Geochemistry and isotope study of the thermal waters from the urban getohermal field, Beijing, China

Abstract

The Urban geothermal field is a famous low-temperature geothermal field in China. Its production history spans more than 30 years. The geothermal system consists basically of three reservoirs, the Tieling Group (Jxt), the Wumishan Group (Jxw) and the Hongshuizhuang Group (Jxh) of the Jixian (Jx) system. Geochemical methods are used to interpret the chemical and isotopic characteristics of thermal waters in the Urban field. Because of the relatively low temperature of the thermal water, the thermal fluid does not seem to be in equilibrium with any hydrothermal minerals in the reservoir, except for chalcedony and quartz. As for mixing, the binary plot and silica-enthalpy mixing model are used, indicating that the fluids in the Urban geothermal field have been mixed with cold water. Several kinds of geothermometers are used to predict the subsurface temperatures in this area. Geothermometer calculations indicate that one can expect the highest temperature, a little over 100°C, in the centre of the sedimentary basin. Therefore, the main potential for further development of the geothermal resources is in that region. From the log (Q/K) diagram, it can be seen that the geothermal water in the Urban field has a potential for scaling. The present study indicates that the thermal water is of meteoric origin. The chemical composition of the water has changed very little during the period 1984-2000.