Estimates of reservoir temperatures for non-magmatic convective geothermal systems: Insights from the Ranwu and Rekeng geothermal fields, western Sichuan Province, China

Abstract

The Ranwu and Rekeng geothermal fields are non-magmatic convective geothermal systems located in the western part of the Sichuan Province. The Ranwu field is a mid-low temperature system with relatively weak geothermal manifestations, whiles the Rekeng system is the high-temperature type with strong manifestations. In this study, we first investigate the flow processes of the thermal fluids in Ranwu by analyzing the hydrogeochemistry of different thermal springs. Next, we ascertain the existence of shallow or deep reservoirs in the two thermal fields and estimate the reservoir temperatures by analyzing the water–rock interactions. Our findings confirm that the Ranwu geothermal field is a mid-low temperature convective system without magma degassing effects. Only a reservoir with a temperature of 116 ℃ formed beneath Ranwu without any relatively shallow reservoirs. Furthermore, we discourage using Na-K geothermometer for non-magmatic convective geothermal systems because it is difficult to achieve Na-K equilibrium between the fluids and the reservoirs rocks at such relatively low temperatures. Also, the Na-K-Ca geothermometer with β = 1/3 fails because of the Na-K imbalance in the two geothermal fields. However, Mg-correction improves its accuracy for Mg-rich thermal waters in Ranwu and Rekeng probably because the Mg-K-Ca system correlates better with the reservoir temperatures than the Na-K-Ca system. Additionally, chalcedony and K-Mg geothermometers are valid for estimating the reservoir temperatures of such geothermal systems. Comparable estimates of the two geothermometers could be used to establish the existence of shallow or deep reservoirs.