||Tight chalk intervals play a major role in North-Sea hydrocarbon fields, by controlling fluid flow pathways of hydrocarbon and water. Recent studies reveal that low-permeability chalk properties are dependent on clay content and cementation. Therefore, in this research, three characteristic samples were selected: (A) a porous micritic chalk, (B) a cemented chalk and (C) an argillaceous chalk. Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) analyses were performed and 3D pore networks were reconstructed for each sample. By using MATLAB® and Avizo® softwares, relevant pore data were extracted, including pore volumes, lengths and network tortuosity. Results show that the pore length is reduced in tight chalks, with 140 nm on average in argillaceous chalk and 533 nm in cemented chalk, compared to 1091 nm in micritic reservoir chalk. Pore shape analysis demonstrates that, when present, clay flakes are predominant. Argillaceous chalk displays 35% of flattened pores, while these represent only 15 and 18% of pores in micritic and cemented chalk respectively. Virtual rock porosity calculated from FIB-SEM is consistent with helium porosity lab-measurements. MICP pore-throat diameters also match calculated pore widths. These preliminary findings confirm the potential of FIB-SEM analyses in characterizing chalks porous media. In order to understand fluid transport, fluid flow was simulated on each sample using the Lattice Boltzmann Method (LBM), which allows visualising fluid pathways and calculating permeabilities. Simple phase LBM-modelled permeabilities are on the same order of magnitude as gas permeabilities measured on centimetric plugs with Klinkenberg correction. This means that the pore network characterized by FIB-SEM has a significant contribution to macroscopic fluid transport. The impact of clay content and cementation degree on total porosity and pore shapes in chalk is a key finding, which requires to be accounted for, e.g. in mesoscale fluid flow modelling.