Quantification of the influences of radiolarian fossils on the pore structure of Wufeng-Lungmachi gas shales (Ordovician-Silurian) in the Sichuan Basin, South China

Understanding the differences among shale pore systems is crucial to exploring the enrichment mechanism of shale gas. In this study, we found that radiolarian micro-fossils are widely distributed in the overmature marine organic-rich Wufeng-Lungmachi formations surrounding the Sichuan Basin, South China. This enables an investigation on the full-scale pore structure characteristics of the two formations by using a combination of qualitative methods (field emission scanning electron microscopy (FE-SEM)) and quantitative methods (low-pressure N-2 and CO2 adsorption and high-pressure mercury intrusion porosimetry (MIP)). The results show that micropores (accounting for 75.81%) and mesopores (accounting for 63.56%) are mainly responsible for the total surface area and the total pore volume, respectively. Furthermore, organic matter significantly contributes to the shale surface area, and the intercrystalline pores of clay minerals make an important contribution to the pore volume of shale. Abundant biological pores can be discovered in radiolarian fossils, and these biological pores belong to mesopores and macropores. The investigation of gas adsorption shows that radiolarian-poor shale reflects the presence of slit-shaped pores, while radiolarian-rich shale indicates the presence of ink-bottle-shaped pores. In addition, despite similar TOC contents, clay minerals and brittle minerals, radiolarian-rich layers have larger pore volumes than radiolarian-poor layers. The results calculated using the threshold method with ImageJ software show that the average surface porosities of the organic matter, mineral matrix, and radiolarians are 15.69%, 1.84%, and 5.34%, respectively. Given that the quantitative calculation results in a greater contribution by the pores developed in radiolarians, the biological pores developed in radiolarians are beneficial for the enrichment of free gas.