Quantitative characterization and controlling factors of organic pores in deep marine shale: A case study of Wufeng–Longmaxi Formation, South Sichuan Basin

Abstract

The strong heterogeneity of deep shale reservoirs poses significant risks to exploration and development. This study is significant for shale gas exploration buried over 3500 m. Based on geochemistry, scanning electron microscopy (SEM), low-temperature nitrogen adsorption (LTNA), X-ray diffraction (XRD), and laser Raman spectroscopy (LRS), characteristics of organic matter and the factors influencing organic pores (OPs) in deep shale have been determined. The thermal evolution of organic matter in deep marine shales is advanced, and the reflectance of vitrinites does not accurately determine organic maturity. The results indicate that the deep marine shales of the Wufeng–Longmaxi Formation in southern Sichuan have a high organic matter content, classified as Type I to II 1 organic matter, with a vitrinitic reflectance of 2.47%. The development of OPs in marine shale is influenced by organic matter type, organic matter content, kerogen type, and thermal evolution. Type I and Type II 1 kerogens promote the development of OPs. Higher organic matter content correlates with increased shale porosity and decreased average pore diameter. Additionally, greater organic matter content results in a larger specific surface area. When organic matter reaches excessive maturity, it undergoes graphitization. As the degree of graphitization increases, the development of OPs deteriorates. Beyond the maturity limit ( Ro = 3.5%), geochemical indicators and mineral composition do not affect changes in pore structure. When Ro is below 3.5%, the sweet-spot evaluation index of deep shale is almost the same as that of shallow- to medium-buried shale. The results provide a reasonable maturity range for the selection of sweet spots in deep shale.