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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.

The evolution of shale gas reservoirs is highly complicated, especially when the reservoir is at a great depth and has undergone complex diagenesis and multiple stages of tectonic activity. However, the evolution mechanism of material composition− pore structure−adsorption property remains unclear. In this study, pyrolysis experiments were used to obtain products of different maturities and determine the evolution processes that took place in a reservoir and total organic carbon (TOC), X-ray diffraction, field emission scanning electron microscopy, low-pressure nitrogen/carbon dioxide gas adsorption, and methane isothermal adsorption experiments were employed to analyze the material composition, pore morphology and structure, and adsorption capacity. The results showed that the TOC value of the pyrolysis samples decreased in comparison to the original sample because some organic matter was converted into hydrocarbons. As diagenesis strengthened, unstable minerals, such as feldspar and carbonate, were converted into clay minerals, while quartz remained basically unchanged. The transformation between clay minerals was quite substantial and inheritable in terms of pore structures and adsorption capacity. The change in the pore structure was mainly caused by mesopores. The evolution of the pore structure and the adsorption property were controlled by clay minerals (mainly illite−smectite mixed layer and illite) and organic matter. Both micro-and mesopores had a controlling effect on the adsorption capacity. When R o exceeds 2.48%, there was a dramatic change in material composition, pore structure, and adsorption property, which should be given more attention in future research.

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