The role of provenance in illitization of deeply buried reservoir sandstones from Haltenbanken and north Viking Graben, offshore Norway

An extensive mineralogical and geochemical database has been compiled to evaluate the relationship between diagenesis and provenance of the Lower/Middle Jurassic sandstones from the Haltenbanken (Mid-Norway) and north Viking Graben. The data from the Haltenbanken area, from burial depths between 2.1 and 4.5 km relative to seafloor (RSF), show a reduction in K-feldspar and kaolin, and a sharp increase in illite content in sandstones which are presently located at depths greater than 3.7 km RSF (120-140 degrees C), However, there is little change in the bulk K/Al molar ratio with depth, which suggests that potassium has been conserved during the precipitation of illite and dissolution of K-feldspar. The K/Al molar ratios in most of the deeply buried Haltenbanken sandstones are greater than 1/4, which corresponds to the molar ratio in illite. This indicates that excess K-feldspar is present, which is confirmed by petrographic observations. By contrast, deeply buried sandstones from the north Viking Graben are mostly characterised by the absence of K-feldspar, a K/Al molar ratio lower than illite and varying amounts of kaolin. This suggests that the amount of K-feldspar present at the onset of illitization (of kaolin) was lower in the north Viking Graben sandstones than in the Haltenbanken area. The degree of illitization of kaolin is higher in the Haltenbanken reservoirs as compared to the north Viking Graben, and this may be directly related to the higher K-feldspar content in the Haltenbanken sandstones. The survival of kaolin below 3.7 km RSF depth, in most of the plagioclase rich and K-feldspar poor sandstones from the north Viking Graben, therefore indicates that illitization of kaolin requires a local presence of K-feldspar in the sandstones. (C) 2000 Elsevier Science Ltd. All rights reserved.