Abstract: Micro-migration process of oil/gas-drive-water has been studied with actual core clip and micro-experiment technology.Research shows that entry pressures of both oil-drive-water and gas-drive-water increase obviously according to porosity decrease.The statistical regularity submits to power function variation regularity.However,there is complex changing relationship between porosity and oil/gas-drive-water entry pressure difference,forming 3 fragment curves with different accumulation significance.When porosity is over 10%-12%,the difference between oil/gas-drive-water entry pressures is small and the probabilities of oil and gas reservoir formation are nearly equal.When porosity is between 5% and 10%-12%,the difference is big,indicating that reservoir can seal hydrocarbon.It is the reservoir for not oil but gas.When porosity is less than 5%,the difference is small and the entry pressures of capillary show the same sealing function for oil and gas.Oil and gas pools are both difficult to form.Experimental results have given rational explanation for the difference of accumulation probability between source-contacting gas and source-contacting oil,and for the mechanism problem of tight sand as cap rock,too.