Abstract The development and evolution of pore space in the impure chalk-type source rocks of the Oligocene Grybów Marls (Outer Carpathians, Poland) have been investigated using high-resolution petrographic techniques (FESEM/BS/CCI). Evidence suggests that the Grybów Marls are the products of the re-deposition of shelf sediments in deeper parts of the Outer Carpathian Basin during the final stages of its closure. The initial shelf sediments had variable levels of clay content ranging from fairly pure to impure chalks. After redeposition, the diagenetic history was similar to that of chalk sediment modified during the initial stages of diagenesis and controlled by the clay content. Stress-induced micro-cracks are restricted mainly to foraminiferal tests. Most coccolith shields were unaffected by compaction as they were protected from crushing by the presence of clay cement; the cement was re-organized under stress, forming less porous lamellar aggregates. The timing of diagenetic processes indicates that the origin of the clay and the eogenetic overgrowth calcite cement influenced the pathway of burial diagenesis and, thus, the pore-space evolution in the impure chalk.