Research shows that entry pressures of both oil-drive-water and gas-drive-water increase obviously according to porosity decrease.

The entry pressure is the minimum driving force which is responsible for oil and gas secondary migration and also major index that measures the secondary migration and the blocking capability of a fault.

The expulsion energy of carbonate source rock mainly come from the increasing pressure within itself caused by the great increase in molar volumes with the generating and further cracking of hy.

In this paper the authors discuss the relationship between the pore fluid pressure and the fluid expulsion under the compaction, and suggests that the abnormal pore fluid pressure results from the increase of the capilary force due to the change of permeability and that a dynimic balance is maintained between them during the pregnancy of the fluid pressure.