Formation of the hydrothermal system from granite reservoir for power generation in igneous rock areas of South China

Abstract

Hydrothermal systems occurring in granite reservoir widely exist over the world, but those can continuously supply large quantities of hot water with certain temperature for power generation are rare, e.g., TangKeng geothermal system in South China. It is unclear how the granite reservoir keeps this kind of large natural yield of geothermal water with stable temperature and the genesis study is of great importance to find similar geothermal fields for further exploitation. This paper aims to discuss the formation of TangKeng geothermal system in granite of Cretaceous period, with emphasis on heat sources, geothermal fluid circulation and reservoir temperature. Results show that average regional heat flow of 72.2 mW/m2 is composed by almost equal contribution from heat production of radioactive elements in crust and conductive heat of mantle. Extension and thinning processes of lithosphere and upwelling of magma in the asthenosphere caused the thermal erosion of the upper mantle and lower crust but doesn't induce the formation of magma chamber or partial molten in the upper crust and had very limited influences on the regional thermal state. Heat flow of the crust is mainly composed by intrusive granite pluton with high heat production value in the surface and the felsic upper crust. Geothermal fluid is recharged by the local precipitation from the surrounding mountains through regional developed NE and NW faults system. The crushed fine-middle grained biotite granite or granite with fractures and fissures of Mesozoic period provide excellent reservoir for water circulation. Interactions between regional faults and fractures in granites provide excellent conduits for additional recharge from shallow groundwater which is continuously recharged by the Rong Jiang River water during the ascending process. Reservoir temperature is calculated to be about 140–143 °C with correction of slightly CO2 degassing and mixing. Missing of caprock for most of the areas and mixing with cold groundwater during ascending process led to the occurrence of geothermal water with lower temperature of 92 °C collected at the surface.Fractured granite reservoir with high heat productivity from radioactive elements, well developed NE and NW oriented faults system and continuous recharge from local precipitation and surface water nearby are essential for the formation of this kind of hydrothermal system.