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 microscopy (SEM) and X-ray computed tomography (CT) methods strongly depend on the observed domain size (analysis scale). In this paper, the influence of the analysis scale on areal 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. The 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 CTbased 3D reconstructions with resolutions of 0.42 (F41#-1) and 0.5 μm/pixel (H172#-1) reflect the 3D submicron pore system. The results indicate that the areal (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. Minimum REAs are proposed to be 2.93×104 (F41#-2) and 0.91×104 μm2 (Y556#-1), and minimum REVs are 0.016 (F41#-1) and 0.027 mm3 (H172#-1). 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. These results provide insights into the porosity and PSD characterization of shales by SEM and X-ray CT methods.