| In 21st century, with the rapid development of global economy and great improvement of people s living standards, the demand for conventional energy such as oil, coal and natural gas is growing, and the available resources are becoming more and more scarcity. As a new energy, geothermal resource can be used directly. High temperature geothermal resources can be used for power generation, while medium-low temperature geothermal resources can be used for bathing, greenhouse cultivation and heating supply. Geothermal water with different qualily have different medical health effect. Thus, geothermal water is really low-cost, high benefit and less pollution resource. The geothermal resource in our country is widely distributed and abundant, almost distributed in all provinces and autonomous regions. Gonghe basin is one of the most abundant geothermal areas in Qinghai province, the heat source of which is hot dry rock. Rehai geothermal field is the largest and most active hydrothermal area in Tengchong, the geothermal resource of which is originated from a still not colling completely magma chamber. The amount of geothermal resource in these two geothermal areas is abundant, and has great developing potentiality in power generation, medical treatment, scientific research, tourism, industry, agriculture and so on.In view of the different heat sources in Gonghe basin and Rehai geothermal field, we will compare these two geothermal systems in hydrogeochemistry and isotope geochemistry on basis of the geology and hydrogeology analysis, and discuss the deep heat source distribution by the use of geothermometer, proposing the conceptual models of formation mechanism and providing basic data for exploitation of geothermal resources finally.Main research contents and some conclusions:1. Hydrogeochemistry(1) The geothermal waters from Gonghe are mainly Na-Cl, Na-Cl-HCO3 and Na-SO4-Cl types, and Rehai acid waters are H-Na-SO4-Cl and H-SO4 types, whereas those from Rehai neutral waters are Na-HCO3-Cl and Na-Cl-HCO3 types. Due to the differences of reservoir types, reservoir depth, extent of water-rock interaction and formation mechanism, the water types are different in these geothermal waters.(2) Contrastively analyzing the hydrogeochemical elements in Gonghe and Rehai geothermal waters, the trends are as follows. In macroelements, the concentrations of Na+, Cl" and HCO3- in Gonghe geothermal waters are not much difference with those in Rehai neutral waters and much more high than those in Rehai acid waters, while SO42- is higher than that in Rehai neutral waters and much lower than Rehai acid waters, whereas Ca2+ and Mg2+ are higher than Rehai on the whole. In characteristic components, the contents of F and B in Gonghe geothermal waters are lower than those in Rehai neutral waters and a little more than those in Rehai acid waters, while Li, Rb and Cs are not much difference with those in Rehai neutral waters and higher than Rehai acid waters, and the concentration of SiO2 in Gonghe geothermal waters is match with Rehai.(3) The Na, B, F, SiO2, Li, Rb and Cs show excellent linear relation with Cl in Rehai neutral waters, indicating there exist a unified parent geothermal liquid in the deep of Rehai geothermal field. Those elements show no linear relationship with Cl in Gonghe and Rehai acid waters, which may be caused by their different sources.2. Isotope geochemistry(1) The characteristics of hydrogen and oxygen isotopes in study area indicate that Gonghe geothermal waters are from the infiltration recharge of meteoric waters and the recharge area is located in border mount fracture zone. Besides the recharge of meteoric waters, there still exist some magmatic water supply in Rehai, and the recharge area of nautral waters are in metamorphic rock area of Gaoligong mountain while acid waters are origined from the local atmospheric precipitation.(2) The main C isotope values in Gonghe geothermal waters are located in crustal carbonate area, and a few are in atmosphere or soil CO2 recharge region, which show that the sources of C in Gonghe geothermal waters may come from dissolution of carbonate minerals, in addition, may be little mixture of atmosphere and soil CO2. The samples of Rehai neutral waters are projected to deep source region, indicating some deep mantle origin. The C in Rehai acid waters are mainly from the mixture of deep mantle CO2 and surface soil CO2, and different mixture proportions lead to the dispersion of C isotope values.(3) The S isotope values in study area are mainly located in marine evaporite and polluted meteoric waters, showing that the main sources of sulfur in geothermal waters are dissolution of marine evaporite (gypsum and anhydrite) and recharge of polluted meteoric waters. The Ca2+: SO42- scatter diagram of geothermal water indicates that besides the dissolution of marine evaporate (gypsum and anhydrite), the content of SO42- may be controlled by some other geochemical processes, such as the oxidation of pyrite in rocks and biogenic sulfur. Compared with Gonghe, the proportion of other sulfur sources may be much more in Rehai geothermal water, such as supply of magma volatile components (SO2 and H2S).(4) The thermochemical sulfate reduction in study area can be divided into 3 groups. Three acid samples in Rehai waters show no thermochemical sulfate reduction, which is coincide with the formation mechanism of acid spring. Gonghe, Guide and Rehai neutral samples show week thermochemical sulfate reduction while Xinhai samples show strong thermochemical sulfate reduction, and Rehai neutral samples are little stronger than Gonghe and Guide waters.3 Distribution of deep heat(1)Gonghe and Rehai acid waters do not reach water-rock equilibrium and the quartz geothermometer is suitable for calculating reservoir temperature. The temperatures in Gonghe spring spot are all under 100 centigrade, and the quartz geothermometer (no steam loss) is suitable. The temperatures in Rehai acid springs are high, and quartz geothermometer (no steam loss) is suitable for not boiling springs and quartz geothermometer (the maximum steam loss) is suitable for boiling springs. Rehai neutral springs reach water-rock equilibrium, and Na-K geothermometer is fit for deep reservoir and K-Mg geothermometer is fit for shallow reservoir.(2) The circulation depth of Gonghe geothermal waters are between 1849.2 to 6072.8 m, while those of Rehai waters are between 1533.8 to 1635.9 m. Thus it can be seen that the heat source in Gonghe is deep and geothermal water can be heated only in a large depth, while the heat source in Rehai is shallow and geothermal water can be heated in a certain depth.4. Formation mechanismBased on the above analysis of hydrogeochemistry, isotope geochemistry and deep heat distribution, we conclude that Rehai geothermal field is a geothermal system with magma heat source, while Gonghe geothermal area is not. Combining the heat source, water source, reservoir, cap rock and heat conduction channel, we put forward the conceptual models of Gonghe and Rehai geothermal system.The heat source of Gonghe geothermal system is from the intense tectonic activety, which is significant to the formation of geothermal system. The granite basement is heated by the friction heat, leading to high heat flow values and abnormal geothermal gradient. Due to the large thickness of overlying stratum, conductive geothermal system is formed inside the basin. Because of the extensive faults, convective geothermal system is formed. Below the Rehai field, there is a magma chamber. The meteoric waters are heated and absorbing volatile components from the magma chamber to form parent geothermal liquid. The parent geothermal liquid is the basic water in Rehai, and ascending to the surface by different channels As Cl, Na, K, B, F, As, Li, Rb and Cs for characteristic components,... |
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