Molecular structure characterization of kerogen in contact metamorphic shales: Insights into the effect of graphitization on organic matter pores

Abstract

The adjustment of organic matter (OM) molecular structure due to graphitization should be of great significance to the evolution and preservation of OM pores at an extremely high maturity stage. In this study, the lower Paleozoic Silurian Longmaxi contact metamorphic shales from the north section of the Xuefeng Mountain tectonic zone of the South China block were taken as an example and the molecular structure of kerogen in these postmature Longmaxi shales was analyzed using laser Raman microprobe, transmission electron microscopy, and Fourier transform infrared spectroscopy techniques. The OM pore structures of shale samples with different thermal maturity were compared using CO2/N2 adsorption and field emission scanning electron microscopy analysis. The results indicate that postmature kerogens have entered the transition stage from amorphous carbon to crystallized graphite. The orderliness and crystallinity of carbon atom layers in these postmature samples continuously increase with maturity, accompanied by decreased disordered graphite lattice. The minimum d (Å) value of carbon layers is close to 0.335 nm, indicating that the kerogens have partially reached the ideal graphite state. Around or between clay platelets, OM develops numerous bubble pores that have diameters of 50 to 200 nm, displaying high plane porosity and multilayer superposition. This OM type has morphological characteristics resembling artificial porous graphite. The OM porosity contributes significantly to the total porosity, and decreases with progressing graphitization. The thermodynamic stability of OM allows homogenization of pores after graphitization, as heterogeneity decreases and orderliness increases. However, graphitization could reduce the compressive capacity of pores, which is not conducive to OM pore preservation.