ABSTRACT

Detrital plagioclase (An₁₂-An₂₈) in Triassic reservoir sandstones of the Lunde Formation in the Snorre Field, offshore Norway has been partially to completely albitized during burial diagenesis (depths of about 2,500-3,000 m; 75-100°C present temperature). Calcite (CaCO₃ > 99 mole %) and kaolinite were formed as by-products of plagioclase albitization. Sodium-rich detrital plagioclase (An₂-An₁₁) are usually unalbitized. Detrital K-feldspar is fresh or, rarely, partly albitized. Differences in albitization behavior of detrital plagioclase and K-feldspar are shown to be related to the chemistry of formation waters, reactivity of kaolinite, and temperature. The diagenetically formed albite pseudomorphs are very pure (Ab > 9 ), well-ordered low-albite, dark-luminescent, vacuolized, mostly untwinned, and comprised of numerous euhedral albite crystals.

Some of the microporosity within the albite pseudomorphs is caused by volume reduction during albitization of the detrital plagioclase. The amount of albite formed in the pseudomorphs is related to the molar proportion of albite component in the detrital unstable Ca-rich plagioclase. The microporosity formed during albitization is proportional to the anorthite component, which is believed to have dissolved and resulted primarily in the formation of calcite and kaolinite. The albitization of detrital plagioclase in the studied sandstones can thus be explained without assuming the consumption of significant amounts of Na⁺ from pore solutions.