The Thermal Evolution History Of Ordos Basin Mesozoic And Its Relationship With Various Energy Mineral Deposit

The generation, migration, accumulation and reservoir of oil and gas are controlled by the basin thermal evolution history, which also plays an important role in the formation process of sandstone-type uranium deposit and coal mine. Therefore, it has an important meaning on analyzing the basin thermal evolution history.In this paper, the studies of present-day geothermal field and paleo-geothermal field were made on the basis of using geological, geophysical and geochemical data comprehensively in the southern region of the Ordos basin. The thermal evolutional history of the Mesozoic in the southern region of the basin was recovered by means of some techniques including vitrinite reflectance and fission track analysis and some software including basinmod and AFTsolve. The relationship between the region thermal evolutional history and the multifarious minerals deposit was also discussed.On the regression analysis from measured thermometric data, results show that the representative value of present geothermal gradients of the region is 2.9℃/100m, and the average value of the heat flow is 64 mW/m².The present geothermal gradients is 2.6℃/100m in Huangling area, which was lower than that in Zhenyuan area where the present geothermal gradients is 2.7℃/100m, and also lower than 2.9℃/100m in Dongsheng area. In the study area, the characteristic of buried depth was deep in the west and shallow in the east on the horizon containing uranium deposit which was at the bottom of Zhiluo formation(J_2z). And the temperature of Zhiluo formation(J_2z) was higher in the west and lower in the east.With the comparison of the Qingshen2 well, the Xuntan1 well and the El well, the difference between the north and the south thermal structure of the basin was investigated. The study showed that the difference was bigger. Further studies indicated that both the heat production and the formation temperature in the south were higher than that in the north.The distribution characteristic of the maturity, which was in the Yanchang formation and Yan'an formation in the south area of the basin, indicated that high in the south while low in the north, and high in the center while low in the east and the west. The organic maturity was obviously higher in Huangling area than that in Dongsheng area according to the data of vitrinite reflectance and the highest paleo-temperature the formation experienced.The erosion thickness of the formation in the southern region of the basin was recovered by means of vitrinite reflectance and acoustic-time difference. The analysis indicates that the general regulation was that the erosion amount increasing from west to east.By means of some software including basinmod and AFTsolve and some techniques including vitrinite reflectance and fission track analysis, the thermal evolution history of some wells has been simulated. Simulation results indicate that the paleo-geothermal gradients are lower during the Palaeozoic-Mesozoic. The paleo-geothermal gradient reaches the maximum at 97Ma ,which are between 3.6℃/100m to 4.4℃/100m. It has became lower and lower gradually since late cretaceous.The simulation results of thermal evolution history indicate that the hydrocarbon Source Rock in thermal abnormal area is earlier to get its hydrocarbon generation threshold and threshold depth is shallow. The father the place to thermal abnormal area, the later Hydrocarbon Source Rock to get its hydrocarbon generation threshold and the shallower threshold depth.Coalification reaches the peak of hydrocarbon generation and hydrocarbon expulsion during Early Cretaceous. Coal-formed Gas begins to generate on large scale during Early Cretaceous. The peak hydrocarbon generation stage of the Mesozoic Hydrocarbon Source Rock and the mineralization period of the uranium deposit are simultaneous. Generated oil and gas provides reductive sufficient conditions for uranium deposit formation .