Abstract: While linear relationship exists between porosity and depth of burial of sandstones at the northern flank of Bei Dagang structural zone,the opposite is true in the case of the sandstones at the southern flank,where at the depth of less than 3,500 meters,the porosity of sandstones decreases with their increasing depth till 3,500 meters when the curve assumes an upward trend,and at 4,000 meters there occur sandstones with an average porosity of 14% to the maximum porosity of 20%.This is attributable to the preservation of a part of the primary porosity in the rocks due to the formation of high porosity pressure.The increase of porosity pressure may effectively check the action of compressive solution at the granular contacts and reduce the extent of secondary enlargement of quartzites and feldspars.It is presumed that the high porosity pressure is resulted from differential compaction of sandstones and mudstones in the process of their diagenesis.