Geochemistry of Porosity Enhancement and Reduction in Clastic Sediments

Abstract

Summary The diagenetic history of many reservoir sandstones appears to be divisible into three stages. The first includes destruction of primary porosity, the second porosity enhancement by chemical leaching, and the third a return to loss by cementation and recrystallisation. In an attempt to account for the solution transport necessarily involved, the several types of water-rock interaction that take place during burial of clastic sediment sequences are reviewed. It is concluded that kinetic factors play an important role: some reactions are spontaneous and fast, others are dependent on ‘initiating’ reactions and are slower. ‘Initiating’ reactions modify pore water composition drastically and are thought to be mainly redox or acid-generating reactions. They take place mostly in mudrocks which contain much more unstable material than sandstones. Comparisons between mudrock and sandstone diagenetic patterns lend support to previous suggestions that much diagenetic alteration in sandstones is affected by pore waters (and solutes) originating in mudrocks. Aluminium is thought to be mobile under these conditions. A brief consideration of mineral/water reactions in soils leads to the view that pore waters derived directly from meteoric water are unlikely to be effective agents for leaching at depth.