An experimental study of organic matter, minerals and porosity evolution in shales within high-temperature and high-pressure constraints

The development and evolution of porosity in organic-rich shales (ORSs) is critical to the commercial exploitation of shale oil and gas resources. In this paper, we present the results of high-temperature and high-pressure experiments on typical marine and lacustrine shales to investigate porosity changes. The samples were taken from the Proterozoic Xiamaling Formation in the North China Platform, the Permian Lucaogou Formation in the Junggar Basin, the Triassic Chang 7 member in the Ordos Basin, and the Silurian Longmaxi Formation in the Sichuan Basin, all in China. We found that the key factors influencing porosity evolution include the extent of compaction, abundance of organic matter, degree of thermal evolution, and organic matter–inorganic mineral framework. The effects of thermal evolution on the pore structure of high-mature shales are more obvious than those on low-mature shales. Although the porosity evolution is positively correlated with maturity, we found evidence for different porosity evolution in ORSs in the oil window. High-resolution scanning electron microscopy observations of experimental and actual core samples indicate that liquid hydrocarbon is adsorbed and dissolved in organic matter in the oil window, leading to swelling of the organic material. This explains why there are few organic matter pores in lacustrine shales in China. The pore structure evolution is similar for marine and lacustrine shales, suggesting that kerogen has a stronger influence on the porosity evolution of shale than does the depositional environment. The lower limit of vitrinite reflectance values (Ro) at which abundant organic pores develop is 1.5%–2.5%, and the degree of pore development in ORSs is highest when Ro values are 2.5%–3.0%. These results have important implications for shale oil and gas exploration.