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

Abstract

During mineral diagenesis, the chemical composition of pore water can be strongly influenced by mineral reactions, especially under geological conditions where water/rock ratios are low. To model and eventually to predict diagenesis, it is important to have an understanding of formation water composition. However, available water analyses in many basins are not complete enough for this purpose. In most cases, reliable values for pH, dissolved silica, and aluminium are lacking. Such data can be reconstructed using various relations developed herein. These data are constrained by petrographic observations. Because of their well preserved, or authigenic character, some alumino-silicates are considered as relatively stable and can be thought of as effectively saturated with respect to them. The saturation state of the other minerals of the matrix can be discussed according to these constraints. The sub-arkosic series of the Mahakam basin resevoirs (Mio-Pliocene) are used as a reference example to perfect a method capable of reconstructing consistent compositions of diagenetic waters, suitable for input into diagenetic modelling.