Microstructure characterization of kerogen in mature shale: Molecular investigation of micropore development

Micropores (<2 nm) are commonly well developed in shale kerogen, which plays an important role for generation and transport of gas in shales. However, the mechanism of micropores development in kerogen structure has not been fully investigated. In this study, we provide an improved methodology to construct 3-D models of kerogen structure, then we construct such molecular models of kerogen for two well-known shales (the Marcellus shale from USA and Shanxi Formation shale from China), and finally we discussed the influences of chemical composition and disorder degree on the formation of micropores in the mature kerogens. Results show that loss of aliphatic structures and the increase in aromatic structures are favorable for the development of micropores. By comparing the kerogen models of M-1 and SX-2, we found that the morphology of micropores in aliphatic structures is more complex than that in aromatic structures. Analyses from ¹³C Nuclear Magnetic Resonance (NMR) and Raman spectroscopy indicate that apart from chemical compositions, spatial arrangement of structural groups can also affect the development of micropores. For mature kerogen with a large proportion of aromatic rings, disordered aromatic rings are favorable for the development of micropores.