This study aims to better understand the evolution of the organic matter (OM) ultrafine structure (designating the nanoscopic structure of the OM/macerals) and porosity with increasing thermal maturity of mudstones source-rocks. To this end, the particulate fractions of kerogen from organic-rich Kimmeridge clay mudstones were isolated by acidic treatment before and after laboratory thermal maturation. The evolution of the composition, the chemical structure and the porosity of kerogen from the immature stage (Ro = 0.42%) to the dry gas zone (Ro = 2.12%) were documented using a combination of elemental analysis, vitrinite reflectance measurements, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), Raman spectroscopy and Small Angle X-Ray Scattering (SAXS). The evolution of the porosity of OM was then compared to the evolution of the pore volume of the total rock measured by nitrogen adsorption. The results show that the progressive densification, reorganization and aromatization of the amorphous kerogen particles into a more ordered, but heterogeneous carbon-rich residue is responsible for significant variations in the kerogen porosity with increasing maturity. Contrary to the previous observations on these Kimmeridge clay mudstones, the variations that occur during the peak of oil generation (Ro of ca. 0.90%) mark the onset of the OM-hosted pores development. There appears to be a natural close relationship between the evolution of OM porosity and total pore volume of organic-rich mudstones during gas generation. Indeed, a similar alternation of pore collapse and pore development is observed in response to gas generation. The development and the evolution of pores in these organic-rich mudstones seems thus mainly driven by the evolution of the chemical structure and the composition of OM during thermal maturation. In the dry gas zone, the porosity and the specific surface area of the kerogen are significant (19% and 57.1 m2.g−2 respectively). This highlights the importance of the OM content, type and composition in the porosity and gas storage capacities of mudstone reservoirs, increasingly discussed in the available literature in recent years.