Deep geological barriers, such as Callovo-Oxfordian argillites, display suitable characteristics for a high level radioactive waste repository. Their low permeability is a key parameter to retain radionuclides locally. However, once the system is closed, gases produced by corrosion of the waste canisters can represent a risk to the installation safety. Gas pressure will build up with time, partially and locally desaturating Callovo-Oxfordian argillites. The pressure rise may fracture the argillites locally if the gas released through the host barrier is not evacuated fast enough. The present work focuses on porous-network identification with regard to gas transfer process. Mercury intrusion experiments were performed to evaluate how argillite reacts to gas intrusion. A sharp threshold is observed for pore diameters of 20 nm. Measured mercury porosity is 14% and is mainly made of mesopores (diameter < 100 nm). These experiments were complemented by water adsorption isotherms. Due to the evapo-condensation phenomenon, the results were different from the mercury intrusion curves. The method reveals that a larger trapped porosity exists (diameters between 20 nm and 1 μm). Pores of 20 nm diameter represent the capillary barrier to gas breakthrough. A network model, based on percolation, namely XDQ was built based on the connectivity and pore size distribution obtained previously. It predicts gas permeability of 10− 18 m2 at atmospheric pressure. The Klinkenberg effect greatly increases the gas permeability compared to water one. Gas permeability measurements confirm XDQ results. According to the XDQ model, permeability depends mainly on the percolation threshold and saturation. The model shows that partially desaturated network have no connected pathways for gas migration. In any case, pore connectivity is the key factor in gas transfer issues.