Pore evolution of organic matter-rich continental shales: a case study of Lucaogou Formation shales in the southern margin of the Junggar Basin

To investigate the pore evolution characteristics of high-maturity continental shale oil and gas reservoirs, thermal simulation experiments were carried out on lacustrine low maturity shale samples from the Middle-Upper Permian Lucaogou Formation in the southern margin of the Junggar Basin, replicating the thermal evolution process of organic-rich, sapropelic continental shale. Based on experimental analyses such as argon ion polishing scanning electron microscopy, X-ray diffraction, low-temperature nitrogen adsorption, and low-temperature carbon dioxide adsorption, this study quantitatively analyzed the distribution of pore structure, mineral composition and content of the shale across different temperature points. Furthermore, the evolution laws of pores, minerals, and organic matter in shales were discussed, and a pore evolution model was established. The results indicate that during the thermal evolution process, pore volume, pore specific surface area, and face porosity of the shale all exhibited a trend of initially increasing and then decreasing, with the maximum values located approximately at 270 °C (R O = 1.3%). Influenced by the pyrolysis of organic matter and transformation of minerals, the pore evolution pattern in the area can be divided into two processes: 1) the low-to middle-maturity stage (0.5% ≤ R O < 1.3%), organic matter undergoes pyrolysis and microfractures form inside quartz, resulting in the formation of pores and an increase in shale pore volume; and 2) the high-to over-maturity stage (R O ≥ 1.3%), the generation of secondary minerals feldspar and expand of clay minerals block the pores, resulting in a decrease in shale pore volume.