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.