Coupled Control of Thermal Maturation and Microscopic Deformation on the Pore Structure Evolution of High-Maturity Coal-Bearing Shale

Abstract

Affected by the global low-carbon strategy, high-maturity shale, as one of the major sources of shale gas resources, exhibits important energy value and economic benefits. However, the microstructure evolution of high-maturity shales under thermal metamorphism and microscopic deformation remains unclear. In this study, we selected 20 high-maturity coal-bearing shales from the western Guizhou to reveal the evolution of shale microstructure during thermal maturation and the effects of microscopic deformation on pore distribution. The maximum pyrolysis temperature (T max) and vitrinite reflectance index (R o) values (averaging 578 °C and 3.08%, respectively) suggest that the Upper Permian Leping shale reaches the dry gas generation stage, and the average hydrogen index (HI) and carbon isotope of kerogen (δ13C) values indicate that all samples have already undergone intense hydrocarbon generation. Brittle minerals represented by quartz have negative relationships with the pore volume (PV) and surface area (SA), and ductile clay minerals significantly improve the PV and average pore size (AP). Layered clay minerals, especially illite/smectite (I/S), are more conducive to the development of nanopores in a shale matrix. Both H/C and O/C ratios are positively correlated with δ13Corg values, suggesting that the variation in organic elements in kerogen is closely related to carbon isotope fractionation. With the enhancement of thermal evolution, the correlation between AP and T max is negative at the first stage and then positive at the second stage. At the high maturation stage, the accumulation of gas leads to the expansion of the organic matter surface and the formation of gas pores, resulting in a rapid increase in AP. The distribution of the pore–fracture system is closely related to the microscopic deformation. Compared with the contact zone of ductile deformation, more microfractures can be observed in the contact zone between brittle deformation and ductile deformation.