Study On Generating Mechanism And Evaluation Of Low-Medium Temperature Geothermal System

The Songliao basin whose area is 62400km2 is an important oil-bearing basin in China, in which occurrence of abundant oil and gas as well as perspective geothermal resources are dependent upon tectonic setting and particular thermal structures of the basin. It is one of large low-medium temperature geothermal field in china. Thus the tectonic evolution of the Songliao basin and geothermal effect, the genesis and occurrence state of geothermal resources, the characteristic of formation and evolution, the method of geothermal reaources evaluation can be studied according to the rich geological data and several successful example of the basin. It is of practical value to explore the basin resource including oil, gas and geothermals. It also can rich the theory of low-medium temperature geothermal system formation and the technique of resource evaluation.This paper focuses on geothermal resources of the Songliao basin, particularly in the Dumeng area. A systematic study of geophysics, tectonics, petroleum and geothermal geology, combined with numerical simulation, was performed for this purpose. The results are shown as below.The history of the Songliao basin is divides into four stages, high-temperature doming and extension, high-temperature rifting, low-temperature depression, and low-temperature shrinkage. Numerical simulation of mechanical process shows that the basin appeared in the early- to late- Jurassic as a result of rapid low-angle subduction of the Pacific plate towards the northwest, of movement of the Siberian block towards the southeast, of many other influences such as internal thermal stress in the mantle, gravitational force, stress of phase transition,etc. Both the Pacific plate and the Siberian block slowed down in the early Cretaceous, and almost stopped in the middle Cretaceous with the rapid decrease of abnormal heat in the mantle below. Afterwards, reactivation of the Pacific plate and thermal in the mantle below became the major factors in controlling the evolution of the Songliao basin during the Cenozoic.The Mohor discontinuity is buried at a depth of 29~33km in the Dumeng area. In a similar configuration, the Curie surface is buried at a depth of 18~24km in the area. It tends in plan view to increase its depth in a semi-concentric pattern to the east of Du401 well. This indicates a tendency that the depth of the Curie surface is lower in the northwest than in the southeast. Interestingly, a broad area of Hercynian and Yanshannian granites is found in the basement.Numerical technique was employed to simulate the present temperature field in the Dumeng area. Calculated temperature ranges from 80~91℃at depth of 2000m,from 63~70℃at depth of 1500m and from 42~48℃at depth of 1000m.The deep temperature of geothermal reservoirs is a parameter that is essential to geothermal resources assessment in the exploration of geothermal resources. Resulting geothermal gradient is 3.8~4.4℃/m and the flux is 62~65 mW/m2. Geotemperature in the Dumeng area is fairly high, especially in the Xiaolingke and Xindian. This suggests the presence of favorite conditions for geothermal resources in the area.Three sources for geothermal resources in the Dumeng area are recognized as heat derived from the mantle, heat derived from the intruded magma, and radioactive heat. The geothermal resources in distribution took place in rifting environment where high connectivity and high openness between the mantle and the crust provided a favorite...