Abstract:

This paper takes the Dongba Sag in the Tianzhu geothermal field in Beijing as an example, in which the thermal groundwater is of the highest temperature and the highest total dissolved solids. Data of the geothermal wells and related geological survey in the recent years were used to better examine the mechanism of the rela-tive high temperature and total dissolved solids by analyzing the geological conditions of the geothermal filed, such as faults, geothermal reservoirs, cap rocks, hydrochemistry and occurrence of the thermal groundwater. The results show that the northern section of the Taiyanggong fault, Liangxiang-Qianmen fault and Louzizhuang fault are big faults, and they deeply cut the Jixian geothermal reservoir. The Jixian geothermal reservoir in the Dongba Sag is approximately 1,000 m lower than two sides of the sag. Thus, the Dongba sag is a relatively closed area like a “black box”. This kind of geological structure leads to a very weak lateral recharge, and the south of Dongba Sag is in a closed reduction environment. The results also show that the deep heat source can flow up into the northern section of Taiyanggong fault along the fault. The high temperature geothermal fluid in the deep part of the sag is ruptured in the geothermal reservoir in the southern Dongba Sag. As a result, the geothermal fluid is of high temperature, high dissolved solids and week yield to wells. This work is of positive significance for the development of geothermal resources in our country.

Key words: geothermal field, geothermal fluid, geothermal reservoir, cap rock, geothermal gradient, Dongba Sag, Beijing