Nanoscale pores have an important role in the accumulation of gas in shale gas reservoirs. Indeed, the formation of nanopores is critical for the characterization and evaluation of a shale reservoir. Moreover, the effect of pyrolysis on the modification of nanopores is not clear. Therefore, this paper focuses on pyrolysis and nitrogen adsorption experiments to examine the nanoscale pore structure and evolution in marine shale strata with low total organic carbon content. All of the examined samples contain micro-, meso-, and macropores. The results show that the number of micropores increased as a result of artificial maturation (i.e., pyrolysis), which resulted in a significant increase in the surface area and the total pore volume. The openness of the pores significantly increased when the maturity was higher than 2.5% R-o (vitrinite reflectance). The 1.5-7.5 and 60-70 nm pores are the most pronounced to change after pyrolysis. Furthermore, liquid hydrocarbons produced during heating were shown to influence pores of approximately 41 nm width. In the overmature stage (R-o = 2.77%), the number of pores and pore volume significantly increased during pyrolysis. The pore structure of the overmature shale was different from that of the shale during the mature and high-maturity stages. Pores less than 20 nm wide nearly provided 90% of the surface area and at least 50% of the pore volume. The transformation of organic matter from the solid state to the liquid and gas states is most closely related to the number of mesopores. The pores with sizes less than 10 nm in width have the greatest change in the proportion of the surface area to pore volume with increasing maturation.