Related Quartz and Illite Cementation in the Brent Sandstones: A Modelling Approach

Summary

DIAPHORE is a water–rock interaction code that simulates short-term diagenetic episodes (few millions of years). It couples rock–water interaction and the transport of dissolved chemical species. It explicitly takes kinetics into account. DIAPHORE is applied here to the diagenesis of subarkosic sandstone reservoirs. It shows the extent to which silica for quartz cementation can be derived from the dissolution of kaolinite and feldspars, and the role of advective transport. Simulation results are controlled by core and fluid data from the Brent Group in the Greater Alwyn area (North Sea) where diagenetic illite and quartz can be considered, at least in part, as being cogenetic and of Eocene age (≈ 105°C and 270 bars). Two different types of diagenetic transformations are depicted by modelling: (i) The first transformation is governed by the instability of minerals present in the system. Approximately half of the quartz formed during modelling corresponds to the rapid, isochemical transformation of kaolinite and K-feldspar to illite and quartz. This transformation can be modelled using a closed-system approach. (ii) The second transformation is governed by disequilibrium between inflowing water and the minerals. It can be addressed only with an open-system model. Once the less abundant phase in the kaolinite and K-feldspar pair has disappeared, additional quartz can precipitate from the residual silicates present in the system (e.g. albite). Reaction fronts form and propagate throughout the reservoir at a slower velocity than the fluid flow. The reactions are no longer isochemical. Moreover, charging the reservoir with hydrocarbons, or any other process that would stop the water flow during the progress of the reactions, can help to explain heterogeneities in the mineral composition observed at the field scale.