CHEMISTRY OF OIL-FIELD BRINES IN RELATION TO DIAGENESIS OF RESERVOIRS.2. RECONSTRUCTION OF PALAEO-WATER COMPOSITION FOR MODELING ILLITE DIAGENESIS IN THE GREATER ALWYN AREA (NORTH-SEA)

Input data for water composition are required in order to numerically simulate diagenetic episodes. In rock-dominated, closed-system modelli ng, the composition of water initially present in the pores has at mos t only a minor influence on the stability of the system, and is contro lled by the mineral composition. In open-system modelling, the composi tion of infiltrating water can play an important role. Brent water sal inities of the Greater Alwyn area exhibit a regional trend from the No rth to the South. The K/Cl ratio of these waters Varies along two tren ds: it is higher in the North than in the South. Alkalinity and publis hed pCO(2) values are used to calculate pH under subsurface conditions . Very few silica measurements are available. Aluminium analyses of po re 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. Satu ration of present-day Brent waters with respect to these minerals is e xamined. The method proposed in Part 1 is applied to reconstruct a ran ge of possible compositions for ancient waters in Brent reservoirs, be tween several extreme conditions. One of these extremes is provided by buffering water with a set of minerals chosen from petrographic consi derations. The other extremes correspond to reasonable states of over- saturation with respect to particular minerals, e.g. quartz, and conce rn the values of dissolved Si and Al, which are the least well charact erised concentrations from the routine analyses of formation waters. ( C) 1997 Elsevier Science Ltd.