3D nanopores modeling using TEM-tomography (dolostones - Upper Triassic)

Abstract

Abstract Transmission Electron Microscopy Tomography (TEMT) stands as a novel and alternative tool to tridimensionally model the nano-scale pores of rocks. In this study it is flanked with traditional Mercury Capillary Injection Porosimetry (MICP), Nitrogen adsorption Porosimetry (NP) and Scanning Electron Microscopy (SEM) analysis, to test a sample belonging to an Upper Triassic dolostone cropping out in Northern Calabria. MICP revealed low values of porosity and permeability, 2.3% and the 0.11 mD respectively. Nanopores account for the 94% of the pore volume, whereas the remaining 6% is represented by micropores. The integrated MICP and NP pore size distribution (PSD) shows the presence of a dominant pore diameter population coincident with 50\u202fnm, with minor peaks at diameters of 12\u202fμm, 850, 25, 13, 6, 2\u202fnm. However, since the PSD peaks of the two techniques do not match, a comparison of the two curves is necessary to better characterized the overlapping area. SEM imaging analysis showed the presence of nano-scale intercrystalline and intracrystalline pores. The first corresponds with the spaces among dolomite crystals and, as suggested by the NP analysis, shows a wedge/slit morphology. The second appears with a prevailing polygonal section or as tight microfractures. Since SEM provides exclusively two-dimensional images, the morphology and development of these pores was obtained through TEMT 3D reconstruction. The 3D model showed the presence of open cavities, fractures and blind/isolated pores. Cavities cross the entire dolomite crystals and can present a constant (sub-prismatic pores) or variable channel section (funnel-shaped pores) that can reduce its aperture, as observed, of also more than a ten factor (e.g. from 300\u202fnm to 20\u202fnm). Fractures, commonly developing on the existing cleavage surfaces, cut the crystal faces for 300–350\u202fnm, reducing its aperture (maximum of 20–30\u202fnm) from the edge towards the inner part of the crystal. Lastly, blind/isolated pores can assume diameters and thickness of 250\u202fnm. They show a sub-cubic morphology and, can be filled by solid, liquid or gaseous inclusions.