Abstract: South Gulong sag possesses a complicated oil- water relationship,from the sag center to the surrounding it presents the characteristic of tight oil reservoir- oil and water transition zone- conventional reservoirs.Through studies on sedimentary microfacies,reservoir microscopic pore characteristics,diagenesis,constant-rate mercury injection experiment and relative permeability experiment,the sag center is mainly composed of delta front sheet sand.The interactive deposition from the north and the west leads to banded or lenticular sand bodies.The shale content of the sag center is high,showing the diagenesis occurs late,and the reservoir rock is lipophilic.At the sag center,the throat radius is greater than that of the structural high areas,but its sorting is poor.The pore radius there is smaller than that of the high areas,but with better sorting.Furthermore,oil reservoirs of the depression center are mainly characterized by big throat radius,the pores,number of throats and the porethroat radius ratios are generally smaller with poor pore connectivity.While in the structural high areas,oil reservoirs are characterized by pores,the numbers of pores and throats are bigger with big throat number-radius ratios and good connectivity.At the sag center,the dynamics of petroleum accumulation mainly comes from throat capillary force and hydrocarbon generation pressure of underlying strata.