Recently, there is a growing recognition of thermal maturity involvement in the development of the organic matter (OM) porosity in gas shales, and thus, in their gas storage capacity. Therefore, a better understanding of the role of OM chemical transformations on shale porosity during maturation is required. For this purpose, the porosity of immature (Kimmeridge Clay, UK) and gas-mature organic rich shales (Vaca Muerta, Argentina) was first characterized. Then, artificial thermal maturation of immature shales using confined pyrolysis experiments was performed to follow the evolution of porosity from the early oil-window to the dry gas-window. Porosity before and after maturation was measured by low pressure nitrogen adsorption and evaluated as function of both thermal maturity and shale composition (XRD, Rock Eval analyses, Organic petrography...). The OM chemical transformations were investigated by GC/MS analysis of oil and gas generated during pyrolysis experiments. First results revealed that the OM porosity, non-existent in immature shales, seems to grow during thermal diagenesis to become predominant in natural mature rocks. According to pyrolysis experiments, the development of the OM porosity appeared in relation with the production of gas from the end of the oil-window. This could be probably caused by reorganizations of the kerogen and residual bitumen structures that would occupy mineral interparticular pores during the oil-window. Moreover, variations of the pore size versus organic carbon content and thus, OM composition in general, suggest that small variations in OM composition could have a great influence on pores genesis during maturation. Future investigations should clarify this.