Analyses of both type II organic-rich low-mature mudstones from the Kimmeridge Clay Formation (Yorkshire, England) and gas-mature mudstones from the Vaca Muerta formation (Argentina) were performed using petrographic and SEM observations to characterize the organic and inorganic components of the rocks and their relationship with porosity. The porosity analyses using nitrogen adsorption measurements and mercury intrusion porosimetry were then evaluated as a function of both composition and thermal maturity. Despite the absence of variation in the pore size distribution and the total pore volume with maturity, which is controlled by total organic carbon content (TOC), this study demonstrates that the pore network of these marine mudstones varies considerably with thermal maturity. Contrary to low-mature samples, whose porosity depends essentially on mineral interparticle pores, the porosity of thermally-mature rocks is mainly influenced by organic-matter-hosted pores located within secondary solid bitumen. Organic matter (OM) porosity, which is rare in the low-mature stage, appears to increase during thermal maturation in response to the thermal cracking of kerogen and oil, to become predominant in gas-mature rocks. It can therefore be considered that thermal maturation is a major process for the development of the OM porosity. However, the OM of high-TOC (>5.5 wt%) gas-mature samples contains smaller pores than gas-mature low-TOC mudstones probably due to a difference in the original OM composition between these high- and low-TOC samples. By analogy with the Kimmeridge Clay formation, which is often considered as time equivalent in terms of OM composition (Uliana et al., 1999), the high-TOC Vaca Muerta samples appear to have originally contained greater proportions of oil-prone amorphous OM and thus a better OM intrinsic quality for oil generation. This difference of OM composition and petroleum potential between Vaca Muerta marine mudstones appears to have dramatically influenced the OM-hosted pore genesis during maturation. Consistent with other studies, these results show that thermal maturity is not the only process that can explain OM-hosted pore genesis in gas shale systems. The OM composition can greatly influence the evolution of this porosity during maturation. Organic composition and OM thermal maturity may therefore have a complementary effect, explaining the diversity of trends observed on porosity in natural formations.