Hot Dry Rocks and Their Geological Significance from Gonghe Basin at Northeast Margin of Qinghai-Tibet Plateau
Jianyun FENG, Ying ZHANG, Jun LUO
[PEPRIS, SINOPEC, China]
Gonghe Basin in northeast margin of Qinghai-Tibet Plateau is a faulted basin formed since Mesozoic era, around which is controlled by large active dextral-slide faults, for instance, the Wahongshan fault in NNW direction to the west, and the Waligongshan and the Duohemao faults in NNW direction to the east. Based on regional geology, geothermal geology and integrated geophysical exploration results, the GR1 hot dry rock (HDR) exploration well was completed in the central part of Qiabuqia Town in Gonghe basin. The GR1 well is the highest temperature HDR exploration well in China, which has laid an important foundation for the first EGS demonstration project of China. The temperature measurement results show that the temperature at the depth of 2500m is 150°C, entering into the hot dry rock section. The temperature of the bottom hole at the depth of 3602.6m is 188.87°C. The average geothermal gradient of 2500-3705m is 71.4°C/km, which is higher than that of the other 3 hot dry rock exploration wells. At 2800-3705m of GR1 well the geothermal gradient is higher than 80°C/km. The types of rocks related with the hot dry rocks are complicated and various, including volcanic and intrusive rocks. Volcanic rocks consist of basalt, andesite, dacite, rhyolite and volcanic breccia; intrusive rocks consist of gabbro, diorite, granodiorite, granite and quartz porphyry, etc. U-Pb isotopic chronology studies reveal that these igneous rocks are mostly crystallized in Anisian period of Middle Triassic (247-242 Ma). And the granitic plutons are considered to be derived from the same magma chamber and intruded into the upper crust contemporaneously. Geochemistry characteristics and rock assemblages suggest that the Middle Triassic igneous rocks are formed in continental margin arc setting by the northern subduction of Paleo-Tethys ocean. The early preexisting horizontal fractures in GR1 indicate that the granite are naturally fractured and are propitious to form a connected fracture network within 3200 m-3705 m depth, which are conducive to the formation of complex heat exchange paths during reservoir stimulation. Consequently, thus far the Qiabuqia site is believed suitable for establishing an EGS power generation system in China.
|        Topic: Geology||Paper Number: 12025|