Characterization of pore structure and fractal dimension of Paleozoic shales from the northeastern Sichuan Basin, China

Abstract

The heterogeneity of pore structure greatly affects gas sorption and flow in shales. The pore structure of shales from the Upper Permian Dalong Formation and Lower Silurian Longmaxi Formation in northeastern Sichuan Basin were investigated using low-pressure N2 adsorption and field emission scanning electron microscopy (FE-SEM) techniques. The effect of the material compositions on fractal feature, using the Frenkel-Halsey-Hill (FHH) method from low-pressure N2 adsorption data, was also discussed. The results show that organic pores, intraparticle pores, and microfractures are well developed in Longmaxi shales, but only a certain amount of intraparticle pores within pyrite framboids or calcite grains formed by partial dissolution exist in Dalong shales. The specific surface area and pore volume of Dalong shales vary from 2.20 to 3.52 m2/g and 1.11–1.77 cm3/100 g, respectively, caused by low maturity, whereas those of Longmaxi shales are in the range of 17.83–29.49 m2/g and 2.53–5.18 cm3/100 g, respectively, which mainly constitute high-maturity kerogen.The fractal dimensions range from 2.474 to 2.534 for Dalong shales, and from 2.694 to 2.76 for Longmaxi shales. The differences in fractal dimensions of the two sets of shales seem to be related to the maturity of kerogen, which controls the development of organic pores. TOC is also a controlling factor of fractal dimension, exhibited by negative correlation between TOC content and fractal dimension, for both Dalong and Longmaxi shales. The fractal dimension increases with increasing soluble hydrocarbon (S1) for Longmaxi shales, but decreases for Dalong shales, due to S1 which may fill in the shale pores and affects the uniformity of pore distribution. Weak positive correlations between the fractal dimension and the content of calcite and feldspar suggest that the increase in carbonate and feldspar could slightly increase the heterogeneity of shale pores. There are negative relationships between the fractal dimension and the specific surface area and pore volume for Longmaxi shales, but no relationships for Dalong shales, illustrating that more micropores and mesopores in Longmaxi shales could decrease the heterogeneity of the pore system.