Characterization of Full-Sized Pore Structure and Fractal Characteristics of Marine–Continental Transitional Longtan Formation Shale of Sichuan Basin, South China

Despite the recent significant progress reviewed in marine and continental shale gas reservoirs, there is no general investigation concerning marine–continental transitional shales, especially in the research of the pore structure and fractal characteristics of marine–continental shale reservoirs. In this study, the pore structure characteristics of 12 typical marine–continental transitional shale samples of the Upper Permian Longtan Formation collected from Southern Sichuan Basin were analyzed, combining techniques of total organic carbon (TOC) content determination, X-ray diffraction, field emission scanning electron microscopy, high-pressure mercury intrusion porosimetry, and low-pressure N₂/CO₂ adsorption. And the pore fractal characteristics of shales were also analyzed using the Frenkel–Halsey–Hill model with N₂ adsorption data. Not only the effects of TOC content and mineralogical compositions for pore structure parameters and fractal dimensions were analyzed but also the relationships between pore structure parameters and fractal dimensions were discussed. The results showed that the macropores (>50 nm) mainly develop within or between clay minerals, while the micropores (<2 nm) and mesopores (2–50 nm) commonly develop within intraparticle pores and organic matter (OM) pores. Marine–continental Longtan shales develop with the characteristic of various pore types, complicated pore structure, and obvious heterogeneity, which mainly consist of silt-shaped pores and ink bottleneck pores. Micropores provide the dominant specific surface area (SSA), whereas meso- and macropores occupy the majority of the pore volume. Both uni- and multimodal pore-size distributions (PSDs) in shales were analyzed; the PSDs of macropores increase rapidly to above 5 μm or below 30 nm, but a flat trend is present within the range of 30 nm to 5 μm; the PSDs of mesopores develop with a peak around 4 nm; and the PSDs of micropores develop with a minor peak around 0.35–0.40 nm and two major peaks around 0.45–0.50 and 0.55–0.60 nm. Micropores and macropores were the most important storage space for shale gas, and the micropores play a pivotal role in shale gas adsorption by providing the dominated SSAs. Two fractal dimensions (D₁, 2.523–2.696; D₂, 2.754–2.886) are positively associated with the pore volumes and SSAs of shales, and the micropores are the dominant factor for controlling the pore structure heterogeneity. The small pores of marine–continental shales mainly consist of OM pores, which are considerably affected by TOC content; large pores are mainly produced in clay minerals. TOC content has a significant positive relationship not only with the pore volumes and special surface areas of micropores but also with the fractal dimensions; and the enrichment of the clay minerals will contribute to the production of macropores (including fractures) while the brittle ones play the reverse role.