The differences noticed in the porosity-permeability values and the clay mineralogy of the Brent and Statfjord reservoirs of two closely related wells from the Central Viking Graben appear to be due essentially to diagenesis. According to their type, nature and relationships, the diagenetic phases are organized into three distinct sequences, each induced by distinct hydrodynamic systems. These systems are related to three main episodes of the geological history of the reservoirs, which were: the compaction-lithilication phase, the Kimmerian to Lower Cretaceous emersive phase, and the hydrologic loading of the deep parts of the basins and/or fault panels. The late diagenetic sequence is expressed by quartz overgrowths, blocky kaolinite and illite authigenesis. With a probable ‘catalytic’ effect of temperature, this sequence is the most intensely developed and so greatly damages the porosity and permeability. In the oil-bearing reservoirs of Well 25/4-1, the stage of evolution of this third diagenetic sequence is not well advanced: illitization was rapidly stopped by the hydrocarbon charge so that permeabilities remained satisfactory (up to 500 mD). In the water-zone of Well 25/4-5, illitization is extensive and permeabilities considerably reduced (lower than 5 mD). K/Ar dating of authigenic illites provides evidence of a late Eocene phase (40–44 Ma) as the maximum age for the hydrocarbon entrapment in Well 25/4-1, whereas the last illite authigenesis from the water-zone of Well 25/4-5 gives an Oligocene phase (28 Ma in the Statfjord Formation). These data indicate that fluid circulation was effective until this time in the down-dip faulted panel of Well 25/4-5, and has therefore been responsible for the drastic permeability reduction.