The Middle Jurassic Garn Formation of the Haltenbanken area has been studied using mineralogical and geochemical data from 21 wells, ranging in burial depths from 2.0 to 4.1 km relative to seafloor (RSF). K-feldspar and plagioclase contents show variations on a regional scale both laterally and as a function of burial depth. The content of pore-filling authigenic illite increases sharply, and the content of K-feldspar and kaolinite decreases in Garn sandstones presently at depths greater than 3.6-3.7 km RSF (120-130°C). The depletion in K-feldspar below 3.7 km RSF is not accompanied by lower potassium values in the bulk chemical composition (wt % K2O). This suggests that the potassium released during K-feldspar dissolution is retained in the sandstones and is precipitated as illite. The variations in bulk contents of potassium and sodium are therefore considered to be related principally to primary variations in sandstone mineralogy.
The shallower sandstones (< 3.7 km RSF) with average wt % K2O greater than 0.95 (K/Al molar ratio > 1/3) have a K-feldspar:kaolinite ratio greater than one. The deeply buried (> 3.7 km RSF) sandstones with similar potassium contents contain excess K-feldspar and most of the kaolinite is illitized. However, deeply buried sandstones containing an average of 0.38 wt % K2O (K/Al molar ratio < 1/4) contain a significant amount of kaolinite but negligible K-feldspar. This suggests that the K-feldspar:kaolinite ratio before the onset of illitization was less than one, and hence that the kaolinite-illite reaction has been restricted by an insufficient supply of potassium (absence of K-feldspar). This illustrates how illitization of kaolinite depends upon K-feldspar as a local source of potassium. Prediction of illitization in sandstones, therefore, must be based on integration of models for provenance, facies, and early diagenesis in addition to burial and thermal history. The formation of pore-filling authigenic illite in these sandstones is an important influence on the total reservoir quality.