In order to determine the effectiveness of the mudstone cap—rocks in the southeast Sichuan basin, both hydrocarbon generation histories of source rocks and sealing histories of the mudstone cap—rocks are reconstructed. The results show that the sealing function of the Cambrian mudstone formed earlier than the onset of gas generation of the Sinian source rock and its maximum displacement pressure was as high as 26 MPa at the end of the Late Cretaceous. Writers, therefore, consider that the Cambrian mudstone was of effective sealing function with respect to the Sinian source rock before Late Cretaceous. Similarly, the Permian and Triassic mudstone cap—rocks were also of effective sealing function before being drastically exhumed or uplifted in the late tectonic events. However, the Silurian mudstone cap—rock was less effective in sealing the gas generated by the Cambrian source rock though its maximum sealing capacity (displacement pressure =16 MPa) is powerful enough to seal high pressure gas reservoir at the end of the Late Cretaceous.