The transformation of biogenic silica (opal-A) to cryptocrystalline opal-CT (cristobalite and tridymite) and subsequent transition of opal-CT to quartz are important diagenetic reactions that may occur in siliceous sediments within many sedimentary basins worldwide. These diagenetic transitions are basin-scale features and can have areal extents of 10⁴ -10⁵ km². These reactions can have a significant effect on the physical properties, especially porosity, seismic velocity and density, of the host sediment. The role played by silica diagenesis within sedimentary basins is poorly understood, but it is becoming apparent that these reactions could have significant implications for basin analysis in terms of the sedimentological and deformational history of the basin as well as for hydrocarbon exploration.