Silica diagenesis can significantly change physical properties of the host strata and release large volumes of water. Predicting these changes and their timing is essential to understanding compaction, fluid flow and rock deformation in sedimentary basins. In this paper, the influence of silica diagenesis (opal-A/CT transformation) on physical properties is determined, the sediment volume affected by these changes is mapped, and a new technique to model silica diagenesis is introduced. A petrophysical analysis of 16 exploration wells shows that the opal-A/CT transformation leads to a porosity reduction of c.20% (from 49 to 29%) in Cenozoic mudstones of the North Viking Graben. Using three-dimensional seismic reflection data, it is shown that the c.50 m thick opal-A/CT transformation zone covers an area of >1500 km2, equating to a minimum volume of 75 km3. The spatial and temporal evolution of opal-A/CT transformation is simulated using an innovative basin modelling approach, the results of which indicate that the transformation started around Middle-to-Late Eocene times and then migrated upwards until it gradually fossilised between the Miocene and present. These findings are important, as they help understanding how these sediments compact and when fluids are released by diagenesis.