Geothermal characteristics and formation mechanism of the Medi River in Bijie City, Guizhou Province

To study the formation mechanism and hydration characteristics of geothermal resources and provide a reference for the comprehensive utilization and development of resources. Collect the water samples from Midi River in Bijie City, Guizhou Province, analyze the hydrochemical characteristics, and adopt the Piper trilinear diagram, geothermal and landmark method and audio magnetotelluric method to study the geothermal hydration and formation mechanism of Midi River. The research indicates: affected by Pingzhai dome structure, the internal joint structure in the dome is very developed in this region, which is conducive to the migration of deep thermal water to the relatively low area in the dome, forming the convective underground thermal water resources of the fold dome structure.Through regional fault detection, it is inferred that there are five shattered fault zones (or fissures) in this region, among which F4 and F13 faults have steep occurrence and shallow downward extension; F9 is more pronounced and tends to be northwest; F1 has a nearly northsouth trend, tends to northwest, and extends deeper; F17 and F18 are more obvious, but the extension is shallow. It is inferred that the F1 fault has good thermal control lability and is a favorable position for underground thermal water upwelling. Heating of the surrounding rock is obtained within 2 500 m depth of runoff after receiving infiltration recharge water from atmospheric precipitation in the rim of Pingzhai dome, and a deep pressure-bearing thermal reservoir is formed in the dolomite of Dengying Formation of Sinian System under the action of overlying Cambrian clastic rock water-resisting and heat-preservation overburden. The analysis of hydrochemical characteristics shows that geothermal water is low-salinity water. Na+ is the main positive ion in geothermal well water, accounting for approximately 45%, followed by Ca2+ and Mg2+, accounting for approximately 49%. HCO3- accounts for the main negative ion, SO42- accounts for 39%, and Cl- accounts for almost no, so the chemical type of geothermal water is HCO3-·SO42--Na·Ca; the geothermal reservoir temperature of the geothermal well is 53.98℃, the geothermal gradient is 2.85℃/100 m, and the circulation depth of the geothermal water is approximately 2 500 m.The research results have a good guiding significance for the exploitation and utilization of geothermal resources in Bijie City, Guizhou Province.