17:15 〜 19:15
[U05-P02] Geothermal potential in Wulu metamorphic mountainous area, Eastern Taiwan
キーワード：geothermal exploration, metamorphic region, eastern Taiwan, slimhole drilling, Wulu area, magneto-telluric survey
In order to achieve the net zero transition goals, the Taiwanese government plan to achieve the geothermal power generation capacity to 20 MW by 2025 and 1 GW by 2030. Throughout the entire Taiwan island, 10 geothermal anomalies has been identified and estimated the geothermal potential to be 30-40 GW, of which conventional geothermal energy (<3 km) is less than 1 GW, and the rest is deep unconventional geothermal energy (3~6 km) (TIGC, 2023). Feasible conventional geothermal power generation capacity is <400 MW, while deep unconventional geothermal power generation capacity is <8 GW.
Wulu area is located in the metamorphic mountainous area of eastern Taiwan and is one of the 10 geothermal anomalies. Due to the scarcity of geothermal geological data, we systematically conducted the surface outcrop, geophysical and geochemical surveys and deep slimhole drilling, downhole testing, fluid sampling and analysis. The survey results are expected to provide the basic and reliable information for geothermal resource assessment and to reduce the resource risk in exploration phase. Fracture zone appears as the major fluid pathway of the geothermal system in the metamorphic region and therefore shows low resistivity characteristics. Two significant low-resistance anomaly zones, Motian and Wulu, were identified from the three-dimensional resistivity model in our survey region. According to the distribution of low-resistance anomaly zones, three slimholes with depth of 1000.4 m (WL01A), 2004 m (WL02) and 1900 m (WL03), respectively were completed in the Wulu area by using core drilling method to confirm the geothermal gradient and geological conceptual model.
The lithology of drilling cores is mainly schist, slate and meta-sandstone with abundant quartz vein and calcite vein. In addition, at the depth of 637 m in borehole WL01A, we drilled granite that has never been discovered before, and this granitic body was not penetrated until the bottom of the borehole (1000.4 m). The highest temperature in the slimhole is: 121 °C at a depth of 650-750 m of WL01A, 140 °C at the bottom of WL02 (2004 m) and estimated 140 °C at the bottom of WL03 (1900 m). The geothermal gradient of WL02 and WL03 is estimated to be about 60 °C/km, and the temperature continues to rise until the planned drilling depth, which implies the active orogeny creates high geothermal gradient by exhumation of the warmer material from depth and Wulu area might have the potential for unconventional geothermal development, such as EGS or deep closed-loops.
Combining the magnetotelluric, airborne magnetic and three slimholes drilling data, a large granitic rock body underneath the survey region is recognized. However, the intact granitic rock body has high magnetic and resistivity characteristics; in contrast, the area affected by hot water shows low resistivity, low magnetic susceptibility, and magnetic reversal. It inferred from the downhole temperature measurements that the igneous rock body seems not a heat source, and the fracture zone controls the flow of hot water. According to the geochemical characteristics of hot water collected in Wulu area, meteoric water heated by the deep circulation is considered the dominant source of geothermal fluids.
Wulu area is located in the metamorphic mountainous area of eastern Taiwan and is one of the 10 geothermal anomalies. Due to the scarcity of geothermal geological data, we systematically conducted the surface outcrop, geophysical and geochemical surveys and deep slimhole drilling, downhole testing, fluid sampling and analysis. The survey results are expected to provide the basic and reliable information for geothermal resource assessment and to reduce the resource risk in exploration phase. Fracture zone appears as the major fluid pathway of the geothermal system in the metamorphic region and therefore shows low resistivity characteristics. Two significant low-resistance anomaly zones, Motian and Wulu, were identified from the three-dimensional resistivity model in our survey region. According to the distribution of low-resistance anomaly zones, three slimholes with depth of 1000.4 m (WL01A), 2004 m (WL02) and 1900 m (WL03), respectively were completed in the Wulu area by using core drilling method to confirm the geothermal gradient and geological conceptual model.
The lithology of drilling cores is mainly schist, slate and meta-sandstone with abundant quartz vein and calcite vein. In addition, at the depth of 637 m in borehole WL01A, we drilled granite that has never been discovered before, and this granitic body was not penetrated until the bottom of the borehole (1000.4 m). The highest temperature in the slimhole is: 121 °C at a depth of 650-750 m of WL01A, 140 °C at the bottom of WL02 (2004 m) and estimated 140 °C at the bottom of WL03 (1900 m). The geothermal gradient of WL02 and WL03 is estimated to be about 60 °C/km, and the temperature continues to rise until the planned drilling depth, which implies the active orogeny creates high geothermal gradient by exhumation of the warmer material from depth and Wulu area might have the potential for unconventional geothermal development, such as EGS or deep closed-loops.
Combining the magnetotelluric, airborne magnetic and three slimholes drilling data, a large granitic rock body underneath the survey region is recognized. However, the intact granitic rock body has high magnetic and resistivity characteristics; in contrast, the area affected by hot water shows low resistivity, low magnetic susceptibility, and magnetic reversal. It inferred from the downhole temperature measurements that the igneous rock body seems not a heat source, and the fracture zone controls the flow of hot water. According to the geochemical characteristics of hot water collected in Wulu area, meteoric water heated by the deep circulation is considered the dominant source of geothermal fluids.