Evolution Mechanism of Material Composition–Pore Structure–Adsorption Property in Marine Shale Based on Pyrolysis Experiments: A Typical Case of the Mesoproterozoic Xiamaling Formation

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.