Geologic framework of the Fang Hot Springs area with emphasis on structure, hydrology, and geothermal development, Chiang Mai Province, northern Thailand

Abstract

Geologic mapping, a magnetotelluric survey, well data, and earlier reports are integrated to guide further development of the Fang geothermal system. The Fang Hot Springs originally flowed ~ 20 l s −1 of 90-99 °C water from a 10-hectare area of crystalline rocks presumed to be of Triassic age. Four wells 92-500 m deep now flow ~ 20 l s −1 of 110-115 °C water and generate 115-250 kWe from the 1989 Ormat binary power plant. Wells are not pumped nor is the spent water re-injected. Temperatures of 130 °C occur in some wells and water chemistry indicates reservoir temperatures of 150 °C. The springs now flow ~ 10 l s −1. The Fang geothermal area is at the west end of the active left-lateral strike-slip Mae Chan fault (MCF). MCF transitions to extensional faulting along the western boundary of the Cenozoic Fang basin. The hot waters emanate from crystalline rocks 0.7 km north of the MCF. Permeable fractures may be tensile fractures at the right-stepping fault tip. The less permeable MCF fault core and Cenozoic sediments of the Fang basin to the SW are not considered to be drilling targets. Unrelated to the fracture system is the Doi Kia detachment fault which places Paleozoic sediments over crystalline rock with a low-angle contact. Electrical resistivity surveys detect low resistivity (< 60 Ωm) only within the upper 50-100 m of the hot springs area. Deeper crystalline rock is > 100 Ωm. Low resistivity is caused mostly by conductive minerals of hydrothermal alteration, and not by the geothermal water of resistivity 5.6 Ωm. No deep resistivity anomaly is detected beneath the seeps or producing wells, although resolution of past surveys would not have imaged narrow zones of alteration. High-resolution resistivity surveys focused on detecting the deeper fracture system are recommended over the hot well area and south over the area underlain by crystalline rocks. Future development should focus on drilling wells (≤ 500 m) with diameters large enough to install submersible pumps to increase flows. Development of several MWe may be possible and should include a designed re-injection well system to sustain pump levels.

Structural interpretation of Mae Suai Basin, Chiang Rai Province, based on gravity data analysis and modelling

“…16; Morley, 2007). Further north the similarly oriented ENE-WNW Mae Chan Fault is a sub-vertical sinistral strike-slip fault (Wood et al., 2018).

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Geothermal Energy for Southern Thailand: Opportunities and Realities

“…For the Fang geothermal power plant cited there, for example, fluid inlet temperatures are 110-115°C, with some wells reaching 130°C. The associated hot springs reach exit temperatures of 90-99°C, [39], in other areas even beyond.…”

Geochemical Modeling of Scale Formation due to Cooling and CO2-degassing in San Kamphaeng Geothermal Field, Northern Thailand

“…The choices of geothermal application, either directly or through electricity generation, depend on a number of factors such as geochemical characteristics of fluids, surface temperature, seepage rate, reservoir size and geologic structures etc. Most geothermal studies in Thailand have focused on the classification of hot spring potential based on their chemical characteristics, enthalpies and flow rates (Ramingwong et al, 1979;Ratanasthien et al, 1988;Raksaskulwong, 2003;Singharajwarapan et al, 2012;Wood et al, 2018). Although most geothermal sites in Thailand have relatively low potential due to either low temperature or flow rate or both, some high potential sites, such as Fang hot springs, have been developed for electricity generation (e.g., Wood et al, 2018).…”

“…Most geothermal studies in Thailand have focused on the classification of hot spring potential based on their chemical characteristics, enthalpies and flow rates (Ramingwong et al, 1979;Ratanasthien et al, 1988;Raksaskulwong, 2003;Singharajwarapan et al, 2012;Wood et al, 2018). Although most geothermal sites in Thailand have relatively low potential due to either low temperature or flow rate or both, some high potential sites, such as Fang hot springs, have been developed for electricity generation (e.g., Wood et al, 2018). San Kamphaeng (SK) hot springs, on the other hand, have relatively low surface temperature (~40-50 °C) but, due to its high seepage rate (~70-80 L/s), it has been famously used for recreational purposes.…”