Pore structure and adsorption behavior of shale gas reservoir with influence of maturity: a case study of Lower Silurian Longmaxi formation in China

Knowledge of pore structure and adsorption capacity provides guidance for better studying the origin, hydrocarbon distribution, and productivity of shale gas reservoir. In this study, pore structure characteristics of six shale core plugs with different maturity from the Lower Silurian Longmaxi formation in south China were investigated using the Rock-eval analysis, X-ray diffraction, total organic carbon (TOC) content test, and scanning electron microscope (SEM) observation. To further investigate the influence of maturity, the adsorption behavior of gas shale was experimentally measured, with the maximal pressure being 20 MPa. Rock-eval analysis indicates that Ro is 0.67 similar to 1.34%. SEM results show that organic matter (OM) pores are abundant in high-maturity shale sample. The OM pores are mainly irregular to elliptical in shape, the size is 8 similar to 100 nm. The TOC content is 0.16 similar to 4.21% and shows a positive correlation with the BET surface area. A negative relationship exists between TOC content and average pore diameter, which indicates that abundant nanometer pores are related to the OM. A noticeable characteristic in the pore size distribution curve is that the content of micropores (pore width < 2 nm) increases with the increasing TOC content. Additionally, the thermal maturity results in significant difference in methane adsorption capacity. Maximal adsorption capacity of shale samples is also lineally correlated with TOC content, which increases with maturity. This study provides a quantitative understanding of how maturity affects pore structure and adsorption behavior of shale gas reservoir.