Formation Model of Granite Geothermal System in the Fujian, Guangdong and Hainan Provinces in Southeast China
Authors: Feng Jianyun , Zhang Ying , Zheng Herong , Luo Jun , Zeng Yan , Yun Xiaorui 
Published: August 2024

ABSTRACT:
Granitic plutons are widespread distributed in Fujian, Guangdong and Hainan Provinces that comprise granite batholith of various rock gradients and multi-phase intrusions formed in a complicated geological setting of geodynamics. It's widely accepted that granite is an ideal heat source and conductor for its highly radioactive and high-thermal conductivity, which facilitates migration and enrichment of deep-seated intermediate-high temperature geothermal system, as long as reservoir and cap-rock develops well. Therefore, how to predict granitic fracture reservoir with a certain scale is most crucial for screening deep-seated geothermal resource in a thermal-abnormal area under a high-value of thermal background. In order to make clear migration and enrichment of geothermal resource controlled by distribution of granite in the study area, and select favorable play for deep-seated geothermal resource, based on analysis of rock and stratigraphic structure, it is concluded that the sedimentary basin-type geothermal system suitable for large-scale geothermal resource development is mainly controlled by the thickness of cap rock and the difference of thermal conductivity between cap rock and reservoir. Based on the above classification, the convective mesothermal geothermal system in the thin cover area of Zhangzhou in southeast Fujian, the convective mesothermal geothermal system in the middle cover area of Yuezhong Depression and the conduction high temperature geothermal system in the thick cover area of Fushan in North Qiongshan are successively identified in the study area.

1. INTRUDUCTION
The Fujian Guangdong Qiong region, located in the southeastern coastal area of the Cathaysia Block, has developed different types of igneous rocks in different periods due to multiple tectonic events such as the closure of the Tethys Ocean, westward subduction of the Pacific Plate, and expansion of the South China Sea. It has become the most active zone of tectonic magmatic activity in the South China Plate since the Mesozoic and Cenozoic eras and belongs to the southeastern coastal thermal anomaly area, The most direct manifestation is the widespread development of granite and moderately acidic volcanic rocks, characterized by multiple rock types and multiple episodes of episodic production. Their formation process and geodynamic background are complex and intricate (Zhu, et al., 2019; Shu et al., 2020; Zhang et al., 2020; Wang et al., 2021; Zhang, et al., 2021; Chen et al., 2022; Luo et al., 2022; Yan et al., 2022a, 2022b). Due to the high radioactive heat generation and good thermal conductivity of granite itself, a deep medium to high temperature geothermal system can be formed under the premise of the development of high-quality granite fracture type geothermal reservoirs and suitable cover layer insulation, and previous studies on geothermal reservoir are mostly focused on hydrothermal reservoirs of sandstone and carbonate rocks, or few granite reservoirs without caprocks(Wollenberg, et al., 1987; Shi, et al., 1990; Zhao, et al., 1995; Wang, et al., 2014; Qiu, et al., 2014). Considering origin and reservoir of geothermal system, the paper uniquely studied on heat-generation, migration and accumulation of the whole geothermal system, and analysis the system in multi-views of geology, engineering and economy different with other studies. This study points out the direction for the exploration and development of deep geothermal resources in the southeast coastal area of China. If in-depth research is carried out, the large-scale beneficial development of deep geothermal resources can be realized in the future, especially the breakthrough of hot dry rock resources, which is conducive to the further promotion and improvement of EGS technology.