Lithofacies classification and its effect on pore structure of the Cambrian marine shale in the Upper Yangtze Platform, South China: Evidence from FE-SEM and gas adsorption analysis

- TOC content has the most significant positive effect on shale pore structure.
- Effect of calcareous mineral on pore structure is negative.
- Organic-rich siliceous shale has the largest storage capacity for gas adsorption.
- Organic-poor calcareous shale has the smallest storage capacity for gas adsorption.

Lithofacies classification and storage capacity are of great significance for shale gas. In this study, shale lithofacies classification and its effect on pore structure from the Niutitang shale in the Upper Yangtze Platform were analyzed by experiments of field emission scanning electron microscope (FE-SEM) and gas adsorption analysis. Results show that Niutitang shale can be divided into 12 kinds of shale lithofacies on the basis of TOC content and mineral composition. Pore types and size distribution are significantly distinct of different shale lithofacies since TOC content and mineral compositions do affect shale pore structure differently in their own ways. Specifically, TOC content has the most significant positive effect on shale pore structure, indicating that the higher the TOC content is, the larger the pore volume and surface area are. Negative correlation has been found between calcareous mineral and pore volume and surface area, indicating the decrease of pore volume and surface area with the increase of calcareous mineral content. Compared to argillaceous mineral, siliceous mineral has stronger effect on pore structure. Pore types are also distinctly different in different shale lithofacies which also significantly affect shale gas storage capacity. Organic matter pore is the most abundant pore type in the organic-rich siliceous shale while no OM pore has been found in the organic-poor calcareous shale. Above all, the organic-rich siliceous shale has the largest pore surface area as well as gas storage capacity. While the organic-poor calcareous shale has the smallest gas storage capacity with the poor pore volume and surface area. It would be an effective method to optimize the favorable shale lithofacies in guiding the exploration and development of shale gas.