Geochemical modelling of diagenetic illite and quartz cement formation in Brent sandstone reservoirs: Example of the Hild field, Norwegian North Sea
Résumé
Deep burial diagenesis in the Brent reservoir sandstones of the Hild Field, Norwegian North Sea, was responsible for the development of illite and quartz, and the extensive dissolution of kaolinite and K-feldspar at T > 100°C. Geochemical modelling and numerical simulations were conducted in an attempt to reproduce these diagenetic processes. Present-day formation water in Hild is a Na-Cl brine (Total Dissolved Solids about 70 g/l) with a calculated pH of about 5.2 in reservoir conditions. Saturation indices (150°C, 800 bar) indicate equilibrium with kaolinite, illite, paragonite and calcite, near-equilibrium with quartz and disordered dolomite, and marked under-saturation with respect to anorthite, albite, K-feldspar and phengite. Numerical simulations performed in closed- and open-system conditions, using sea water, present-day formation water and freshwater as pore water, indicate that illitisation in Hild is controlled by the closed-system reaction : K-feldspar + kaolinite * illite + 2 quartz + H2O. The results indicate further that the initial proportion of K-feldspar and kaolinite in the reacting rock constitutes the primary factor controlling the amount of diagenetic quartz and illite generated numerically. Variables such as T (between 100-150°C), pCO2 and aqueous acetate concentration have little influence on simulation outcomes. The amount of diagenetic illite (average=6%) present in the reservoir can readily be reproduced numerically by adjusting the K-feldspar content and K-feldspar/kaolinite molar ratio (which must be <1) in the initial assemblage reconstructed from petrography. Spatial variation in the present-day diagenetic illite content most likely reflects primarily variation in K-feldspar abundance at the time of illitisation. In contrast, the amount of diagenetic quartz observed in the reservoir (average=10%) can not be achieved numerically by the reaction described, suggesting that a contribution by pressure-solution was significant. Numerical simulation further illustrates that the illitisation process is not responsible for the loss of porosity that affected the Hild Brent sandstones. Mechanical compaction and quartz cementation are more likely causes. This study illustrates that numerical modelling in closed-system or open-system conditions is useful in simulating diagenetic transformations observed in illitised Brent reservoirs provided that sufficient constraints can be placed on mineralogy, fluid chemistry and pressure-temperature conditions of the system.