鄂尔多斯盆地剥蚀厚度恢复及其对上古生界烃源岩热演化程度的影响

运用镜质体反射率-深度剖面恢复出鄂尔多斯盆地中生界地层的剥蚀厚度,并结合烃源岩加热降温热模拟实验研究上古生界烃源岩的热演化特征。研究认为,反射率-深度剖面模型可分为3类,即单段式、双段式和三段式。盆地东部剥蚀厚度大,为1400～2200 m;西部剥蚀厚度小,为400～1000 m。据烃源岩时温互补热演化模拟实验,岩样在加热到一定温度后降温,随时间增加Ro值虽在增加,但Ro值增加速率不断降低,且在相同时间段内,降温幅度越大,生烃能力越低,当降温到一定程度时,随时间增加,生烃能力非常小。表明了源岩受热的时间尺度与热效应之间,温度起着主导作用。由于中东部剥蚀厚度远大于烃源岩热演化停滞时的最小剥蚀厚度,烃源岩热演化作用已处于停滞状态,西部剥蚀厚度较小的布1-天1地区,烃源岩的热演化作用在新生代仍有可能继续缓慢进行。

Restoration of eroded thickness and its influence on thermal evolution of Upper Paleozoic source rocks in Ordos basin

The eroded thickness of Mesozoic in Ordos basin is restored by using vitrinite reflectance-depth profile(R_(o)-H),and the thermal evolution of the Upper Paleozoic is comprehensively studied based on thermal simulation experiment of source rocks.The R_(o)-H profile models can be divided into 3 types: single segment,double segments and three segments.The eroded thickness in the eastern part of the basin is large,in the range from(1 400m) to 2 200m;while that in the western part is relatively small,in the range from 400m to 1 000m.Isothermal-complementary thermal evolution simulation experiments of source rocks show that the R_(o) increases with time,but the incremental rate decreases when heating the rock samples to a certain temperature and then cooling them;in the same time interval,the larger the temperature drop is,the lower the hydrocarbon-generating capacity would be and the hydrocarbon generating capacity would be getting very low with time when the temperature lowers to a certain value.These results show that temperature plays a key role between the time scale of heating and thermal effects of source rocks.In the eastern and middle parts of the basin,the erosion thickness is much larger than the minimum erosion thickness causing the stagnation of thermal evolution of source rocks, which indicates that the thermal evolution of source rocks in these areas would have been in stagnation.While in the western part,the thermal evolution of source rocks might have lasted slowly till the Cenozoic in the area from Bu1 to Tian1 wells where the erosion thickness is relatively thinner.