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

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 nm, with minor peaks at diameters of 12 pm, 850, 25, 13, 6, 2 nm. 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 nm to 20 nm). Fractures, commonly developing on the existing cleavage surfaces, cut the crystal faces for 300-350 nm, reducing its aperture (maximum of 20-30 nm) from the edge towards the inner part of the crystal. Lastly, blind/isolated pores can assume diameters and thickness of 250 nm. They show a sub-cubic morphology and, can be filled by solid, liquid or gaseous inclusions.