Closed-system burial diagenesis in reservoir sandstones: Examples from theGarn Formation at Haltenbanken area, offshore mid-Norway

The Middle Jurassic Garn Formation of the Haltenbanken area has been studie d 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 plagi oclase contents show variations on a regional scale both laterally and as a function of burial depth. The content of pore-filling authigenic illite in creases sharply, and the content of K-feldspar and kaolinite decreases in G arn sandstones presently at depths greater than 3.6-3.7 km RSF (120-130 deg reesC), 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 su ggests that the potassium released during K-feldspar dissolution is retaine d in the sandstones and is precipitated as illite, The variations in bulk c ontents of potassium and sodium are therefore considered to be related prin cipally 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 tha n one. The deeply buried (> 3.7 km RSF) sandstones with similar potassium c ontents contain excess K-feldspar and most of the kaolinite is illitized, H owever, deeply buried sandstones containing an average of 0.38 wt % K2O (K/ Al molar ratio < 1/4) contain a significant amount of kaolinite but negligi ble K-feldspar, This suggests that the K-feldspar:kaolinite ratio before th e onset of illitization was less than one, and hence that the kaolinite-ill ite reaction has been restricted by an insufficient supply of potassium (ab sence of K-feldspar), This illustrates how illitization of kaolinite depend s upon K-feldspar as a local source of potassium. Prediction of illitizatio n in sandstones, therefore, must be based on integration of models for prov enance, facies, and early diagenesis in addition to burial and thermal hist ory. The formation of pore-filling authigenic illite in these sandstones is an important influence on the total reservoir quality.