MATERIAL FLUX AND POROSITY CHANGES DURING SEDIMENT DIAGENESIS

UNDERSTANDING the changes in porosity that occur during burial of sediments is of importance both in modelling fluid flow in sedimentary basins and for prediction of petroleum reservoir quality. When deposited, up to 50% of the volume of sands is intergranular pore space. Porosity generally decreases with depth because of compaction and precipitation of diagenetic minerals, but some sandstones retain an anomalously high porosity. Whether this is due to dissolution and removal of material or to its local redistribution is not clear. We originally intended to quantify losses of major chemical components during burial of a complete sedimentary unit. We have investigated ten reservoir sandstones, of Permian to Tertiary age, from oilfields worldwide. These show consistent results, and here we present detailed data from four of them. We found that the silica content of these sandstones actually increased, by 220 to 350 kg m-3, following compaction. No statistically significant changes were observed for aluminium, potassium or sodium. The flux of silica cannot be modelled satisfactorily using currently accepted values of permeability, silica solubility and flow rates, posing a challenge for existing models of fluid flow and ore emplacement.