Oil expulsion is an important process for the evolution of shale, especially in the oil-generative window. Low oil expulsion efficiency will cause the retention of oil and gas in mature source rock. This study used semi-closed pyrolysis to simulate the hydrocarbon generation and expulsion process of the Xiamaling Formation marine shale at various conditions. Low-pressure nitrogen (N-2) and carbon dioxide (CO2) gas adsorption isotherms were obtained for the original shale sample and its thermally evolved solid residues. The results showed that the amount of residual bitumen first increased with increasing expulsion efficiency (EE < 13.43%) and then remained constant with a further increase in EE. This finding implied that the saturation threshold for the source rock must be reached before oil expulsion can proceed. Meanwhile, the evolution of pore volumes with EE can be divided into two stages. The pore volumes decreased sharply in the first stage (EE < 13.43%), whereas they decreased slowly in the second stage (EE > 13.43%). The evolution of volume for micropores and fine mesopores with EE is very similar to that of expelled hydrocarbons. This similar evolution trend was further confirmed by the abovementioned oil expulsion model. This study enhanced understanding of the generation and evolution of shale gas in the oil-generative window.