Mature-stage lacustrine shale was sampled from the Late Triassic Chang 7 member in the Ordos Basin, China. Two aliquots separated from the original sample were heated by hydrous pyrolysis to high- and overmature stages. The three samples were analyzed by nanometer-scale resolution X-ray computed tomography (nano-CT). From the distribution and geometry of the organic matter pores (OM pores) in the two-dimensional (2-D) nano-CT images, this study calculated the total organic matter content and porosity of the OM pores (2-D TOC and 2-D OM porosity, respectively) and characterized the OM pores and throats. The results suggest the following: (1) The OM pores tended to distribute centrally rather than sparsely throughout the organic matter, and large-area pores existed above a 2-D TOC threshold of 2%. (2) The main diameter range of the OM pores was 100-700 nm, and the number of OM pores increased with maturity. (3) The amount of coordination numbers corresponding to a given pore diameter interval and the number of pores corresponding to the same range of coordination numbers (1-10) were greater in the high-mature and overmature samples than in the mature samples, indicating that the OM pore connectivity improved with maturity. The three-dimensional volume data confirmed that the OM pore connectivity decreased in the following order: high-mature, overmature, and mature samples. Highly mature samples may be more conducive to the diffusion of natural gas within the OM pores.