CHEMISTRY OF OIL-FIELD BRINES IN RELATION TO DIAGENESIS OF RESERVOIRS.1. USE OF MINERAL STABILITY FIELDS TO RECONSTRUCT IN-SITU WATER COMPOSITION - EXAMPLE OF THE MAHAKAM BASIN

During mineral diagenesis, the chemical composition of pore water can be strongly influenced by mineral reactions, especially under geologic al conditions where water/rock ratios are low. To model and eventually to predict diagenesis, it is important to have an understanding of fo rmation water composition. However, available water analyses in many b asins are not complete enough for this purpose. In most cases, reliabl e values for pH, dissolved silica, and aluminium are lacking. Such dat a can be reconstructed using various relations developed herein. These data are constrained by petrographic observations. Because of their w ell preserved, or authigenic character, some alumino-silicates are con sidered as relatively stable and can be thought of as effectively satu rated with respect to them. The saturation state of the other minerals of the matrix can be discussed according to these constraints. The su b-arkosic series of the Mahakam basin reservoirs (Mio-Pliocene) are us ed as a reference example to perfect a method capable of reconstructin g consistent compositions of diagenetic waters, suitable for input int o diagenetic modelling. (C) 1997 Elsevier Science Ltd.