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Scale-Dependent Nature of Porosity and Pore Size Distribution in Lacustrine Shales: An Investigation by BIB-SEM and X-ray CT Methods


Due to heterogeneous pore distributions within shales, petrophysical properties of shales determined by scanning electron m icroscopy (SEM) and X-ray computed tomography (CT) methods strongly depend on the observed domain size (analysis scale). A vital limitation of SEM and CT methods is that low-resolution SEM and X-ray CT cross-sections mainly display microstructures in relation to larger pores, whereas cross-sections imaged at high magnification only allow smaller nanopores to be observed, which directly impacts the results of petrophysical property extraction from SEM images and CT data. In this study, the influence of the analysis scale on surface and bulk porosities and pore size distribution (PSD) for lacustrine shales from the Dongying sag of Bohai Bay basin, China were investigated using broad ion beam (BIB)-SEM and X-ray CT methods. BIB-SEM cross-sections with high imaging resolution (10 nm/pixel) and a large field of view (> 1 mm2) mainly describe the 2D nanoscale pore system in the two shales (samples F41#-2 and Y556#-1), while CT-based 3D reconstructions with resolutions of 0.42 μm/pixel (sample F41#-1) and 0.5 μm/pixel (sample H172#-1) reflect the 3D sub-micron pore system. The results indicate that the surface (bulk) porosity exhibits a multiple power-law distribution with increasing analysis area (volume), which can be used to extrapolate the porosity of a given area (volume). Based on SEM and CT investigations, the sizes of the minimum representative elementary areas (REAs) and volumes (REVs) were determined respectively, which are closely associated with the heterogeneousness of the pore system. A larger REA or REV corresponds to a much more heterogeneous pore system. Minimum REAs are proposed to be 0.0293 mm2 (F41#-2) and 0.0091 mm2 (Y556#-1), and minimum REVs are 0.016 mm3 (F41#-1) and 0.027 mm3 (H172#-1). Both the 2D and 3D PSDs extracted from SEM images and CT data were also influenced by the analysis scale due to heterogeneities in the pore system. As the analyzed areas (volumes) are larger than the minimum REA (REV), obtained 2D (3D) PSDs are comparable to each other and can be considered to reflect the shale PSD. In addition, compared to porosities obtained by He and mercury injection porosimetry, the SEM-extracted surface porosity clearly underestimates the actual porosity and does not accurately reflect the 3D pore system.