Evaluating the Chingshui geothermal reservoir in northeast Taiwan with a 3D integrated geophysical visualization model

In the current study, we assess the Chingshui geothermal reservoir with a three-dimensional visualization model that integrates geophysical measurements with well logs. To re-evaluate the geothermal reservoir quantitatively, we reprocessed resistivity measurements from a series of studies conducted nearly 40 years ago, as well as from the magnetotelluric (MT) explorations performed recently in the Chingshui area. We established a three-dimensional (3D) visualization model that integrates these different geophysical survey results as well as the well-logs to better perform the spatial relationships between them. From the orthogonal bipole-bipole resistivity surveys, we have identified several regional conductive structures with resistivity of less than 50 Ohm-m representing the major fault zones of the Dahsi, Xiaonanao, and Chingshuihsi faults. Among them, the Chingshui fault is located along the Chingshuihsi River valley and is associated with hot spring features. The collinear Schlumberger survey along the Chingshuichi Valley identified three relatively conductive regions with resistivity of less than 20 Ohm-m. The MT interpretation shows that the structure associated with the geothermal reservoir extends from these near-surface fractures to a depth of −1500 m toward the south in the fault zone. The identified production zone from the core drilling records is consistent with the conductive structure in the MT inverted image. In addition, the structure seems to consist of two sub-regions: a somewhat shallow one at a depth of between −400 and −800 m in the north and a somewhat deep one at a depth of between −600 and −1500 m in the south. From the 3D model, we estimate that the volume of the Chingshui geothermal reservoir is about 9.54 × 107 m3. Given a gross porosity of 0.1 and 100% saturation for the fracture zones from the core logs, the inferred Chingshui geothermal reservoir contains about 10 million cubic meters of geothermal fluids.