The microscopic pore structure of reservoirs provides the basis for shale oil exploration and development. In our analyses of the pore development characteristics of shale oil reservoirs, we integrated the argon-ion scanning electron microscopy test and a statistical method to determine the parameters of the pore images of the reservoir. Eighteen rock-core samples of the Shahejie Formation in the Dongying Sag, Bohai Bay Basin, China, were used in the analyses. The results indicate that the main pore types of the shale oil layer in this formation are inter-particle pores, dissolved pores, inter-crystal pores, and intra-crystal pores. The largest rate of contribution to plane porosity is from the argillaceous inter-particle pores, with the dissolved pores making a high contribution, and the inter-crystal pores and intra-crystal pores making the lowest contributions. The pore diameter in the shale oil reservoir is of nano and micron scale, with the nano-scale pores being the dominant quantity. However, the non-dominant micron-scale pores provide porosity to the reservoir plane. Therefore, the micron-scale pores provide the main reservoir space to the shale oil layer in the Shahejie Formation.