Comprehensive Understanding of OM-Hosted Pores in Transitional Shale: A Case Study of Permian Longtan Shale in South China Based on Organic Petrographic Analysis, Gas Adsorption, and X-ray Diffraction Measurements

Marine-continental transitional (hereinafter referred to as transitional) Permian shales are the important targets for shale gas in China. However, the nature and formation of organic matter (OM)-hosted pores in this shale facies have not yet been studied in detail from the perspectives of organic petrology and geochemistry. In this paper, we selected typical transitional shale samples from the Permian Longtan Formation in the northwestern Guizhou province to investigate the development of OM-hosted pores mainly through organic petrographic analysis, gas adsorption, and X-ray diffraction (XRD) measurements. Petrographic observations under an optical microscope and a scanning electron microscope (SEM) reveal that the OM in the Longtan shale is composed predominantly of type III kerogens (vitrinite and inertinite), with minor amounts of amorphous solid bitumen. SEM imaging reveals that secondary OM pores (as a result of devolatilization of gaseous hydrocarbons) were very rare in Longtan shale, being only occasionally observed in individual type III kerogens with a bubble-like morphology. This phenomenon is most likely attributed to the chemical-inert body of type III kerogens that cannot depolymerize during thermal degradation. On the other hand, the high methane adsorption values and their strong linear correlation with total organic carbon contents in the Longtan shale indicate that the type III kerogens contain numerous SEM-invisible micropores (<2 nm). By virtue of the combined N-2 and CO2 adsorption on the type III kerogens separated from the bulk Longtan shale, we demonstrated the existence of SEM-invisible OM micropores (distributed at 0.34-0.36 nm) and quantified their contribution to the surface area and pore volume of OM as 87 and 13%, respectively. XRD further correlated these micropores to the spacing between the aromatic rings of type III kerogens. As such, the interlayer pores distributed among the aromatic rings are responsible for the sorptive capacity of type III kerogens and the bulk Longtan shale. Overall, the revelation of OM-hosted pores in the transitional shales provides a further understanding of shale gas generation and its occurrence in China.