Abstract:
The precipitation of kaolinite in reservoir sandstones is a relatively early diagenetic reaction, which depends on the initial proportions of mica and feldspar varieties in the sand, and on the nature and flow rate of the interstitial water. A water–rock interaction code was used to explore the influence of the mineralogical composition and of the water characteristics, on the formation and stability of kaolinite in subarkosic sandstones. The water compositions tested were meteoric or, alternatively, marine. The detrital aluminosilicates considered were plagioclase (anorthite and low-T albite), K-feldspar, phengite and quartz. From this assemblage, below 75°C plagioclase was the main control for kaolinite formation. Anorthite in all cases acts as a precursor of kaolinite, whereas low-T albite was destabilized only by influx of marine or meteoric water. Phengite dissolved only in the marine case, which also induced some K-feldspar overgrowth. After burial, in more and more confined conditions, K-feldspar and some kaolinite were altered, and consequently illite precipitated. The assemblage of kaolinite, K-feldspar and residual phengite was destabilized at c. 95°C, and illite began to form. If the precipitated clay mineral was an Fe, Mg-illite and if the system was closed, this reaction was of limited extent at this temperature. A more efficient illitization process occurred between 120 and 130°C.The preceding scenario was compared with petrographical and geochemical data obtained from samples of the Brent Group reservoirs cored in the Hild Field. The main discrepancy between the simulation results and the data is a generation of kaolin-mineral polytype (seemingly dickite) that according to the petrographical and geochemical constraints occurred in the 105–125°C range. The conditions explored in the modelling approach were not able to reproduce the formation of this mineral above 90°C.