青藏高原中-新生代沉积盆地热体制与古地温梯度演化

尽管前人对青藏高原隆升机制、地块拼合和陆内俯冲、中-下地壳流动以及岩浆活动等过程做了大量研究, 但对工区发育众多的中-新生代沉积盆地热体制和古地温梯度演化很少涉及, 而这些对中生代海相烃源岩油气生成过程以及已生成的油气命运具有重要影响.在总结前人有关青藏高原温度场背景和盆地类型演化成果基础之上, 运用流体包裹体均一温度测定数据, 综合建立了高原腹部中生代海相盆地古地温梯度演化曲线, 认为在中生代至古近纪的被动陆缘-弧后盆地-前陆盆地演化过程中, 中生代海相盆地处于相对低的地温梯度条件下(＜3.0 ℃/100 m)有利于成熟油的生成; 在新近纪至第四纪的青藏高原隆升阶段, 这些中生代海相盆地不仅演化成残留盆地, 而且还伴随着新的热事件使得盆地地温梯度不均匀急剧上升(~6.5 ℃/100 m), 同时会导致大部分中生代海相烃源岩生成的油再度裂解成气和存在二次生烃(气)的可能性.因此, 古地温梯度演化决定了在"冷盆"区域可能还存在找油潜力, 但在大部分的"热盆"区域只能以找气为主. 

Thermal Regime and Paleogeothermal Gradient Evolution of Mesozoic-Cenozoic Sedimentary Basins in the Tibetan Plateau, China

Although many scholars have done a lot of research on the uplift mechanism of Tibet plateau, intercontinental subduction, mid-low crust flowing and magmatic activities, the thermal regime and paleogeothermal gradient evolution in the Mesozoic-Cenozoic sedimentary basins have rarely been studied. But the thermal regime and evolution of paleogeothermal gradient have great significance for the hydrocarbon generation processes and hydrocarbon preservation of the Mesozoic marine source rocks. On the basis of achievements of the previous research on the temperature field of Tibet plateau and evolution of basin types, using the fluid inclusion homogenous temperature, the paleogeothermal gradient evolution curves of the Mesozoic marine basins in Tibet plateau are formed in this paper. And it suggests that the Mesozoic marine basins in the low paleogeothermal gradient (< 3.0 ℃/100 m) when they were in the evolution process from continental margin rift basins to back-arc basins and then to foreland basins during Mesozoic to Eocene, which were beneficial for the generation of mature oil. After that these basins evolved into residual basins and the paleogeothermal gradient rising sharply (~6.5 ℃/100 m) because of continuous and periodic uplifts of the Tibet plateau during Neogene to Quaternary, which resulted in the possibility of that the oil reservoir was cracked into natural gas and secondary hydrocarbon generation of the Mesozoic marine source rocks. Therefore, the paleogeothermal gradient evolution determines the exploration potential for oil in the cooling basin and the exploration potential for gas in the heating basin.