Abstract: The Dongying depression is one of the highly petroliferous basins in the Bohai Bay, with its major cycle of oil generation occurring in the Shahejie formation of Oligocene. The whole basin is characterized with the presence of independent oil pools all of low saturation(saturation pressure of less than 110 atm. in general)buried at medium depth of 1800-3200 meters, where no gas pool has yet been found; and highly saturated oil pools/oil-gas pools(saturation pressure as high as 170 atm. to less than 100 atm.)buried at less than 1800 meters. Data indicate that the oil in the deep and shallow pools had the same source, that the oil in the shallow beds came from the deep pools by migration due to tectonic disturbances. Intense faulting conducive to vertical, migration of oil and gas provided conditions for their differential separation at different spill points. It may serve as the basis of calculating the approximate time of oil and gas accumulation.