Compositional controls on pore-size distribution by nitrogen adsorption technique in the Lower Permian Shanxi Shales, Ordos Basin

Abstract

The purpose of this article is to quantitatively reveal the compositional controls on pore-size distribution (PSD) within the transitional Lower Permian Shanxi Shales.Our results demonstrate that the average pore-size (APS) of Shanxi Shales is approximately 12–24 nm and that the size is essentially controlled by clay minerals. The ‘closed area’ of the hysteresis loop, the most prevalent feature in microporous materials, can qualitatively reflect the PSD. A rule regarding the PSD is confirmed that the larger the closed area is, the smaller the pores are, and vice versa. Type III kerogen in the Shanxi Shales is shown to be composed of nearly nonporous organics, whereas the clay minerals possess large amounts of inter-layer pores; therefore, the PSD of the Shanxi Shales is determined by the clay-hosted pores, whose pore-size intervals differ by clay mineral type. Specifically, I/S-hosted pores possess both intra-aggregate pores with a characteristic 3–4 nm pore size and inter-aggregate pores with a characteristic 50–60 nm pore size; of these, the former predominates. On the other hand, the pores associated with the other clay mineral types (including kaolinite, illite, and chlorite) are mostly inter-aggregate pores with a characteristic 50–60 nm pore size. Overall, our research contributes to understanding the physical properties of the transitional Shanxi Shales and revealing their nano-scaled pore-size distribution and variation.