The genesis and controlling factors of micropore volume in transitional coal-bearing shale reservoirs under different sedimentary environments

The micropores (pore size < 2 nm) that develop in coal-bearing shale reservoirs are of great significance to the adsorption and storage of shale gas. To investigate the characteristics and genesis of micropore volume in transitional coal-bearing shale, we selected 18 coal-bearing shale samples deposited in four types of sedimentary environments (carbonate tidal flat, pro-delta, delta front, and delta plain), using low-pressure CO2 adsorption, X-ray diffraction (XRD), vitrinite reflectance (R-o), and total organic carbon (TOC) analysis. The results first indicate that two meaning peaks are present in the distribution curve of micropore volume in transitional coal-bearing shale, which are peak 2 (0.4577-0.6272 sun) and peak 3 (0.8216 nm), while the pore volume of peak 1 may be a false peak caused by the testing principle. The hydrodynamic intensity in different sedimentary environments is the key variable in determining the characteristics of micropores. In a weak hydrodynamic environment, the higher the hydrodynamic intensity, the lower the pore volume of peaks and total micropore volume, while the opposite is true in a strong hydrodynamic environment. When the hydrodynamic force reaches a certain intensity, the pore volume of peaks and the total micropore volume decrease rapidly. The quartz in transitional shale typically contains a small number of micropores, resulting in a reduction in the micropore volume. Meanwhile, the clay minerals process substantial micropores and work together with organic matter (OM) to change the pore structure of shale, resulting in an increase in the micropore volume. Moreover, the influence of clay minerals on the total micropore volume and pore volume of peak 2 is slightly higher than that of TOC, while the opposite was found for peak 3. Overall, the OM micropores of peak 2 are interlaminar pores of aromatic rings or other types of OM pores, the OM micropores of peak 3 correspond to the columnar pores formed by ordered stacking of aromatic rings, and the micropores in clay minerals are the spacings in the inner layers of kaolinite and chlorite.