Abstract: This is a study of compaction of sand and mud stones in North China, including Dongying, Dagang and Bohai Sea Based on the variations of porosity of the rock and their relationship to the volume of rock, equations for the thickness of rocks at any depth intervals before compaction are derived. According to the results of electronic computer's computation, the process of compaction of and squeezing out of water from the rocks is divided into four stages, with rapid compaction of mud stones and further expulsion of large amount of water occurring during the 3rd stage at 2, 100-3,200 meters depth. This agrees with the depth of oil occurrence in North China, indicative of the significance of this stage to oil and gas migration. Mercury injection tests have been carried out to study the changes of peak values of pore throats with increasing depth, while the data obtained from x-ray diffraction analysis have been used to study the changes of clay minerals with increasing depth with a view to illustrate further the character of compaction and water expulsion in different stages. The elastic effect of rocks, the time of large scale accumulation of oil and gas into the reservoir rooks.