Exploring the shallow geothermal fluid reservoir of Fang geothermal system, Thailand via a 3-D magnetotelluric survey.

After early exploration during the 1980s and 1990s, the 0.3 MW Fang geothermal power plant was built as a demonstration to supply electricity to the local community. The shallow well (100 m) drilling program produced about 22 l/s of 125 °C water, and two wells to 500 m produced about 10 l/s. Due to the lack of detailed information on the geothermal system, the plan to expand to a larger power plant was halted to avoid the drilling missing the hot fluid. The plan was resumed in the last ten years starting with the magnetotelluric (MT) survey. Thirty three MT sites were deployed on the southern part of the Fang geothermal area. A remote site was installed about 600 km south of the study area for better data quality. After data processing, the data was inverted with WSINV3DMT to produce the 3-D resistivity model which clearly matches the near-surface geology and is also in agreement with the conceptual geology of the Fang geothermal system. The high resistivity zone is interpreted as the crystalline granitic rock, while the intermediate resistivity zone is associated with the Fang sedimentary basin. The resistivity contrast between the higher and lower resistivities helps reveal the orientations of the major Mae Chan Fault (MCF) and the two local faults of the area. The two main conductors of interests, C1 and C2, are directly linked to the hot fluid found at the surface. C1 is shallow (<50 m), and found only beneath the Fang hot spring, and so it is interpreted as the fracture reservoir. C2, which was not discovered in previous studies, extends from near the surface to a depth of 500 m, and at a depth of 200 m, it is about 1 km wide. It is about 1 km south of the Fang hot spring where the warm water was found to have seeped to the surface through the MCF. Two possible interpretations are proposed for the C2 conductor. The first is that there is an impermeable clay zone trapping a relatively high resistivity geothermal fluid reservoir beneath, like the caprock of a magmatic geothermal play type. This would require a deeper well to extract the hot fluid. As with the C1 conductor, the other interpretation is that the C2 conductor is a fracture geothermal reservoir where hot fluid from the deep resides within the pores of the sedimentary rock and fractures of the altered granite. This would require a shallower well. Both interpretations suggest that the C2 anomaly is of value. Since it has never been explored, a drilling over the C2 anomaly is recommended to probe its characteristic and also to extract more hot fluid for the future expansion of the geothermal power plant. [ABSTRACT FROM AUTHOR]