In order to explore the petroleum reservoir forming laws in Nanpu Sag, the research method of the spa- cial distribution relation between the petroleum and its migration-accumulation process is adopted to study the process and their main controlling factors. The obtained processes for different horizons can be divided into two parts: vertical migration and lateral diverting accumulation. The former is mainly controlled by the following two factors :1)faults in the oil source rock are the principal transporting path ways for the oil and gas migration ; 2)size of the connected fault thicknesses of the regional caprock controls the enriched or accumulated positions of the pe- troleum. The latter is generally dominated by the following three factors:1)strata with high sandstone-formation ra- tios on and beneath the regional caprock are favorable for oil and gas to migrate laterally;2)positive structures near the faults connected with the oil source rock are prone for oil and gas to accumulate;and 3)different combined forms of the faults result in the different degrees of petroleum accumulation.