A comparative study of the specific surface area and pore structure of different shales and their kerogens

Abstract

The pore structures and controlling factors of several different Paleozoic shales from Southern China and their kerogens were studied using nitrogen adsorption and scanning electron microscopy methods. The results indicate that: 1) The specific surface area is 2.22–3.52 m2/g and has no correlation with the TOC content of the Permian Dalong Formation shales, nanopores are extremely undeveloped in the Dalong Formation kerogens, which have specific surface areas of 20.35–27.49 m2/g; 2) the specific surface area of the Silurian Longmaxi Formation shales is in the range of 17.83–29.49 m2/g and is positively correlated with TOC content, the kerogens from the Longmaxi Formation have well-developed nanopores, with round or elliptical shapes, and the specific surface areas of these kerogens are as high as 279.84–300.3 m2/g; 3) for the Niutitang Formation shales, the specific surface area is 20.12–29.49 m2/grock and increases significantly with increasing TOC and smectite content. The Niutitang Formation kerogens develop a certain amount of nanopores with a specific surface area of 161.2 m2/g. Oil shale was also examined for comparison, and was found to have a specific surface area of 19.99 m2/g. Nanopores are rare in the Youganwo Formation kerogen, which has a specific surface area of only 5.54 m2/g, suggesting that the specific surface area of oil shale is due mainly to the presence of smectite and other clay minerals. The specific surface area and the number of pores present in shales are closely related to TOC, kerogen type and maturity, smectite content, and other factors. Low-maturity kerogen has very few nanopores and therefore has a very low specific surface area, whereas nanopores are abundant in mature to over-mature kerogen, leading to high specific surface areas. The Longmaxi Formation kerogen has more developed nanopores and a higher specific surface area than the Niutitang Formation kerogen, which may be due to differences in the kerogen type and maceral components. A high content of smectite may also contribute to shale surface area. The pore volume and specific surface area of low-maturity kerogens are mainly attributable to pores with diameters above 10 nm. By contrast, the pore volume of mature kerogens consists predominantly of pores with diameters above 10 nm with some contribution from about 4 nm diameter pores, while the specific surface area is due mainly to pores with diameters of less than 4 nm. Through a comparative study of the specific surface area and pore structure characteristics of different shales and their kerogens, we conclude that the Longmaxi Formation shales and Niutitang Formation shales have greater sorption capacities than the Dalong Formation shales.