Geochemistry and genesis of geothermal well water from a carbonate–evaporite aquifer in Chongqing, SW China

Abstract

Thermal water is an important natural resource. The hydrogeochemistry and geothermometry of thermal water from the Qianyi well (TWQW), which is associated with the Tongjing Warm Springs located in the Eastern Sichuan Fold Belts in Chongqing, SW China, was investigated. The reservoir of this region consists of upper and middle Triassic carbonate and evaporite rocks. The TWQW was of Ca–Mg–SO4 type with water temperature of 47.3 °C. Plotting on the Giggenbach Na–K–Mg diagram indicates that the TWQW was immature due to a mixing of shallow karst groundwater and thermal water. The mixing model indicates that the TWQW was composed of 49% deep thermal water and 51% shallow karst groundwater. Geothermometers suggest a reservoir temperature of about 79 °C, locating this aquifer at a depth of ~ 2.5 km. The δD and δ18O values plot near the local meteoric water line, suggesting that the TWQW originates from local meteoric water at recharge elevations of ~ 780–1160 m a.s.l.. A conceptual model of the genesis for Tongjing Warm Springs was developed. Rainfall infiltrates via karst outcrops in the elevated areas of the northern Tongluoxia anticline, extracts heat from reservoir rocks at depth, dissolves minerals and becomes thermal water. The thermal water is driven by gravity, flows through the carbonate–evaporite aquifer, and follows the Tongluoxia anticline of the Eastern Sichuan Fold Belts to the southwest. The thermal water ascends to the surface along the incision created by the Wentang River and mixes with shallow karst groundwater to create warm springs. This study may be relevant to other karst geothermal reservoirs in China.