A study of the characteristics of geothermal reservoir and genesis of thermal groundwater in the Tongluoshan anticline near Guang'an in east Sichuan

Abundant geothermal water of low-medium temperature occurs in the karstic geothermal reservoirs, which are the major target strata for the exploration and exploitation of geothermal resources in the city of Guang'an. In this paper, methods and means of geothermal geology, geophysics, geochemistry and environmental isotopes are used and the characteristics and genesis of the geothermal reservoirs in the Tongluoshan anticline are discussed. The comparative studies of regional geology, geological structure and geothermal wells suggests that (1) the regional geothermal gradients, water temperatures, and hydrochemical type are 18~20℃/km, 26~42℃ and SO₄-Ca·Mg type, suggesting a neutral and low-temperature thermal mineral water. (2) The thermal groundwater is of meteoric origin, and the recharge area is located in the Daba Mountain area to the north with an elevation of more than 1 100 m, indicating a deep groundwater circulation and a long flow path indicated by oxygen and hydrogen isotopes. (3) Results of the audio frequency magnetotelluric method and measured temperature curve of a geothermal well drilled to a depth of 2 503 m suggest a water conducted zone at depth of 800~1 100 m, since there is an area with low resistance belt and remarkable inflexion. Based on the results of geothermal drilling, the audio frequency magnetotelluric method, hydrogeochemistry and environmental isotopes, the genetic model of thermal groundwater in the Tongluoshan anticline are summarized. Groundwater receives recharge from precipitation in valley areas of the high-ridge anticline, flows from north to south through dissolved-fissure, karst conduit, and structural channels. Thermal groundwater receives heat from heat flow and discharges naturally in the deep section of the canyon and/or tapped artificially by geothermal wells. This study may provide a scientific basis for geothermal exploration, development and utilization for the local government in the city of Guang'an.