Geothermal explorations of Paolai, Taiwan: integrated results from geological, geophysical, and geochemical surveys

Taiwan, one of the most active orogens in the world, sits on the convergent plate boundary between the Luzon Arc of the Philippine Sea Plate and the continental margin of the Eurasian Plate. The Philippine Sea Plate continuously moves northwest and leads to the intense processes of mountain building. Consequently, the island presents a variety of tough landforms and bears high heat flows with potential for geothermal energy. The Single Service Window for Taiwan Geothermal Power estimated total geothermal potential around the island of Taiwan to be about 33.64 GW. The study area, Paolai Hot Spring, is known as one of the potential geothermal sites in Taiwan. The village situates upon the Laonung River, which drains across the low degree metamorphic strata, and is bounded by the Chaozhou Fault to the west. The Chaozhou Fault is an east-dipping thrust fault that brought up the low degree metamorphic argillite-to-slate strata over the sedimentary formations. To the east of the Chaozhou Fault, some researchers also suggested the occurrence of the Meilunshan Fault as orientation of strata and foliation appear to be different across the suspected fault. However, fault displacement is not evidenced in the field and the extent of the fault is still ambiguous. In this study, we conducted extensive geological, geophysical (including Magneto-Telluric, seismic, and gravity surveys), and geochemical surveys in this region to discuss the geothermal features across the Chaozhou Fault and the Meilungshan Fault, in terms of fluid, heat, fractures and the reservoir.

Two hydrological circulation models are justified here: 1) the single fault (the Chaozhou Fault solely) model and 2) the dual fault model. In the single fault model, the Chaozhou Fault serves as the controlling boundary with groundwater replenished from the eastern Backbone Range area. The deeply circulated fluids are heated and circulating along the highly ruptured, permeable zone within the Chaozhou Fault system. In the dual fault model, the Chaozhou and Meilunshan Faults divide the hydrological circulation into two systems. To the east of the Meilunshan Fault, groundwater is replenished from the Backbone Range area, circulating downward, heated, and migrates to the surface along the open fissures on the hanging wall of the Meilunshan Fault. To the west of the Meilunshan Fault, the circulation would be restricted by the two faults. Our field results concur occurrence of the Meilunshan Fault while the geophysical and geochemical data conformably suggest distinct characteristics of water and migration of fluids across the fault. Overall, our data show complex characteristics of multiple domains bounded by the two faults and suggest that the dual-fault model is more appropriate for further evaluations of geothermal potential. In addition, fractures are identified on the hanging wall of the Meilungshan Fault, which may serve as the near surface conduits of fluids. These areas hence have potential for shallow geothermal energy development because they both show evidence of deep reservoirs and shallow fractures. Test drilling and further investigation towards the complex structures in this region should be thoroughly considered in the future.