Pore Structure Characterization of the Lower Permian Marine Continental Transitional Black Shale in the Southern North China Basin, Central China

The Carboniferous-Permian Taiyuan-Shanxi coal-bearing formations, as the most promising shale-gas reservoir in North China, comprise a typical shale-gas system developed in a transitional environment. To investigate the pore structure of the Taiyuan-Shanxi shale in the Southern North China Basin, field-emission scanning electron microscopy (FE-SEM), low-pressure nitrogen adsorption (LNA), and high-pressure mercury intrusion (HMI) were employed to characterize the pore type, volume, and size distributions of 11 shale samples from the Mouye-1 well. Organic-matter-hosted pores, interparticle pores, intraparticle pores, and shrinkage cracks were observed in the FE-SEM images. Compared to matured marine shale, the number of organic-matter-hosted pores within the Taiyuan-Shanxi shale was much smaller, probably because of the high content of inertinite. The nitrogen total pore volume was found to range from 20.36 x 10(-3) to 31.23 X 10(-3) mL/g, whereas the mercury total pore volume was found to range from 2.1 x 10(-3) to 6.2 x 10(-3) mL/g. The surface area was found to range from 8.66 to 19.38 m(2)/g. The pore size distribution curves suggest a significant contribution of macropores (>50 nm) to the total pore volume. The micropore and meso-/macropore volumes obtained from LNA were found to be separately associated with plagioclase and dolomite. The volume of larger micrometer-sized pore obtained from HMI shows a positive relationship with quartz and a negative relationship with chlorite. However, these correlations are generally weak. The lack of organic-matter hosted pores highlights the importance of pores associated with inorganic material in coal-bearing transitional shale. Rather than organic content, mineral content and chemical/mineral transformation during diagenesis play more important roles with respect to the pore structure. The inhomogeneous pore abundance caused by different degrees of chemical transformation among samples migt be the reason for the weak correlation between the pore volume and the contents of shale components.