A MODEL FOR THE EFFECT OF ILLITIZATION ON POROSITY AND QUARTZ CEMENTATION OF SANDSTONES

Quartz cementation is commonly the most important porosity-reducing pr ocess in deeply buried quartzose sandstones on the Norwegian continent al shelf. The quartz cement is mostly sourced from stylolites, but sil ica may also be derived from illitization of kaolinite. Illitization c an proceed at silica activities several times higher than quartz satur ation, whereas dissolution of quartz at stylolites is very sensitive t o silica supersaturations, and the illitization reaction may consequen tly stop or retard dissolution of quartz at stylolites. Since the illi tization reaction does not cause a net porosity loss, porosity loss du e to precipitation of quartz cement is reduced or totally prevented ov er the depth/temperature range where illitization takes place. Illitiz ation affects albitization of K-feldspar, because both reactions consu me K-feldspar. Oil emplacement during illitization may conceivably cau se greater porosity loss than if oil were not present, since oil empla cement might stop or retard illitization but not quartz cementation so urced from stylolites.