Geological, Hydrogeological and Geophysical Survey of Xumai Geothermal Field in Tibet, China

Abstract

Xumai geothermal field locates in the southern of Naqu-Nimu geothermal belt, which is the most important geothermal belt in Tibet, even in the whole country. There are lots geothermal fields in the belt, including the famous Yangbajing geothermal field. In order to evaluate the geothermal resources for further exploration, geological, hydrogeological and geophysical survey were carried out in the Xumai geothermal field. The thermal activities are still present on the surface with the highest temperature of 76°C. The geological works show that four group fractures exist in this area, and the strikes are SN, EW, NE and NW. Among them, the SN-striking fracture F1 is considered to control the thermal activities due to the fact that most hot spring spots occur along this fracture The hydrogeological work shows that the hydrochemical type is HCO3-Na in the northern wihle SO4·Cl·HCO3-Na in the southern of the Xumai geothermal field. The source of the hot spring water is proved to be atmospheric precipitation by the hydrogen and oxygen isotopes. The highest temperature of the geothermal reservoir is 164.4°C, which is calculated by the SiO2 geothermometer. Geophysics works, including magnetotelluric (MT), Audiomagnetotelluric (AMT) and gravity surveys were also carried out in this area. The fracture F1 and its secondary fractures can be identified clearly by banded low resistivity anomaly. Based on the above works, we can conclude that: 1) the reservoir is zonal, controlling by the F1 and its secondary fractures. 2) The surrounding rocks of the reservoir are granite and granodiorite, and no obvious cap was recognized. 3) The Xumai geothermal field is a good candidate for the exploitation of geothermal resource, and drilling works are suggested to validate deep geothermal resources.