Exhumation,Cooling and Erosion History of Triassic Oil Shale since Late Cretaceous,Binxian-Tongchuan Area of Ordos Basin

Yu Qiang; Ren Zhanli; Zhu Zengwu; Tao Ni; Wang Baojiang; Li Changchun

Abstract：The recovery of exhumation history of oil shale with the method of geothermometer and thermochronology has scien-tific significance for the study of thermal background of oil shale mineralization and its reformation.Based on the theory of tec-tonothermal evolution,the paleogeotemperature,the exhumation-related cooling and the erosion of the Triassic oil shale since Late Cretaceous in the Binxian area and the Tongchuan area were recovered with the fission track data from the well Yongcan1 and the (U-Th)/He data from the drilling well ZK900,respectively.The tectonothermal evolution difference of the two areas are discussed.The average (U-Th)/He ages of the Chang6,Chang9 and Chang10 samples are 43.83 Ma,31.87 Ma,and 22.88 Ma respectively.Three stages of exhumation-cooling since Late Cretaceous of the Tongchuan area were recovered:97—40 Ma,fast exhumation;40—8 Ma,slow exhumation,and 8 Ma to the present,extremely fast exhumation,respectively.The erosion thickness and exhumation rate are 600 m,10.5 m/Ma and 10 m,0.3 m/Ma and 1 290 m,161.3 m/Ma,respectively. The paleotemperature and cooling rate are 100—60 ℃,0.70 ℃/Ma and 60—50 ℃,0.22 ℃/Ma and 50—25 ℃,2.90 ℃/Ma, respectively.Three stages of exhumation-cooling since Late Cretaceous of the Triassic in the Binxian area were recovered:97—40 Ma,increasing exhumation-cooling (130—75 ℃),cooling rate of 0.96 ℃/Ma,exhumation rate of 14.4 m/Ma,erosion thickness of 820 m;40—8 Ma with little changed as 75—70℃,exhumation rate/cooling rate,0.16 ℃/Ma and 1.9 m/Ma,ero-sion thickness of 60 m;8 Ma to the present with sharp cooling as 70—31 ℃,exhumation rate of 125 m/Ma,cooling rate of 4.88 ℃/Ma,erosion thickness of 1 000 m,respectively.There are three stages for the exhumation-cooling history of the oil shale (Triassic in the Binxian-Tongchuan area).The Eocene (40 Ma)and the Late Miocene (8 Ma)are the critical moments for the cooling of the oil shale during its mineralization.More attention should be paid to the differences of exhumation,cooling, erosion between Binxian area and Tongchuan area when discussing the mineralization of the oil shale and its reformation in the Ordos basin.