Structure and Fractal Characteristics of Nano-Micro Pores in Organic-Rich Qiongzhusi Formation Shales in Eastern Yunnan Province

Eastern Yunnan in southern China has become an important area for studying Qiongzhusi Formation shale, which is an organic-rich shale that formed in a deep-water shelf environment. Using field emission scanning electron microscopy and low-pressure N-2 adsorption, we qualitatively and quantitatively analyzed the structural characteristics of nano-micro pores in this formation. Our results show that the pores in the Qiongzhusi Formation shale are mainly organic, intragranular and intergranular. We quantitatively described the pore structure using surface fractal models and the Image J software for image processing. The mean Mandelbrot fractal dimension and the box-counting fractal dimension were different, with values of 1.2543 and 1.5203 for organic pores, 1.4520 and 1.5781 for intragranular pores, and 1.5111 and 1.5171 for intergranular pores, respectively. The organic pores had the most regular pore morphology and a moderately homogenous pore distribution. The intragranular pores had moderately irregular morphology and the most homogenous distribution. The intergranular pores had the most irregular morphology and the least homogenous distribution. Based on the pore development characteristics of the Qiongzhusi Formation shale, we divided the shale minerals into three categories: easily dissolved minerals, braced minerals, and clay minerals. Low-pressure N2 adsorption results showed that the organic matter content, easily dissolved mineral content and braced mineral content determine the pore volume and specific surface area. In addition, Frenkele Halseye Hill fractal analysis indicated that the TOC and easily dissolved mineral content are most important for pore sizes <4.5 nm and the easily dissolved mineral and braced mineral content are important for pore sizes >4.5 nm. The negative correlation between the TOC content and easily dissolved mineral content and the pore heterogeneity result from the relatively simple structures and uniform distributions of organic and intragranular pores. The positive correlation between the braced mineral content and the pore heterogeneity results from the complex structure and non-uniform distribution of intergranular pores. In addition, shale samples with larger fractal dimensions have lower porosity and higher permeability. Thus, the fractal dimension reflects the gas storage capacity and migration ability of shale. In this study, we successfully characterized the structural and fractal characteristics of nano-micro pores in Qiongzhusi Formation shale. Our results have implications for understanding the occurrence and enrichment of shale gas.