To enhance the science and effectiveness of the subsequent exploration and development efforts targeting the Chang 6 oil reservoir in the Fenchuan block of Nanniwan oilfield. Core observation, cast thin section identification, scanning electron microscope and other analytical test data were applied to carry out reservoir characterization of the Chang 6 oil formation group in Fenchuan block. Petrel and tNavigator were used to perform 3D geological modeling and numerical simulation of the oil reservoir, and the residual oil distribution study was conducted. The results show that the reservoir's rock type is predominantly feldspathic sandstone, with a minor amount of debris-arkosic sandstone. The pore type is mainly residual intergranular pores and dissolution pores, as well as a small amount of fracture pores. The porosity is mainly distributed between 4% and 9%, and the permeability is mainly distributed below 0.3 × 10^-3 μm², which belongs to low porosity, extra low porosity and extra low permeability reservoirs. The distribution of residual oil is influenced by the sand body boundary, physical property differences, and the injection and production well network. It is primarily located at the boundary of the sand body or the overlapping part of the sand body. The residual oil distribution in the Chang layer is found to be better than that of the Chang layer in terms of contiguity. In future development, residual oil can be effectively utilized through encrypted well deployment, water injection, or secondary development using a three-dimensional well network of "high-angle wells + horizontal wells," thereby improving recovery rates.