Since Paleozoic, the tectonic evolution of Chinese mainland has gone through Ocean Lu differentiation and opposition stage, Carboniferous-Permian soft collision transformation stage and Mesozoic-Cenozoic basin-mountain confrontation stage. Since Mesozoic, the whole continent has been connected, and the evolution and development of basin-mountain pattern control the evolution and development of thermal storage conditions in various regions. Multi-cycle tectonic movements and multi-stage basins overlap to form different geothermal fields. The evolution of the above structures, accompanied by magmatic activities in different periods, formed strata with different lithology and structures, which made the distribution of geothermal flow values in China have obvious regularity (Figure 1-3). According to Geothermal Resources in China-Stratigraphic Characteristics and Potential Evaluation (Chen Moxiang, Wang Jiyun, etc., 1994), the geothermal flow value in China can be divided into five tectonic regions (Figure1-4; Table 1- 1). Among the five geothermal flow tectonic zones, the southwest tectonic zone is the highest, reaching 70 ~ 85 MW/m2; The northwest structural area is the lowest, which is 43 ~ 47 MW/m2. The average heat flux in the North China-Northeast tectonic region is 59 ~ 63 MW/m2, which is close to the national average. The average heat flow in South China tectonic region is 66 ~ 70 MW/m2, which is slightly higher than the national average. The average heat flux in the middle is 40 ~ 60mw/m2. In southwest China, along the suture zone of Yarlung Zangbo River, the heat flow value is relatively high (9 1 ~ 364 MW/m2), which decreases northward with the structural steps, and only reaches 33 ~ 44 MW/m2 in Zhungeer Basin, becoming a "cold basin". The eastern part of China is the geographical zone of Taiwan Province plate, with a high heat flow value of 80 ~ 120 MW/m2, which decreases to 60 ~ 100 MW/m2 after crossing the Taiwan Province Strait to the Yanshan orogenic belt on the southeast coast, and only 57 ~ 69 MW/m2 to Jianghan Basin. It shows the characteristics of gradual change from a high heat flow zone with strong modern tectonic activity to a low heat flow zone with weak tectonic activity. In addition, in large basins, the distribution of geothermal flow is directly related to the structural form of the basement, with the uplift area being a relatively high heat flow area and the depression area being a relatively low heat flow area.
Figure 1- 1 medium distribution map of thick shell (according to Yuan Xuecheng et al. 96 1)
Fig. 1-2 China 1× 1 weighted average Bragg anomaly map (according to Ma Xingyuan, Yin Xiuhua and others 987 1).
Figure 1-3 Mainland China geothermal flow value map (according to Qiu Nansheng, Hu Shengbiao, etc. , 042)
Fig. 1-4 statistical tectonic zoning map of Chinese mainland geothermal flow (according to Chen moxiang and others, 1994).
Table 1- 1 geothermal flow statistics table of Chinese mainland and various structural areas: mW/m2.
(According to Chen Mou Xiang 1994)
Geothermal distribution reflects the characteristics of deep geological structure and the evolution history of geological structure to a certain extent, and is the basic parameter for evaluating geothermal resources, delineating geothermal abnormal areas and zoning geothermal resources development and utilization. According to the basic characteristics of ground temperature distribution in China (ieee fellow, Huang Shangyao, etc. , 1990), the ground temperature distribution in China has obvious regularity: the ground temperature in the east is high, the ground temperature in the west is low, the ground temperature in the south and southwest is high, and the ground temperature in the northwest and north is low; The ground temperature is low in hilly areas and high in large and medium-sized basins. The ground temperature in each basin also conforms to the general law of ground temperature distribution, that is, the ground temperature in the eastern basin is higher than that in the western basin, especially in great basin in the northwest, which is one of the lowest in all basins in China (table1-2; Figure 1-5 ~ Figure 1-7).
Table 1-2 Geothermal Distribution Characteristics in China
sequential
(According to ieee fellow and Huang Shangyao, 1990)
The distribution of geothermal gradient in different regions of China is different, and the general trend is high in the east and low in the west, high in the south and low in the north, which is consistent with the distribution law of geothermal energy (Figure 1-8).
Geothermal resources in China can be divided into two categories according to their distribution and formation conditions: convective geothermal resources in uplift mountains and conductive geothermal resources in sedimentary basins. The formation of geothermal resources is closely related to geological structure, magmatic activity, stratigraphic lithology and hydrogeological conditions. According to Formation Characteristics and Potential Evaluation of Geothermal Resources in China (Chen Moxiang, Wang Jiyun, etc. , 1994), China geothermal system can be divided into two types (geothermal convection and tectonic subsidence conduction in tectonic uplift area) and five types (volcanic type, non-volcanic type, deep circulation type, faulted basin type and depressed basin type). See table 1-3 for various types of geological structure, thermal background, structure and scale, heat source and water source, hot water salinity, geothermal energy utilization direction, representative areas and geothermal fields.
Conductive geothermal resources in (1) sedimentary basins
Conductive geothermal resources in sedimentary basins are conductive medium-low temperature geothermal resources, which are mainly distributed in large basins such as North China Plain, Wei Fen Basin, Songliao Plain, Jianghuai Basin, Subei Basin, Jianghan Basin, Sichuan Basin and Hetao Plain. The sediment is very thick. Among them, there are not only a large number of reservoirs with high porosity and permeability composed of coarse clastic materials, but also a large number of caprocks composed of fine-grained materials, which play a role in heat storage and insulation of reservoirs. The low salinity hot water reservoirs in the Mesozoic-Cenozoic large sedimentary basins in eastern China are structurally composed of interbedded sandstone and mudstone. The Neogene thickness of North China Basin, North Jiangsu Basin and Jianghan Basin ranges from several hundred meters to 2,000 meters respectively. The Cenozoic in Songliao basin is not developed, and the Upper Cretaceous is the main hot water reservoir. Triassic and Jurassic, basin margin facies and channel sandstone facies in the central Ordos basin are suitable for the occurrence of light and low salinity hot water. Triassic in Sichuan basin is composed of marine sand, mudstone and carbonate rocks, while Jurassic is composed of deep lake carbonate rocks and clastic rocks, which is a brine-rich layer. There is enough space in great basin to make the hydrodynamic environment show obvious zonation: the outer ring zone is an active alternating zone of runoff, and the inner ring zone is a slow lagging zone of runoff. After passing through the outer ring belt, the groundwater flowing into the basin turns into a long-distance horizontal migration into the inner ring belt, and the groundwater can fully absorb the heat of the rock, so that the water and the rock are at the same temperature. Therefore, the inner zone of large basins (such as North China Basin) becomes an ideal environment for heat accumulation. The bedrock thermal reservoir in North China Plain is composed of carbonate strata of Lower Paleozoic, Mesoproterozoic and Neoproterozoic, which is consistent with a series of basement uplift areas and constitutes many geothermal fields with economic value. In addition, the formation of geothermal resources in sedimentary basins is closely related to magmatic activity and tectonic activity. The early eastern China basin was characterized by rift basin, multi-stage magmatism and high heat flux. In the later stage, it turned into a hot-cold depression, and the thermal reservoir caprock, basin ridge structure and active deep faults at the bottom of the basin developed well, forming a regional hot water runoff channel and becoming a multi-stage superimposed hot water basin. According to the research on the formation of geothermal energy in Tianjin, its heat source comes from the heat generated by the transformation of radioactive elements in the granite crust at a depth of 8 ~ 16 km and the heat transferred from the lava flow in the upper mantle to the shallow crust (Wu Tiejun et al., 2004).
Figure 1-5 China 1000m deep geothermal distribution map (according to ieee fellow and Huang Shangyao, 90 1).
Fig. 1-6 geothermal distribution map at the depth of 2000m in China (according to ieee fellow and Huang Shangyao, 90 1).
Fig. 1-7 geothermal distribution map at the depth of 3000m in China (according to ieee fellow and Huang Shangyao 90 1).
Figure 1-8 Medium geothermal distribution map of Guo Tie (according to ieee fellow and Huang Shangyao, 90 1).
Table 1-3 List of Basic Types of Geothermal System in China
(According to Chen Moxiang 94 1)
Geothermal resources and their development and utilization in China (Tian Tingshan, Li Minglang, etc. , 2006), according to the mechanical properties and thermal storage characteristics of the basin, the thermal storage of China sedimentary basin is divided into eastern extensional basin, central cratonic depression basin and western compressional basin. Because each basin is a complete thermal storage system, it can be divided into independent thermal storage sub-regions and thermal storage sub-regions (Table 1-4).
Table 1-4 Description of Zoning Conditions of Geothermal Resources in China
sequential
sequential
sequential
(According to Tian Tingshan et al., 2006)
The total area of Mesozoic and Cenozoic basins in China is 340× 104km2. Among them, there are 9 large basins with basin area greater than 5× 104km2, and 39 medium basins with basin area of 1× 104km2 (Figure 1-9). The thermal storage conditions of basins in China from east to west are from good to bad. The eastern basin is a "hot" basin with multi-layer thermal storage, the central basin is a hot brine basin, and the western basin is basically a "cold" basin. From south to north, mountains are brought from high-temperature water to low-temperature water belt.
(2) Convective geothermal resources in uplift mountain areas
Uplift mountain refers to the area with uplift as the main tectonic activity since Miocene, and the mountain is the skeleton of modern landform, including intermountain fault basins and river valleys. Hot water is formed and distributed along the deep fault zone, which is generally an open pulse deep circulation convection system, and there are also conduction-convection systems in which layered fault blocks overflow along the fault, and most of them are discharged and overflowed in the form of springs. The surface thermal display of most hydrothermal areas in China appears in the form of a single spring point or spring group, and boiling springs, boiling fountains, spouts and hydrothermal explosions coexist in a few areas. Geothermal resources and their development and utilization in China (Tian Tingshan, Li Minglang, etc. , 2006), according to the structural characteristics and hydrothermal activity intensity of mountains in China, the convective heat storage in uplift mountains can be divided into high-temperature heat storage in modern plate collision zone, moderate-temperature heat storage in deep and large faults in fault-folded mountains, moderate-temperature heat storage in fault-block karst mountains, moderate-temperature heat storage in Quaternary volcanic afterheat and low-temperature heat storage in fault-folded plateau mountains (table 1-4). According to the exposure of hot springs, there are four hydrothermal activity intensive zones in China: ① South Tibet-West Sichuan-West Yunnan; ② Hydrothermal activity intensive zone in Taiwan Province Province; ③ Hydrothermal activity intensive zone in the southeast coastal area; ④ Hydrothermal activity intensive zones in Jiao Jiao and Liao Peninsula.
Convective geothermal resources are mainly distributed in southern Tibet-western Sichuan-western Yunnan and Taiwan Province Province, and medium-low temperature geothermal resources are mainly distributed in the southeast coastal areas and Jiaodong Peninsula. The formation of geothermal resources in uplift mountains is closely related to structure. China is located in the eastern part of Eurasia Plate, sandwiched between Indian Plate, Pacific Plate and Philippine Sea Plate. Since Cenozoic, due to the collision between Indian plate and Eurasian plate, a convergent continental margin active zone in southern Tibet has been formed in southwest China. On the east side, the collision boundary between the Eurasian plate and the Philippine Sea plate is formed on both sides of the central mountain range of Taiwan Province Island. Although the characteristics of these two collision boundaries and their adjacent areas are different, they are both one of the regions with the strongest tectonic movement in the world today, showing high heat flow anomalies together, and have the necessary geological structural conditions for generating and breeding high-temperature geothermal resources. Away from the plate boundary, the tectonic activity is weakened or a stable block, the thermal background is normal or even low, and the hydrothermal activity is weakened, generally forming medium-low temperature geothermal resources, mostly low temperature geothermal resources. The formation of geothermal resources in uplift mountains is closely related to magmatic activity. Most of the low-temperature hot springs in China are consistent with the distribution area of carbonate rocks, while most of the high-temperature hot springs are exposed in non-carbonate rocks or on the contact interface between carbonate rocks and granite. According to China Hot Spring Resources (Huang Shangyao et al., 1993), the geological types of hot spring resources in China are divided into three categories and six types, and their formation characteristics are shown in Table 1-5.