VARIATIONS OF PORE STRUCTURE IN ORGANIC-RICH SHALES WITH DIFFERENT LITHOFACIES FROM JIANGDONG BLOCK, FULING SHALE GAS FIELD, SW CHINA: INSIGHTS INTO GAS STORAGE AND PORE EVOLUTION

The pore structure of the gas shale reservoir strongly influences the gas occurrence, migration and enrichment, which has become an important fundamental issue for shale gas exploration and production. To investigate the variations in the pore structure of organic-rich shale and provide some insights into gas storage and pore evolution, this work focused on three dominant types of shale lithofacies in the Wufeng (WF) and Longmaxi (LMX) Formations in the Jiangdong Block of Fuling Shale Gas Field. TOC content, X-ray diffraction, high-resolution scanning electron microscope, and low-temperature gas (N₂, CO₂) adsorption experiments were conducted to qualitatively and quantitatively characterize the nano-scale pore structure of organic shale in the study area. Based on the N₂ adsorption experiment and fractal theory, the fractal dimension of micropore (<2nm), mesopore (2–50nm) and macropore (50–300 nm) were respectively calculated, and the relationships between pore structure parameters and mineral composition, TOC content, and fractal dimension of shale in different lithofacies were discussed.

Results show that both the organic pores, inorganic pores (intergranular pores and intragranular pores), and microfractures are developed in the WF and LMX shales. The pore size distribution is multi-peaked, and the total pore volume is mainly provided by mesopore and macropore (contribution rate is about 80%), while the pore specific surface area is mainly provided by micropore (about 70%). Due to the various contents of clay minerals, quartz, and TOC content, shale samples with different lithofacies show different pore evolution characteristics, resulting in strong heterogeneity and complex pore structure. Pores in the WF and LMX formations have obvious multifractal and are characterized by their corresponding fractal dimension values for micropore, mesopore, and macropore. The fractal dimension of micropore in argillaceous shale is the largest than other shales, suggesting a rougher micropore surface in the argillaceous shale. In contrast, the fractal dimension values of mesopores in mixed shale and siliceous shale are comparable, but are larger than those of argillaceous shale, indicating the much complex mesopore structure in mixed shale and siliceous shale, which could be conducive for gas adsorption and accumulation. The fractal dimensions of macropore in siliceous shale shales are relatively larger and can provide a considerable number of pore volume and pore specific surface area for the storage of free and adsorbed gas.