Karst Geothermal system in Taiyuan Basin of North China

The Taiyuan Basin of North China, as a typical intermountain fault basin, is located in the middle part of the Fen-Wei graben system-an extensional faulting belt of the Chinese mainland. With an area of about 1 000 km, it is one of the urban areas where the distribution of the exploitable karst geothermal reserves matches the demand of heating market in Taiyuan City. By the end of 2018, 54 geothermal drilling wells have been completed. Most of the wells have a water temperature of 61~74°C, a single well water volume of 79~150 m/ h. And the geothermal heating area has reached 3.5 million square meters. Therefore, the study on the genetic model of karst geothermal system and the distribution of karst thermal reservoir is of great significance for the overall development of geothermal resources in Taiyuan Basin. In this paper, based on the previous research results and the latest geothermal well data, the formation, distribution and hydrothermal characteristics of the karst geothermal system in Taiyuan Basin are analyzed, and its geothermal resources are evaluated by 8 effective stuctural units. The results show as flow: (1) The strata of karst thermal reservoir in Taiyuan Basin are mainly the Ordovician system of Lower Paleozoic, which is widely distributed in North China Plate. And the evolution of the karst geothermal system has gone through five stages, i.e. the epigenic karstification at the end of the early Paleozoic, the direct caprock deposition in the late Paleozoic, the initial formation of the karst geothermal system during Mesozoic, the transformation during Himalayan and the final setting during the Quaternary. (2)The heat source of the geothermal system comes from the high terrestrial heat flow (> 1.7HFU) of the Cenozoic asymmetric fault basin, and the heat transfer mode can be divided into two different types: the strong convolution type (at the edge of the basin) and the heat conduction type (inside the basin). (3)The residual thickness of Ordovician is 600~750 m, and as an geothermal reservoir, the cumulative thickness of effective reservoir section is 160~180 m. The main water-bearing section of 3~ 4 layers can be identified, where "overflowing" is easy to occur during migration. (4)Controlled by the structure and geomorphology of the intermountain fault basin, the recharge-migration mode of geothermal water in the karst geothermal system has the characteristics of two-way, near-source and rapid. The time from the recharge area to the basin pressure area is only 2000 a, and the type of hydrochemistry changes from HCO3-Ca type to HCO3-Ca•Mg type, then to SO4-Ca type with the increase of their salinity. (5) According to the method of geothermal reservoir volume evaluation, the total geothermal resources of the karst geothermal system in Taiyuan Basin are estimated to be 8.303 billion GJ, which is equivalent to million tons of standard coal. The annual exploitation of geothermal resources can meet the heating area of 15 million square meters with broad prospects for development.