A Correlative Approach for the Assessment of Porosity in Shale Rocks by Scanning Electron Microscopy

A correlative approach to compute porosity at the nanoscale in areas as large as 3 mm × 3 mm through field emission scanning electron microscopy (FESEM) is presented. This approach overcomes the limitation of FESEM related to its representativeness on larger areas. The strategy is based on imaging an area of 3 mm × 3 mm where a set of regions of interest (ROIs) of 600 μm × 600 μm with similar mineralogy is selected and imaged. A second selection of areas of 150 μm × 150 μm inside the ROIs is performed. The three sets of images, linked through their mineralogy and organic matter, account for pore sizes within their given resolution range, providing an efficient computational method. The cumulative porosity plot, in addition of showing the contribution of the different pore sizes to the total porosity, enables the detection of fissures. The pore size distribution function allows to discuss the practical limit for counting small pores, around 30-40 nm for the samples studied in this work, under the given experimental conditions. This value is appreciably larger than the resolution of FESEM and the minimum pore size defined considering either 1 pixel/1 pore or 9 pixels/1 pore.