Impact of Brittle Deformation on Pore Structure Evolution in Shale: Samples Collected from Different Fault Positions

Abstract

Tectonic deformation has an important influence on the pore structure and physical properties of shale, and different parts of the same structural style also have great differences. In this study, eight shale samples were collected from two different faults based on the distance from each fault. Comparative analysis of the micro/nanopore structural characteristics and physical properties of these samples was carried out by scanning electron microscopy, liquid nitrogen adsorption, and carbon dioxide adsorption. The results show that as brittle deformation is enhanced (from far away from the fault to inside the fault), the number of organic pores decreases overall, and the connectivity of microfractures and different pore types increases. The total pore volume and total pore specific surface area of shale both increase, the pore volume of mesopores decreases by 31%, and the macropores increase rapidly by 29%. The storage capacity of shale-related folds is higher than that of shale in faults, which is more conducive to the adsorption of shale gas. The permeability of shale in faults is higher than that of shale-related folds, which is more conducive to the seepage and migration of shale gas. The organic layer structure, which is a unique and very rare microstructure at the shale fault site (under shear), was observed inside the fault. It is believed that the organic matter in the shale at the fault site has transformed from the amorphous state at the initial stage of formation to the orientation of the aromatic lamellae to stretching and extension to an increase in the pore size and the enhancement of connectivity. In addition, inorganic nanoparticles were also observed at the fault site. Both structures are important for the small pores in the fault and the increase in reservoir space. When the fault forms a closed environment or tectonic activity ceases in the later period, the early fault location likely becomes a potential high-quality hydrocarbon generation reservoir. These results help improve the understanding of the physical properties of shale reservoirs and shale gas reservoir migration within fault structures and are of great significance for shale gas exploration, development, and prediction.