Hydro-Geochemistry of Anning Geothermal Field and Flow Channels Inferring of Upper Geothermal Reservoir

Carbonatite geothermal reservoir with freshwater is widespread in Southwest China. Anning low temperature geothermal field, which has a dual vertical structure, is approximately 40 km adjacent to the west of Kunming geothermal field. For exploring formation of geothermal field and discovering flow channel through which deeper thermal water flows upwards to the upper reservoir under a poor-drilling data condition, three methods are employed in this paper. (1) Hydro-geochemical characteristics of the deeper reservoir (Dengying Formation) is determined with an analogy to the similar reservoir (also Dengying Formation) of Kunming geothermal field. (2) Sodium is recognized as a possible indicator of flow channel to the upper reservoir by elaborating the difference and connection between upper reservoir (Qixia-Maokou Formation) and deeper reservoir in consideration of water-rock interaction, hydrochemistry type and correlation of main ions. (3) An inverse numerical model, including exploitation quantity and measured thermal water temperature, is used to verify the rationality of channel location. Thermal water in upper reservoir is mixture, whose ion concentration is distinctive from the deeper thermal water, and its hydrochemistry type (HCO₃-Ca·Mg) is controlled by cold water around obviously. There are slight differences of hydrochemical characteristics among geothermal producing wells, except for sodium distribution, which is corresponding with the distribution of water temperature and exploitation status. The locations of flow channel obtained through hydrochemical data are well matched with numerical simulation results, which suggests that the inferring of flow channels through synthetical analyses mainly based upon the distribution of sodium may be feasible.