Geochemical modelling of mineral diagenesis in the Brent sandstone reservoirs, Alwyn South and Alwyn North areas, East Shetland Basin, North Sea

Abstract

The purpose of this study was to examine mineral diagenesis at Alwyn using the computational code KINDIS in order to simulate mineral dissolution and precipitation, the chemical evolution of the formation water, and the evolution of the host rock during its progressive reaction with the evolving formation water. Mass and volumetric balances are calculated from the simulations. Petrographic study indicates an extensive kaolinitization followed by significant illitization in Alwyn South. In contrast, secondary illite is almost absent at Alwyn North. Quartz precipitation and K-feldspar dissolution are observed in both the Alwyn North and the Alwyn South areas. The simulated interaction between Alwyn South water or Alwyn North water between 90 and 120°C and an idealized sandstone yields significant dissolution of kaolinite, significant to complete dissolution of K-feldspar and significant precipitation of illite. These simulated reactions correspond well with those recognized at Alwyn South reservoirs but fail to reproduce the observations at Alwyn North, where secondary illite is almost absent. Additional simulations indicate that solutions supposed to be supersaturated with respect to quartz or more acidic do not lead to the formation of illite. However, the origin of these chemical conditions remain to be explained. In these simulations K-feldspar and kaolinite are the secondary precipitates.