2.5.1.1 Reserves and Resources

China is rich in geothermal resources. The total resources within the whole territory of 9.6 million km2 , estimated by 50 % of per unit area average heat reserve of 44.15 ¡Á1012 kJ /km2 in the sedimentary basins, are about 211.92¡Á1018 kJ, equal to the caloric value of 7.2310 ¡Á1012 tons of standard coal. The total resources stored in the major sedimentary basins at the depth of less than 2,000 m are about 40.184 ¡Á1018kJ, equal to the caloric value of 1.3711¡Á1012 tons of standard coal. The total exploitable (recoverable) geothermal resources , computed by 1% of the total resources as the exploitable resources, are 401.84 ¡Á1015 kJ, equivalent to the caloric value of 13,711 ¡Á106 tons of standard coal(Table 2.5.2).

According to the statistical data of 1997, altogether 761 geothermal fields(occurrences) have been discovered through prospecting. Among them 43 have been subjected to exploration, 83 to detailed survey and 635 to reconnaissance and regional surveys. It has been proved that the total reserves of exploitable geothermal water of different categories are about

Table 2.5.2 Estimated geothermal resources in China's major basins

2.5238 million m3/d, with usable heat energy of 4£¬333.89 MW, which can provide annually a heat quantity equal to the caloric value of 4.660¡Á106 tons standard coal. The reserves of exploitable geothermal water include:
    Category B: 784,700 m3/d of exploitable geothermal water, with 735.68 MW of usable heat energy;

    Category C: 691,800 m3/d of exploitable geothermal water, with 2,222.92 MW of usable heat energy;

    Categories D+E: 1,047,300 m3/d of exploitable geothermal water, with 1,375.29 MW of usable heat energy.

    For identified geothermal resources in different provinces, autonomous regions and municipalities, see Table 2.5.3.

China's geothermal resources are geographically unbalanced in distribution. In terms of usable geothermal resources, Southwest China is most abundant with a volume of explored geothermal energy of 2£¬204.45 MW, accounting for 50.87% of the national total. Lessabundant are North and Central-south China with explored geothermal energy of 734.72

Table 2.5.3 Identified geothermal resources in different provinces, autonomous regions and municipalities

MW and 687.75 MW respectively, amounting to 16.95% and 15.87% of the national total. Next is East China, making up 10.22% while Northeast and Northwest China  have only 2.52% and 3.57%  of the national total(See Table 2.5.1 and 2.5.4).

Table 2.5.4 Explored recoverable geothermalresources in different regions of China

2.5.1.2 Characteristics

(1)The distribution of geothermal resources is obviously controlled by China's geological structure and its position in the global tectonics

China is situated in the eastern part of the Eurasian plate, and the main body of the China Continent sits among the Indian plate(including Burma plate), Pacific plate and Philippines plate. The mechanism of collision and subduction of those plates resulted in the uplift of the present Qinghai-Tibet Plateau, and the formation of some down-warped basins like Tarim and Junggar, and Cenozoic fault depressional and stretch structures with North China as the representative as well as many complicated but orderly intraplate fracture pattens. Such a tectonic framework has exerted an important influence on the formation and distribution of China's geothermal resources, resulting in the formation of two geothermal and high heat flow value zones: one is in Tibet and Yunnan, the other is along the southeastern coast. The high heat flow value zone itself is an area where geothermal hot springs are concerntrately distributed(see Fig. 2.5.1).On such a geological background, geothermal resources occur in two forms: one is geothermal hot springs outcropping along the major fractures and controlled by the areas of tectonic upheaval; the other is geothermal water occurring in the Mesozoic and Cenozoic sedimentary basins. The former resources are exposed directly on the surface in the form of hot springs in zonal distribution, obviously controlled by fracture structures, with their usable geothermal resources mainly dispersed over fracture zones as well as the zones of their influence;and the latter are buried in the deep geothermal reservoirs with their geothermal heat provided by the conductive heat in the interior of the earth and the usable geothermal resources distributed over the central parts of large sedimentary basins and their buried upheaval zones.

(2)Geothermal resources are widely distributed

The exploration of geothermal fields shows that exploitable geothermal resources are scattered all over the whole country except Shanghai and Ningxia, which has provided a vast space for the development of geothermal resources.

(3)Geothermal resources of low-moderate temperature are predominant and are the key objectives for development and utilization in China

According to the statistical data of exploration in the existing 761 occurrences, there are only 2 high temperature geothermal fields(the Yangbajain and Yangyi geothermal fields in Tibet, and all the rest are lowmoderate temperature geothermal fields. Of them 26 are moderate temperature fields(90~150¡æ), accounting for 3.4%; and 733 are low temperature geothermal fields under 90¡æ,  amounting to 96.3%. The average temperature of explored geothermal fields is 54.8 ¡æ for the whole country, with the highest average of 88.6¡æ in Tibet and the lowest average of 37.7¡æ in Hunan(Table 2.5.5).

Table 2.5.5 Average temperature of geothermal fields in different areas of China

(4)Most of the geothermal fields are medium and small in size

In the 761 geothermal fields which have been explored, 55 are large and medium-sized ones, occupying 7.2%, but their usable heat energy is 3£¬288.10MW, making up 75.87% of the total usable heat energy; 706 are small-sized geothermal fields, 92.8% of the total , but the usable heat energy is only 1£¬045.79MW, 24.13% of the total,  and averages only 1.48MW.

(5)The geothermal water has a low degree of mineralization in most cases and is suitable for a multiple of uses

Water analyses of 493 geothermal fields show that 327 geothermal fields have geothermal water with the content of dissolved solids less than 1.0g/L, occupying 66.3%, and 42 geothermal fields have water containing dissolved solids over 3.0g/L, 8.5% of the total(Table 2.5.6).

Table 2.5.6 Degree of mineralization of geothermal water in the explored geothermal fields in China

(6)Medium-and small-sized geothermal fields with surface thermal manifestations in the uplift area can be developed and utilized more economically

Those geothermal fields have shallow heat reservoirs, usually 300~500m deep and are easy for development. Geothermal water receives some recharge and is good in quality, so it can be used for various purposes. With some further exploration work, the distribution scope, quantity of resource and the conditions for development will be basically defined. Exploration indicates that the explored exploitable geothermal resources can be increased by several times its natural discharge, which shows that further exploration work would make geothermal resource effectively utilized.

(7)Geothermal fields in large sedimentary basins have great potential for development and utilization

Geothermal resources in those sedimentary basins, the North China Plain and the Song-Liao Basin in particular, have great potential for development. However their development is restricted by the buried depth of hot reservoirs, lithology and recharge conditions. For geothermal water over 40¡æ, exploitation depth generally should be no more than 1,000 m. Now in some areas, however, exploitation depth has already exceeded 3,500 m and the depth and scope of the development of geothermal water of 70~90¡æ are showing a tendency of daily increase.