Describing the characteristics of shale pore structure is vital for the assessment of shale reservoir, which has significant influence on the storage and seepage mechanisms of gas shale. To profoundly understand the shale pore structure characteristics of continental shale reservoir, fractal analysis was performed on 45 continental shale samples from the Ordos Basin, NW China, via low-pressure N2 adsorption experiments. The characteristics of N2 adsorption isotherms revealed that slit-shaped shale pores are dominant among the geometric shapes of shale pores. During N2 molecules adsorption process, different characteristics were displayed at two regions where relative pressures (P/P 0) were 0–0.45 and 0.45–1. The Frenkel–Halsey–Hill (FHH) method was used to calculate fractal dimensions (D) at these two regions. In addition, the fractal exponents “(D – 3)/3” and “(D – 3)” were compared adequately. The results show creditable fractal characteristics for continental shale. Fractal exponent D – 3 is more suitable for the calculation of the fractal dimension in the study area. The surface fractal dimension (D 21) and pore structure fractal dimension (D 22) were further investigated. Results indicate that D 21, ranging from 2.04 to 2.50, was affected by shale constituents and provided a site for gas shale adsorption. D 22 reflects the irregularity and heterogeneity of the shale structure, varying from 2.20 to 2.65, and is higher overall than D 21. Furthermore, the value of D 22 negatively correlates with the average diameter of the shale. In addition, the comparisons of shale pore structure characteristics between the reservoirs Chang-7 and Chang-9 show that the shale pore structure of Chang-9 reservoir is more irregular and nonhomogeneous and is favorable for gas shale storage but unfavorable for seepage.

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A series of experiments including porosity and permeability measurements, thin section and scanning electron microscopy (SEM) observations, incremental pressure mercury injection (IPMI), and nuclear magnetic resonance (NMR) were conducted to systematically characterize the pore structure of tight sandstone from the Lower Shihezi Formation of Permian (P2x) in the northeastern Ordos Basin, China. The influences of pore types, pore size distribution, and fractal characteristics on reservoir quality of tight sandstones are also investigated. Results show that the studied tight sandstones generally possess poor quality and complex pore structure. The porosity and permeability range from 4.08% to 17.56% (average 9.22%) and from 0.05 to 16.66 mD (average 2.49 mD), respectively. Five pore types were observed in thin section and SEM images: primary intergranular pores, intergranular dissolution pores, intragranular dissolution pores, micropores within clay aggregates, and microfractures. The pore throats are mainly hairy/fibrous, inhibiting the connectivity between pores. Three types of pore structures were identified in the mercury-injection curves and pore size distribution curves from the IPMI experiment and in the T 2 relaxation time spectrum obtained by NMR. Both experiments yielded consistent classifications, and their combination was necessary to analyze the pore structure effectively. In general, permeability and porosity are positively related and depend on pore types. Large numbers of small pores confer high storage capacity, whereas small numbers of larger pores improve the flow capability. In the high porosity–permeability zone, larger pores also determine the storage capacity. The P2x tight sandstone is fractal, and macropores are more heterogeneous while micropores are more homogeneous. The fractal dimensions of macropores are good indicators of the reservoir quality of the P2x tight sandstone as larger fractal dimension values of macropores reflect poor reservoir quality.

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Mature-stage lacustrine shale was sampled from the Late Triassic Chang 7 member in the Ordos Basin, China. Two aliquots separated from the original sample were heated by hydrous pyrolysis to high- and overmature stages. The three samples were analyzed by nanometer-scale resolution X-ray computed tomography (nano-CT). From the distribution and geometry of the organic matter pores (OM pores) in the two-dimensional (2-D) nano-CT images, this study calculated the total organic matter content and porosity of the OM pores (2-D TOC and 2-D OM porosity, respectively) and characterized the OM pores and throats. The results suggest the following: (1) The OM pores tended to distribute centrally rather than sparsely throughout the organic matter, and large-area pores existed above a 2-D TOC threshold of 2%. (2) The main diameter range of the OM pores was 100–700 nm, and the number of OM pores increased with maturity. (3) The amount of coordination numbers corresponding to a given pore diameter interval and the number of pores corresponding to the same range of coordination numbers (1–10) were greater in the high-mature and overmature samples than in the mature samples, indicating that the OM pore connectivity improved with maturity. The three-dimensional volume data confirmed that the OM pore connectivity decreased in the following order: high-mature, overmature, and mature samples. Highly mature samples may be more conducive to the diffusion of natural gas within the OM pores.

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