Contribution of clay on storage and adsorption capacities of the Middle Permian marls, South China

A significant gas show appeared in JY66-1 well when it was drilled into the Middle Permian limestone-marl alternation interval, high-resolution scanning electron microscopic (SEM) observation shows that clay-related pores were developed extensively. There have been a lot of comprehensive studies on the origin of organic-matter pores and their pore size distribution, but few studies on the scales of clay-related pores and their contributions to the total pore volume and surface area. In this paper, we conducted thin section and SEM observation, mercury intrusion, low pressure N2 adsorption and high-pressure CH4 adsorption experiments, to characterize the pore volume and surface area of Mg-clay-bearing samples. The result suggests that the pore volume of clay-related pores mainly derives from the Intra-aggregate pores of 5–100 nm pore size, while the absolute and relative contributions of Interlayer pores (smaller than 2 nm) and Intra-tactoid pores (2–5 nm) to the total pore volume are small. Based on TOC contents and inorganic mineral compositions, a multiple regression analysis was carried out on the contributions of organic-matter and clay-related pores to pore volume in the pore-diameter ranges of smaller than 2 nm, 2–5 nm, 5–100 nm and larger than 100 nm. Although the contribution of clay-related pores (including inter-aggregate pores, Intra-aggregate pores, Intra-tactoid pores, Interlayer pores) to the total pore volume is comparable to that of organic-matter pores, the latter still contributes most of the pore surface area and CH4 adsorption site. It can be interpreted by the fact that the specific surface area of the dominant Intra-aggregate pores is significantly smaller than that of organic-matter pores. The conclusion of this study can be applied for the analyses of storage and adsorption capacity in all gas shales containing both organic and clay minerals.