Diagenetic Albitization of Detrital K-feldspar in Jurassic, Lower Cretaceous, and Tertiary Clastic Reservoir Rocks from Offshore Norway, I. Textures and Origin

Abstract

ABSTRACT The origin of albitized grains in sandstones has been the topic of considerable discussion in recent years. Some workers believe that the albitization of feldspar can occur in situ during burial diagenesis, but others argue that albitization is a feature inherited from source clastic grains. Our petrological and chemical studies of subsurface samples of the Jurassic, Lower Cretaceous, and Tertiary reservoir sandstones from offshore Norway show that detrital grains of potassium feldspar have been albitized during burial diagenesis. Criteria suggesting that the albitization has occurred during burial diagenesis include 1) euhedral habit of albite crystals with sharp edges and comers and markedly smooth crystal faces; 2) generally untwinned albitized grains, mostly riddled with minute br wnish inclusions; 3) lack of cathodoluminescence in albite; 4) homogeneous and pure albite composition (> 99% Ab); 5) absence of albitized grains in carbonate-cemented zones; and 6) increase in the percentage and degree (partial to complete) of albitized K-feldspar with depth. Under SEM, albitized grains display two types of fabrics. Type I albitized grains are composed of numerous tiny albite crystals (1-30 µm) growing along cleavage planes in parent grains, and they have abundant dissolution porosity, while Type II albitized grains show blocky albite crystals (5-80 @m) forming pseudomorphs of detrital K-feldspar, and they lack any dissolution porosity. Type I albitized grains are found commonly at shallower depths (between 2.2 and 3 km and temperatures varying from 65° to 90°C), whereas Type II albitized grains dominate at greater depths (> 3.5 km and at temperatures > 90°C). Albitization proceeds by a dissolution-precipitation mechanism. At shallower depth, dissolution of the parent K-feldspar is more rapid than the precipitation of albite, and rates of albite precipitation increase with depth, favoring pseudomorphic replacement of K-feldspars. Other factors that influence albitization processes are aNa+/aK+, permeability reduction, and surface-dissolution kinetics of the parent grain.