Temperatures of albitization of plagioclase in sandstones from the Norwegian continental shelf

ABSTRACT Albitization is a widely recognized aspect of sandstone burial diagenesis, but its temperature of occurrence has been only partly constrained by previous studies. Temperatures of albitization of plagioclase in sandstone samples of Devonian to Miocene age from the Norwegian continental shelf were therefore studied by combining thin-section petrography with temperature measurements from wells. The investigated samples cover burial depths of 504–5120 m and a temperature range of 23–183°C. Albite grains with a morphology indicative of a diagenetic origin are present at temperatures of 88°C and higher, whereas calcic plagioclase was found solely at temperatures below 88°C. Oil inclusions in albitized plagioclase are with one exception present only at temperatures above 87°C. Moreover, albitized grains with a characteristic brown cathodoluminescence (CL) color that differentiates them from detrital albite with red and yellow CL colors are in all but one of the wells examined with CL confined to samples where temperatures exceed 88°C. The non-albitized grains of calcic plagioclase show a variety of CL colors; pink, pale green, yellow to orange, and pale brown, and these CL colors are seen only at temperatures less than 88°C. The data therefore suggest that albitization has in practically all cases taken place around 88°C, and that the reaction is rapid on a geological time scale. These results have several useful applications. Fixing the temperature of albitization constrains sandstone provenance and magnitude of uplift because albitization in a sandstone located at the surface or at shallow depth indicates uplift from depths where temperature was at least 88°C, and in situ albitized plagioclase suggests that the sandstone is not sourced solely from older sandstones once buried to temperatures in excess of 88°C, because such a sand source would not provide reactive plagioclase. Many of the examined reservoirs are presently filled with oil or wet gas, and most of these hydrocarbon-filled reservoirs contain oil inclusions within albitized plagioclase, indicating oil emplacement at temperatures less than 88°C.