Organic nanopore structure and fractal characteristics of Wufeng and lower member of Longmaxi shales in southeastern Sichuan, China

Organic nanopores in shale gas reservoirs are the main storage space for free gas and adsorbed gas and directly affect the occurrence mode and movement of shale gas. In this study, field emission-scanning electron microscopy (FE-SEM), low-pressure gas (N-2 and CO2) adsorption and mercury injection capillary pressure (MICP) analyses were performed and interpreted with statistical and fractal analyses to investigate the organic pore structure in the Upper Ordovician Wufeng shale (O(3)w) and the Lower Silurian lower member of Longmaxi shale (S(1)l). It was found that organic pores in 15 samples from 8 layers of the Wufeng-Longmaxi shale section are usually developed in discrete organic matter particles, organic matter associated with clay minerals and organic matter associated with pyrite framboids. The size, shape and quantity of organic pores vary greatly. The organic pores in Wufeng Formation (layer 1) are densely developed in organic matter particles with relatively small sizes and irregular shapes. The organic pores in the lower part of the lower member of Longmaxi Formation (layers 3, 4 and 5) are less developed and have relatively larger pore sizes with an elliptical shape. In contrast, the organic pores in the upper part of the lower member of Longmaxi Formation (layers 6, 7, 8 and 9) are the least developed, with elliptical shapes and pore sizes in between the two cases above. A higher value of fractal dimension refers a more complex form and this value of whole organic pores with full range of sizes (D-wop) is the largest and followed by organic macropores (D-op (> 50)), organic pores with pore sizes between 4 nm and 50 nm (D-op4.50), and organic pores with pore sizes less than 4 nm (D-op < 4). The fractal dimension of the upper part of the lower member of Longmaxi Formation (layers 6, 7, 8 and 9) is lower than that of both the Wufeng Formation (layer 1) and the lower part of the lower member of Longmaxi Formation (layers 3, 4 and 5). Possible causes leading to pore complexity and heterogeneity include the TOC content, pore size, mineral composition and tectonic effect. Wufeng shale is located at the bottom of Wufeng-Longmaxi shale weakness zone and had experienced more tectonic compression, nappe-slip and reconstruction, which may be the main reason that organic nanopores in Wufeng shale (layer 1) are more complex than other layers.