Combining geophysical, geochemical and geological studies toward plausible geothermal models at the Hongchailin site for geothermal power generation in northeastern Taiwan

<p>In this study, we conduct a multi-disciplinary study, including geophysics, geochemistry, and geology, in an attempt to reconstruct geothermal geological model(s) at the shallow 3 km, for a project of potential geothermal power generation at the Hongchailin site in the Ilan plain of northeastern Taiwan. Our geophysical techniques include seismic imaging from natural earthquakes as well as ambient noise. Three seismic arrays deployed at different time periods in the past decade were used. We also incorporate geophysical imaging results from previous studies, in particular a series of seismic reflection profiles. For the magnetotelluric (MT) technique, two major surveys have been conducted, including 2015-2018 AMT by National Central University and 2021-2022 MT by GERD and Academia Sinica.&#160; Three test holes were drilled around the Hongchailin site in 2016-2019. Logging and on-site measurements were conducted at increment depths, including stratigraphy and rock types, P/T measurements, fractures analyses, geochemical analyses (e.g., hydrogen/oxygen/helium isotope, fluid inclusion, etc.), and so on.</p><p>&#160;&#160;&#160;&#160;&#160;&#160;&#160; Based on regional geology and structures, and incorporating geophysical subsurface imaging, we reconstruct geothermal geological models in line with detailed geological cross sections at the shallow 3 km level. We interpret that there exits a shallow geothermal reservoir within the massive quartz sandstone layers (Szeleng sandstone) at 1-2 km depth with the downhole temperature of 80-100 C.&#160; Geologically, the reservoir is located at the regional Songlo syncline and its south limb; and geophysically, it corresponds to a relatively low resistivity area. Isotope results show that the cool meteoric water came from nearby higher altitude mountain area, then flew through the Szeleng sandstone, which plunges 1-2 km depth below the Ilan plain. Hot fluid is interpreted to be derived from deeper heat source and to be upflowed along a N-S trending vertical faults system near the Hongchailin area.&#160; In addition, two E-W trending major faults, identified by the seismic reflection profiles, seem to be acted as hydrothermal fluid barriers to confine the hot fluid within the reservoir area.</p>