Borehole temperature logging and water dynamic system in the Xianyang geothermal field, Northwest China
Rao Song1,2, Li Weiwei2, Jiang Guangzheng2, Tang Xiaoyin2, Luo Lu2, Wang Jiangcun3
1. Key Laboratory of Exploration Technologies for Oil and Gas Resources, Ministry of Education, Yangtze University, Wuhan 430100;
2. Institute of Geology and Geophysics, Chinese Academy of Science, Beijing 100029;
3. SINOPEC Star Petroleum Co., Ltd., Beijing 100083
Research of temperature pattern of geothermal field, is not only the important basis for the classification and source mechanism study, but also provides direct evidence for the identification of advantageous thermal field area and depth. This paper reports the systemic (quasi-) steady temperature data of 13 boreholes in Xianyang geothermal field. Then, the vertical temperature distribution characteristics and the fluid dynamic system have been analyzed. The results show that the Xianyang geothermal field is a conduction-dominated geothermal field in sedimentary basins, with the geothermal gradient between 26.2~40.1 ℃/km, and 32.4 ℃/km for average. But compared with the typical conductive geothermal field, the Xianyang geothermal field shows the special properties on the borehole temperature-depth curve. On the whole, there are four segments on the borehole temperature-depth curve. The shallow region of the temperature-depth curve exhibits a marked serration which is clearly related to the activity of surface water. The upper region is characterized by linear temperature-depth curve and conductive geothermal pattern by the lack of infiltration of shallow surface water and deep hydrothermal upwelling. The concave temperature-depth curve in the lower region reveals that the infiltration and lateral flow of the ground water along the Weihe faul has reduced formation temperature along with the water supplement to the geothermal field. At the bottom of the boreholes, the temperature anomaly increases with depth, which indicates the presence of fluid compartment with abnormal pressure. The high resolution temperature data reveal that the Xianyang geothermal field has multilayer structure of fluid dynamic system in vertical, that is vertical gravity-driven flow in the shallow region, normal compaction-driven flow in the upper region, lateral gravity-driven in the lower part and fluid compartment at the borehole bottom. Therefore, planning and exploitation separately according to the fluid dynamic system layers of Xianyang geothermal field is recommended.