Enhanced understanding of carbonate-rich shale heterogeneity through multifractal characterization based on N2 adsorption data: A case study of the Permian Wujiaping Formation in the Sichuan Basin

Abstract

The carbonate-rich shale of the Permian Wujiaping Formation in Sichuan Basin exhibits significant heterogeneity in its lithology and pore structure, which directly influence its potential for shale gas extraction. This study assesses the factors that govern pore heterogeneity by analyzing the mineral composition of the shale, as well as its pore types and their multifractal characteristics. Three primary shale facies—siliceous, mixed, and calcareous—are identified based on mineralogy, and their multifractal characteristics reveal strongly heterogeneous pore structures. The brittleness of siliceous shale, rich in quartz and pyrite, is favorable for hydraulic fracturing; while calcareous shale, with higher levels of calcite, exhibits reduced brittleness. Multifractal analysis, using nitrogen adsorption isotherms, reveals complex pore structures across different shale facies, with siliceous shale showing better pore connectivity and uniformity. The types of pores in shales include organic matter pores, interparticle pores, and intraparticle pores, among which organic matter pores are the most abundant. Pore size distribution and connectivity are notably higher in siliceous shale compared to calcareous shale, which exhibit a predominance of micropores and more isolated pore structures. Pore heterogeneity of the carbonate-rich shale in the Wujiaping Formation is primarily governed by its intrinsic mineral composition, carbonate diagenesis, mechanical compaction, and its subsequent thermal maturation with the micro-migration of organic matter. This study highlights the importance of mineral composition, especially the presence of dolomite and calcite, in shaping pore heterogeneity. These findings emphasize the critical role of shale lithofacies and pore structure in optimizing shale gas extraction methods.