Microstructural investigation of different nanopore types in marine-continental transitional shales: Examples from the Longtan formation in Southern Sichuan Basin, south China

Abstract

Variations in pore types, the complexity of pore structures, and strong heterogeneity complicate the thorough characterization of pore networks in marine–continental shales. In this study, a novel 2D image processing method was proposed, expounded, and verified to clarify the geometrical morphologies, formation mechanisms, the size distributions, proportions and the fractal characteristics of different types of pores in marine–continental shales. Six types of pores (inter-particle, intra-particle, organic matter (OM), inter-crystalline, dissolved pores, and micro-fractures) were classified and morphologically characterized. The different types of shale pores had distinct relatively restricted morphologies, formation mechanisms, and size ranges. Nano- and micron-sized pores were controlled by OM pores and micro-fractures, with the size ranging of 6–80 nm and 200–350 nm and 6–10 μm, respectively. Furthermore, the fractal dimension characteristics were investigated on the basis of the 2D binary images, and the results were verified by the fractal findings obtained from the N2 adsorption data. These fracture dimension results demonstrated that the Longtan shale pore structures had strong complexity and heterogeneity, which was strongly and positively controlled by micro-fractures, inter-crystalline and OM pores. In addition, the total organic carbon content strongly and positively influences the development and anisotropy of the nanoscopic pores, which were dominated by OM pores; while the clay minerals content has a minor effect on the total pore development and heterogeneity of shales but a significant effect on the dissolved pore development of shales, whereas brittle minerals inhibit total pore development and heterogeneity of shales but contribute greatly to the development of the inter-crystalline pores.