Characterization of Pore Structure and Thermal Evolution of Shale

The porosity and pore structure of mud shales are crucial for shale oil and gas storage and transportation. Although the change of organic matter state in thermal evolution has a significant effect on pore formation, the mechanism and main controlling factors still need to be deeply explored. In this paper, we selected the shale of the Yanchang Formation in the southeastern Ordos Basin as an object, and used low-pressure N₂/CO₂ adsorption, He density method, high-pressure mercury compression, and field FE-SEM to deeply investigate the relationship between thermal evolution and pore development in organic matter-rich shale. The experimental results show that FE-SEM observation reveals that the organic matter pores of the shale in the Long 7 section of the Extension Formation show four developmental states, in which the organic matter in the undeveloped pores is mainly of higher plant origin, solid asphalt and laminated organic matter. The organic matter with highly developed pores is nanofibrous, similar to extracellular polymers. Comparison of thermal simulation experiments shows that the high-pressure semi-closed system has stronger hydrocarbon drainage strength than the closed glass tube system. In the oil generation stage, the pore volume of both systems decreased, mainly due to oil filling; while the oil cracking stage was the main pore development period, especially in the high-pressure semi-closed system; Simulation experiments with different organic matter types and contents showed that the mud shale with better organic matter type and TOC content was more prone to hydrocarbon drainage and pore growth. Compared with type II/II mud shale, type I and II are more likely to form macropores, and compaction has an inhibitory effect on the formation of macropores. This study reveals the close relationship between the pore development of mud shale and its thermal evolution of organic matter, which provides an important reference for shale oil and gas exploration.