This work investigates how slight differences in the relative proportion of OM components and maceral types of marine mudstones throughout a same formation can influence the development and the evolution of OM-pore network during thermal diagenesis of organic-rich marine mudstones. To this end, laboratory gold-tube anhydrous confined thermal maturation, geochemical characterization, organic petrography (palynofacies and macerals analysis), nitrogen adsorption porosimetry, and SEM observations were applied on low-mature marine Kimmeridge Clay (KCF) mudstones. The main properties of organic-rich KCF samples which exhibit a slightly different total particulate OM assemblage were compared before and after maturation at 325°C, 390°C and 470°C for 72h. The results show that organic-rich KCF samples containing higher proportion of oil-prone orange amorphous organic matter (AOM) derived from phytoplanktonic components have developed smaller pores during thermal maturation resulting in lower pore volumes than samples containing higher proportion of brown AOM derived from the selective preservation of cell-walls of green microalgae. Contrary to brown AOM, orange AOM appears thus to be less favorable to the formation of large mesopores. This result suggests that no clear positive correlation exists between the oil-prone quality (HI) of OM and its ability to develop pores during thermal maturation