Abstract
This study defines reasonable reservoir temperatures and cooling processes of subsurface geothermal fluids in the Chabu high-temperature geothermal system. This system lies in the south-central part of the Shenzha-Xietongmen hydrothermal active belt and develops an extensive sinter platform with various and intense hydrothermal manifestations. All the geothermal spring samples collected systematically from the sinter platform are divided into three groups by cluster analysis of major elements. Samples of group 1 and group 3 are distributed in the central part and northern periphery of the sinter platform, respectively, while samples of group 2 are scattered in the transitional zone between groups 1 and 3. The hydrochemical characteristics show that the geothermal waters of the research area have generally mixed with shallow cooler waters in reservoirs. The reasonable reservoir temperatures and the mixing processes of the subsurface geothermal fluids could be speculated by combining the hydrochemical characteristics of geothermal springs, calculated results of the chemical geothermometers, and silica-enthalpy mixing models. Contour maps are applied to measured emerging temperatures, mass flow rates, total dissolved solids of spring samples, and reasonable subsurface temperatures. They indicate that the major cooling processes of the subsurface geothermal fluids gradually transform from adiabatic boiling to conduction from the central part to the peripheral belt. The geothermal reservoir temperatures also show an increasing trend. The point with the highest reservoir temperature (256°C) appears in the east-central part of the research area, which might be the main up-flow zone. The cooling processes of the subsurface geothermal fluids in the research area can be shown on an enthalpy-chloride plot. The deep parent fluid for the Chabu geothermal field has a Cl− concentration of 290 mg/L and an enthalpy of 1550 J/g (with a water temperature of 369°C).
Highlights
As a part of the Mediterranean-Himalayan geothermal belt, Tibet has abundant geothermal resources
The dendrogram constructed by the software SPSS17.0 shows that all the geothermal samples are divided into three clusters, groups 1, 2, and 3, and group 2 can be further divided into two subclusters (Figure 4)
Contour maps were applied to measure emerging temperatures, mass flow rates, and TDS of the spring samples and the reasonable temperatures of geothermal reservoirs as determined in “Selected Reservoir Temperatures.”. This shows that the measured emerging temperatures and mass flow rates gradually decrease from the central part to the peripheral belt (Figures 9(a) and 9(b)), while the opposite change is found with TDS (Figure 9(c)). This indicates that, from the central part to the peripheral belt, the major cooling processes of the subsurface geothermal fluids gradually transform from adiabatic boiling to conduction
Summary
As a part of the Mediterranean-Himalayan geothermal belt, Tibet has abundant geothermal resources. There are a series of S-N-trending normal faults distributed in the Tibetan Plateau, which are the result of the collision between the India and Eurasian plates [1]. These S-N-trending normal fault systems crosscut the Yarlung Zangbo River and Pangong Tso-Nu River suture belts, forming the famous hydrothermal active belt of the Tibetan Plateau [2, 3]. There are four major hydrothermally active belts: Tangrayumco-Gucuo, Shenzha-Xietongmen, Yadong-Gulu, and Sangri-Cuona (Figure 1). Yadong-Gulu is the most active hydrothermal belt and has the most concentrated geothermal reserves, followed by the Shenzha-Xietongmen hydrothermal belt. The Chabu high-temperature geothermal system lies in the south-central part of the Shenzha-Xietongmen hydrothermally active belt and has developed an extensive sinter platform with various and intense hydrothermal manifestations
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