Quartz cementation is commonly the most important porosity-reducing process in deeply buried quartzose sandstones on the Norwegian continental shelf. The quartz cement is mostly sourced from stylolites, but silica may also be derived from illitization of kaolinite. Illitization can proceed at silica activities several times higher than quartz saturation, whereas dissolution of quartz at stylolites is very sensitive to silica supersaturations, and the illitization reaction may consequently stop or retard dissolution of quartz at stylolites. Since the illitization reaction does not cause a net porosity loss, porosity loss due to precipitation of quartz cement is reduced or totally prevented over the depth/temperature range where illitization takes place. Illitization affects albitization of K-feldspar, because both reactions consume K-feldspar. Oil replacement during illitization may conceivably cause greater porosity loss than if oil were not present, since oil emplacement might stop or retard illitization but not quartz cementation sourced from stylolites.