The hydrodynamic system within the Chalk of the Greater Ekofisk area and its effect upon pore pressure distribution and hydrocarbon trapping has been analysed. The Chalk of the area is a relatively closed system for fluid movement and has large pressure cells characterized by their own pressure regime, fluid type and trapping potential. The aquifer is near static and there is no significant regional hydrodynamic flow. Overpressure is higher at Valhall than at Eldfisk, Ekofisk or Albuskjell, but Valhall is structurally shallower than Albuskjell. This trend corresponds to a maturity trend for the reservoir fluids, and is related to burial depth, temperature and hydrocarbon charging. As total pore pressure at the top of the structures reaches fracture pressure, leakage occurs. The gas cloud observed on seismic data above Valhall may be due to present-day leakage, whilst the gas clouds above other fields could be related to leakage which occurred when the fracture pressure was lower than today. Overpressuring is a dynamic process dependent on burial, hydrocarbon generation and migration, diagenesis and seal integrity and it controls both chalk porosity and hydrocarbon trapping.