Diagenesis influencing the porosity of Upper Jurassic reservoir sandstones, Danish North Sea

Abstract

The quartz-rich Upper Jurassic reservoir sandstones in the Hejre-2 well are characterised by relatively high amounts of detrital K-feldspar, which most likely come from a volcanic source. Authigenic K-feldspar grew on detrital K-feldspar, perthite and albite grains and possibly Ca-rich plagioclase. The potassium for the K-feldspar overgrowths probably comes from marine pore fluids, detrital clay minerals and the underlying volcaniclastic conglomerates. The K-feldspar over-growths were initially abundant, but were later dissolved except for the calcite-cemented parts of the sandstones. The formation of secondary porosity in the sandstones in the Hejre-2 well was mainly caused by dissolution of detrital feldspars and authigenic K-feldspar. Quartz diagenesis prevails in the Upper Jurassic sediments in the Hejre-2 well where carbonates are absent and where illitic clays are restricted. Pressure solution and stylolite formation provide abundant silica, which is precipitated as quartz overgrowths. The presence of small amounts of illitic clays and mica may have enhanced the development of stylolites. On the other hand, relatively large amounts of illitic coatings seem to have inhibited or retarded the formation of quartz overgrowths, which otherwise could have reduced the porosity. Consequently, sandstone intervals in the Hejre-2 well with high porosities are the result of dissolution and illite replacement of detrital feldspars and early authigenic K-feldspar cement, thus secondary porosity was created; together with illite coatings that inhibited the formation of quartz over-growths. Future investigations may reveal whether the diagenetic evolution of Upper Jurassic sediments in other wells is similar to that encountered in the Hejre-2 well. Another aspect worth pursuing is to investigate if bulk rock geochemistry can be correlated with major diagenetic phenomena. © GEUS, 2008.