The Thermal Structure And Geothermal Genesis Mechanism In Guide Basin

Study areas are located in the northeastern margin of the Tibetan Plateau, in the Qilian-Kunlun-Qinling orogenic belt, where there have good geothermal geological conditions. Many kinds of geothermal manifestations are distributed in study area. The hydrothermal activities in the study areas are iolent, especially the zhacangsi thermal field, the water temperature is as high as 93℃. The study areas are rich in geothermal resources overall. But the geothermal exploration in the area is mainly focused on the shallow depth of the 3000m due to objective conditions and the research degree is low, the study of deep geothermal distribution and thermal structure has not been done systematically. These severely restricts the development and utilization of geothermal resources in the area. In the paper, we choose two typical geothermal areas in Guide basin as study object, collected and gathered hydrochemistry, hydrogen and oxygen isotopes data, and its isotopes data in the study area to study the source of the geothermal fluid supply, the geothermal fluid migration pathways and the thermal reservoir characteristic; gathered rock samples at different depths to test the heat rate and thermal conductivity and analyze the distribution characteristics of the parameter values in vertical direction. The concept model of geothermal genesis was improved and the concept modes of crustal thermal structure in study area was established for the first time based on the analytical results. Obtained the following understanding:Analysis hydrochemistry of geothermal fluids, the geothermal fluid is alkaline water, the chemistry type of thermal water were mainly Cl·SO4-Na, then the HCO3·SO4-Na and the chemistry type of thermal water was changing along the flow direction. Hot water was rich in F, Li, SiO2 and showed a positive correlation with Cl. With chemical geothermometer of thermal water, estimated the geothermal reservoir temperature of zhacangsi thermal field was about 133℃ and the depth of thermal cycle was about 1800m. There were 60% to 68% cold water mixed with thermal water, estimated with silicon enthalpy diagrams, and the geothermal reservoir temperature was about 222℃, the depth of thermal cycle was about 3280m before the cold water mixed.From stable isotopes data, thermal water showed mainly meteoric recharge, the direction of thermal water runoff was from the surrounding mountains to the center of the basin. The recharge altitude was about 3300m, thermal water presence slight "Oxygen-18 drift" in zhacangsi thermal field. Analysis carbon and sulfur stable isotopic characteristics of thermal water, revealed carbon and sulfur sources in thermal water mainly from leaching of the deposited layer, the age of thermal water changed from 6.16±0.2 to 28.55±1.74ka.The analysis results of rock thermophysical parameters showed that the rock thermal conductivity and heat generation rate of zhacangsi thermal field were higher than sanhe plain that in the center of the basin. The thermal energy produced by the decay of radioactive elements in the strata of zhacangsi thermal field was two times as much as sanhe plain before 2000m. Three terrestrial heat flow values were obtained by related data, respectively 74mW/m2、76mW/m2、79.5mW/m2. The spatial distribution of terrestrial heat flow in Qinghai province showed that the geothermal condition in the northeast of Qinghai province was better than that in the northwest.Compared to the sanhe plain in the center of basin, the water and heat activities were to be more severe and the water temperature was higher of zhacangsi thermal field due to the different geological and tectonic conditions. Zhacangsi thermal field was convective geothermal system, sanhe plain which in the center of basin was convective-conductive geothermal system.Established the heat generation rate model by the seismic wave data, the ratio of crust/mantle heat flow is 1.2, showing hot crust and cold mantle.Finally, comprehensive analysis of regional geological data, deep exploration and the analyze results of geothermal fluid geochemical, improved the concept model of geothermal genesis of different types of geothermal systems in the area, established the conceptual model of thermal structure, for the latter depth research and development and utilization provided an important scientific basis.