Chemistry of oil-field brines in relation to diagenesis of reservoirs—2. Reconstruction of palaeo-water composition for modelling illite diagenesis in the Greater Alwyn area (North Sea)

Abstract

Input data for water composition are required in order to numerically simulate diagenetic episodes. In rock-dominated, closed-system modelling, the composition of water initially present in the pores has at most only a minor influence on the stability of the system, and is controlled by the mineral composition. In open-system modelling, the composition of infiltrating water can play an important role. Brent water salinities of the Greater Alwyn area exhibit a regional trend from the North to the South. The K Cl ratio of these waters varies along two trends: it is higher in the North than in the South. Alkalinity and published pCO 2 values are used to calculate pH under subsurface conditions. Very few silica measurements are available. Aluminium analyses of pore water are not available. Some reservoirs of the Greater Alwyn area experienced a major reduction in permeability due to illite and quartz precipitation, coeval with kaolinite and K-feldspar dissolution. Saturation of present-day Brent waters with respect to these minerals is examined. The method proposed in Part 1 is applied to reconstruct a range of possible compositions for ancient waters in Brent reservoirs, between several extreme conditions. One of these extremes is provided by buffering water with a set of minerals chosen from petrographic considerations. The other extremes correspond to reasonable states of over-saturation with respect to particular minerals, e.g. quartz, and concern the values of dissolved Si and Al, which are the least well characterised concentrations from the routine analyses of formation waters.