compositions adjusted to the shallow reservoir temperatures during their re-equilibrium processes.In contrast,cationic solute geothermometers are unsuitable for SQ20 and RH07,which are categorized as immature water in the Na-K-Mg diagram;a quartz geothermometer was adopted to evaluate the corresponding subsurface temperatures of these samples.According to the reservoir temperature estimation made in this study,there is at least one high-temperature reservoir below Rehai with a possible temperature range of 210-270 ℃.

Characteristic Solutes in Geothermal Water from the Rehai Hydrothermal System,Southwestern China

Rehai, a high-temperature hydrothermal system located in the southern part of the Tengchong volcanic geothermal area of Yunnan Province, is characterized by intensive hydrothermal activities. The hot springs at Rehai that have been sampled so far are Na-HCO3-Cl or Na-HCO3 springs except for the one at Diretiyanqu （experience geothermal area） which is an acid sulfate spring. As typical characteristic solution constituents in high-temperature hydrothermal systems with magma as heat source, CI, B and As in the Rehai geothermal waters originate mainly from the addition of magmatic fluid. However, both the mixing of magmatic fluid and the dissolution of reservoir hostrocks contribute to the enrichment of fluoride in the Rehai geothermal waters, although their fluoride concentrations are primarily controlled by the solubility of fluorite as indicated by a clear negative relation between solution fluoride and calcium concentrations. The much higher concentration of SO4^2- in the Diretiyanqu Spring as compared to the other springs outcropping at Rehai implies a quite different geochemical genesis for this spring. The H2S-rich vapor, separated from the deep geothermal fluid during boiling process （i.e., adiabatic cooling）, can ascend to shallow aquifers where it is mixed with cold groundwaters and oxidized. Acid sulfate-rich hot springs are generally formed in this manner although only one spring of this type has been samnled during the field investigation of this studv.