Impact of Clay Mineral Formation in High-Temperature Geothermal System on Accuracy of Na-K and K-Mg Geothermometers

Traditional geochemical geothermometers have limitations when used to estimate the temperature of shallow reservoirs(100-200 ℃)in high-temperature geothermal systems, and it remains unclear about the cause of the error due to the widely used geothermometers of Na-K and K-Mg. In this study, the hydrogeochemical data of 201 water samples from geothermal wells in the typical hydrothermal areas across the world were collected, based on which the corresponding geochemical compositions of reservoir fluids are calculated using the code WATCH. An evaluation of Na-K and K-Mg geothermometers was further made. The results show that K-feldspar and common dioctahedral potassium-rich clay minerals are likely to control the content of potassium in geothermal fluid from shallow reservoirs and dioctahedral magnesium-rich clay minerals can be equilibrated with geothermal fluid as well, whereas the content of sodium in geothermal fluid is little affected by water-rock interactions. Hence, the Na-K ratio of shallow geothermal fluid doesn't match well with reservoir temperature. K-Mg geothermometer is capable of indicating shallow reservoir temperature to some degree, but still not accurate enough.