Pore network changes in opaline siliceous shale during diagenesis

The siliceous shale of the Nenjiang Formation in the Songliao Basin and the Wufeng-Longmaxi formations in the Sichuan Basin were analyzed using X-ray diffraction and N₂ isothermal adsorption to study pore network changes during diagenetic evolution. The specific surface area and pore volume of shale with a low evolution degree are much higher than those with a high evolution degree. Pores larger than 2 nm in lower evolved shale mainly concentrated in 2-3 nm and 20-30 nm, while those smaller than 2 nm mainly concentrated in 0.6-0.8 nm, and their pore volume is mainly contributed by 10-30 nm. Pores larger than 2 nm in higher evolved shale mainly concentrated in 2-3 nm, while those smaller than 2 nm mainly concentrated in 0.6-1 nm, and their pore volume is mainly contributed by 2-30 nm. Compared with siliceous shale in the Wufeng-Longmachi formations, nano pores were greatly reduced during diagenesis, and pore volume reduced to 1/3-1/10 of the original. Pores of 2-30 nm are the major contributor to pore volume in the Wufeng-Longmaxi formations, and the next is 30-200 nm, and pores smaller than 2 nm almost disappeared, which indicated that the influence of diagenesis on various size pores is different.