Formation mechanism of a carbonate geothermal system and calculation of the reservoir temperature under complex conditions

• The source of geothermal water in Tangshan area is atmospheric precipitation, and the heat source is terrestrial heat flow. Faults are the main channels for the flow of geothermal water. • The reservoir temperature in the northeast of Tangshan is close to 90 °C, while the temperature in the west is only 50 °C. The reservoir temperature has obvious zoning characteristics. • Rainfall flows down from the Tangshan mountain to a depth of about 2.6 km. Then, groundwater transports heat through faults to the east and west sides of Tangshan, forming geothermal resources. Carbonate geothermal systems are widely distributed and have huge mining potential. They have complex and diverse mechanisms for their genesis and strong regional characteristics. Therefore, the detailed description of the mechanisms for the genesis of carbonate geothermal systems is not only an important scientific issue but also a practical issue for the scientific and sustainable exploitation of geothermal resources. Based on the analysis of hydrochemical characteristics and isotopic characteristics of different types of water and hydrogeological characteristics of the study area, the geothermal water source, temperature distribution and water flow channels are described in detail. The accuracy of estimates of temperature has been increased and the geological model of the geothermal system has been well determined. The results show that the Tangshan area is mainly composed of a mid to low temperature hydrothermal carbonate convective geothermal system. Geothermal water is replenished from atmospheric precipitation, flow down and then, under the influence of terrestrial heat flow, it moves upward along multiple sets of structural channels and forms geothermal resources in the groundwater drainage belts on the eastern and western sides of Tangshan. The zoning characteristics of the heat storage temperature are shown by the high temperature area where the reservoir temperature is approximately 84 °C and the low temperature area where it is only 40 °C. This research provides some evidence for determining the conditions controlling the formation of carbonate geothermal systems and provides a theoretical basis for the scientific and sustainable exploitation of geothermal resources in the future.