Abstract: Petroleum was reserved in pores of source rocks(usually mudstones) after generation.The pore volume of source rock controlled the total amount of hydrocarbon generation.When petroleum had moved out from source rock,the pore volume of source rock decreased due to the compaction of overlying strata,and the decrease amount was equal to the petroleum expelling amount.In this way,we can calculate the total pore volume and the decrease amount so as to find out the generation and expelling amounts of petroleum.If the overlying strata were eroded,sandstones would rebound since pressure had changed.Pore increases resulted from sandstone rebounding caused the decrease of formation fluid pressure,providing both driving force and room for petroleum migration and accumulation.Case studies in the Biyang Sag have indicated that,from Hetaoyuan Formation to the end of Liaozhuang Formation,the sag kept subsiding and pressure increased.Petroleum mainly generated during this stage,accounting for over 50%.The regions such as Guian,Shuanghe,Houzhuang and Wangji are main accumulation locations,where sandstone rebounding amount and pressure decrease resulted from erosions during Liaozhuang stage are bigger.