The micromigration of oil-drive-water and gas-drive-water in oil and gas fields was studied, by using core slices and micro-experimental technology, and the migration processes and characteristics of oil and gas in pores and throats were observed, as well as entry pressures of oil/gas migration. Research shows that entry pressures of both oil-drive-water and gas-drive-water increase obviously as porosity decreases, and the statistical regularity observes the power function variation. However, there is a complex changing relationship between porosity and different entry pressure values of the two replacement processes, forming three curve sections, each representing a different accumulation significance. When the porosity is over 10%-12%, the difference between oil-drive-water and gas-drive-water entry pressures is small. Both oil and gas can migrate and accumulate in this kind of reservoir. The probabilities of oil and gas reservoir formation are nearly equal, forming conventional oil/gas pools. When porosity is between 5% and 10%-12%, the difference between the two is obvious, which indicates that this kind of reservoir can seal oil, but can also be a reservoir for gas, easily forming unconventional hydrocarbon pools (deep-basin gas pools). When porosity is less than 5%, the difference is indistinct and the entry capillary pressures show the same sealing function for oil and gas. In this condition, both oil and gas pools are difficult to form. Experimental results give a rational explanation for the difference of accumulation probability between deep-basin gas and deep-basin oil, and also for the mechanism of tight sand acting as cap rock.