Evolution of pore structure in organic shale with type III kerogen and high kaolinite content in Ningwu Basin

Special deposition environment makes organic-rich shales in Ningwu Basin have type III kerogen and high kaolinite content, which are also famous as the kaolinite ore. The specific composition of shale in Ningwu Basin can change the pore structure and thus, influence the shale gas storage and transport. This study focuses on the pore structure and its evolution in shales with type III kerogen and high kaolinite content. In this study, 14 Upper Paleozoic shale samples, whose total organic matter contents (TOC) range from 0.39% to 5.91% and maturities (represented by vitrinite reflectance) range from 1.22% to 2.06%, were collected. Scanning electron microscopy (SEM), high-pressure mercury injection, and low-temperature N2 adsorption experiments were used to analyze pore structure of these shale samples. Results show that when the TOC content is smaller than 1.4%, the kaolinite content decreases linearly and quartz content increases linearly with increasing the TOC content. In contrast, when TOC content is > 1.4%, the clay content tends to increase with increasing TOC. Based on the SEM images, organic pores and clay pores were identified in shale samples with type III kerogen and high kaolinite content. During the maturation process, the kaolinite content decreases and illite content increases with increasing the vitrinite reflectance. At the same time, the pore volume and pore surface area both increase with increasing the vitrinite reflectance, and it may be because more organic pores and clay pores in the illite were generated during the maturation process. This study can provide further understandings of shale gas accumulation in shale with type III kerogen and high kaolinite content.