Impact of tectonism on pore type and pore structure evolution in organic-rich shale: Implications for gas storage and migration pathways in naturally deformed rocks

Tectonism significantly affects the pore type and pore structure evolution of organic-rich shale. The Lower Cambrian Lujiaping Shale in the northeastern of the upper Yangtze area is a significant marine shale being studied for shale gas exploration in South China. Here, we use two groups of Lujiaping Shale samples, such as brittle deformation shales (BDS) and ductile deformation shales (DDS), to investigate the relationship between tectonism and pore structure using a combination of mineralogy, organic geochemistry, low-pressure nitrogen adsorption, low-field nuclear magnetic resonance (NMR), optical microscopy, and high-resolution scanning electron microscopy (SEM). Results indicate that the samples are quartz, carbonate, and clay rich. Total organic carbon (TOC) content ranges between 2.27% and 5.88%, and vitrinite reflectance (R-o) measured between 2.95% and 4.11%. Helium porosity lies between 3.02% and 4.66% in BDS and 3.16%-4.29% in DDS. In addition, four major pore types can be identified and classified by their formation mechanism, location, geometry, and connectivity, which we named type I (pore within or between organic and mineral particles), type II (pore in admixtures), type III (micro-channel), and type IV (micro-fracture). Under the BDS, the interparticle (interP) pores, micro-channels, and micro-fractures are more developed, the macropores (>50 nm) increase, the connectivity and porosity are enhanced. The intraparticle (intraP) pores, micro-channels, and micro-fractures are dominant in DDS, the micropores (< 2 nm) increase and the connectivity is enhanced, whereas the porosity decreases. Meanwhile, pore network within the admixtures of organic matter (OM) and clays can also contribute to gas storage and migration pathways in both BDS and DDS, in addition to other pore types. However, OM pores are poorly-developed in samples. Our findings provide a key component for further evaluation of the exploration and development potential of the Lujiaping Shale in the complex tectonic areas of South China.