Formation of Diagenetic Illite in Sandstones of the Garn Formation, Haltenbanken Area, Mid-Norwegian Continental Shelf

hydrocarbon reservoirs in the Haltenbanken area. XRD and petrographic analyses of core samples from 11 wells show that a transition from incipient to extensive illitization in these sandstones is associated with a present burial depth of 3.7 km below the sea floor, corresponding to a formation temperature of 140 ~ Illite has formed by reaction between K-feldspar and earlier diagenetic kaolinite and probably also by alteration of mica. Although several of the cores are filled with hydrocarbons that probably were in place before base-Eocene (57 Ma), there are no indications that illitization was inhibited by the presence ofhydrocarbons i pore spaces. It is therefore suggested that 20-30 % residual water saturation issufficient for extensive illitization to occur by short-range diffusive transport in a compositionally closed system. There is an unresolved problem regarding both the timing and cause of illitization. On the one hand, the extent of illitization in the 11 wells correlates with variations in both present formation temperature and thermal maturity, implying that he present burial depth of 3.7 km below the seafloor coincides with a critical thermal threshold for illitization. According to this interpretation, the times of illitization in the different wells should correlate with variations in burial history and should in several cases be as young as 3 Ma, when deposition of one kilometer