GEOLOGICAL-GEOPHYSICAL CHARACTERISTICS OF ENHANCED GEOTHERMAL SYSTEMS (HOT DRY ROCKS) IN GONGHE-GUIDE BASIN
XUE Jian-Qiu1, GAN Bing1, LI Bai-xiang2, WANG Zi-lin2
1. Bureau of Environmental Geological Exploration, Xining 810000,China;
2. No.2 Institute of Geology and Mineral Exploration, Gansu Bureau of Geology and Mineral Resources, Lanzhou 730020,China
The Gonghe-Guide basin of Qinghai Province is located in a Cenozoic graben basin of the Qingling-Kunlun orogenic belt, where there are many active faults,magmatites and intense hydrothermal activities. There are 6 hot springs whose temperatures are over 60 ℃, with the hottest one reaching 93.5 ℃, which is beyond the local boiling point (92 ℃) and is hence defined as a boiling spring. The geothermal field is high in this basin. According to three geothermal drill holes that uncovered granite, the drill hole named QR1 969 m in depth exposed 70 ℃ at the bottom, the drill hole named DR1 1 455 m in depth exposed 87 ℃ in the bottom, the drill hole named R2 1 709.56 m in depth even exposed 97 ℃ at the bottom. Geothermal gradient is up to 6-7 ℃ per 100 meter, being two times of the normal gradient. The geothermal warming increases with the depth, and the authors inferred that the temperature in the depth of 3000 m is about 150 to 200 ℃. The basement depth inferred by low gravity anomaly is not consistent with the CSAMT and seismic reflection results. With corresponding magnetic anomaly and drilling evidence, the authors believe that the gravity and magnetic anomaly is caused by Yanshanian granite. As the deep drill holes intersect heat but no water, the authors hold that there may exist hot dry rock as the heat source of this basin.