Deciphering multiple controls on mesopore structural heterogeneity of paralic organic-rich shales: Pennsylvanian–Lower Permian Taiyuan and Shanxi formations, Weibei Coalfield, southwestern North China

Abstract

This study investigates the various factors that affect the mesopore structural heterogeneity of paralic organic-rich shales in the Pennsylvanian–Lower Permian Taiyuan and Shanxi formations, in the Weibei Coalfield of southwestern North China. This assessment is based on an integrated analysis of facies, mineralogical, petrographic, geochemical, and low-temperature N2 adsorption data. Facies analysis leads to the recognition of five facies associations that record the following depositional systems: medial epeiric shelf (FA1), proximal epeiric shelf (FA2), open-coast tidal flat (FA3), river-dominated delta (FA4), and coastal lake (FA5). Shales from the different depositional facies are classified into five different lithotypes: fossiliferous calcareous shale (FCS), fossiliferous mixed shale (FMS), argillaceous siliceous shale (ASS), illite-dominated argillaceous shale (IAS), and kaolinite-dominated argillaceous shale (KAS). FCS appears to be confined to the carbonate shelf in the medial epeiric shelf. Its mesopore structure is severely damaged by extensive micritization and local micrite recrystallization, and only a few plate-like large mesopores (4.34–50 nm) between the carbonate grains survived. FMS is typical of seaward flank deposits of the proximal epeiric shelf. Although abundant calcite cements are formed by the calcification of aragonite shells, FMS still has a few inkbottle-shaped mesopores due to the input of terrigenous clastic sediment. IAS is widely developed in the open-coast supratidal marsh due to volcanic ash kaolinization and kaolinite illitization. Illite-smectite group clays determine the presence of fine mesopores (3–4.34 nm), which are the main contributors to the specific surface area. Thus, IAS has the most abundant homogeneous fine mesopores, as well as the largest specific surface area and pore volume. Conversely, KAS is formed in the delta-plain interdistributary swamp and has a dense structure. ASS is mainly composed of terrigenous clasts and can be formed in multiple environments. Silt-sized brittle minerals determine the number of large mesopores, which are conducive to the total pore volume. Therefore, ASS has a bimodal pore size distribution, as well as medium pore structure parameters and fractal dimensions. This study contributes to a better understanding and prediction of the mesopore structure evolution of organic-rich shales that formed in various paralic sub-environments.