Constraining Geothermal Reservoir Geometry and Development Potential Using Integrated Well-Logging Data: A Case Study from the Chulai Area, Taitung, Taiwan

The Chulai area is situated within the tectonically active longitudinal valley zone, located between the Central Mountain Range and the Coastal Mountain Range in eastern Taiwan. The terrain is steep and undulating, with frequent geological structures and seismic activity. Influenced by multiple active faults and long-term orogenic processes, the subsurface geologic structure is dominated by metamorphic rocks, with widespread development of schist, slate, and related metamorphic rocks, indicating intense tectonic compression and metamorphism. To better constrain the geometry and characteristics of the geothermal reservoir system in this structurally complex setting, this study integrates comprehensive well-logging datasets acquired by Schlumberger from a geothermal exploration well drilled to a depth of 1850 m. The logging program includes gamma ray, spontaneous potential, multi-array resistivity, caliper, deviation, FMI resistivity imaging, dipole sonic, temperature, pressure, and fast-shear azimuth measurements. The well-logging results are synthesized with existing geological, stratigraphic, and active-fault surveys conducted by the Geological Survey and Mining Management Agency (GSMMA) to delineate lithologic boundaries, fracture zones, and potential fluid-flow pathways. This information, integrated with geophysical logging data, is then incorporated into a three-dimensional geothermal reservoir model developed using PetraSim, allowing for the simulation of subsurface temperature distribution under geological conditions characteristic of the Chulai region. Model outputs accurately reproduce the observed downhole temperature gradient, demonstrating the reliability of well–logging–constrained reservoir geometry and supporting the inference that the region exhibits a high geothermal gradient and favorable reservoir properties. These findings confirm that integrated well-logging data provide essential information for accurately characterizing subsurface structures, evaluating geothermal reservoir potential, and guiding future development strategies in one of Taiwan’s most promising geothermal prospects.