OVERPRESSURE AND HYDROCARBON TRAPPING IN THE CHALK OF THE NORWEGIAN CENTRAL GRABEN

The hydrodynamic system within the Chalk of the Greater Ekofisk area a nd its effect upon pore pressure distribution and hydrocarbon trapping has been analysed. The Chalk of the area is a relatively closed syste m for fluid movement and has large pressure cells characterized by the ir 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 Albusk jell, but Valhall is structurally shallower than Albuskjell. This tren d corresponds to a maturity trend for the reservoir fluids, and is rel ated to burial depth, temperature and hydrocarbon charging. As total p ore pressure at the top of the structures reaches fracture pressure, l eakage occurs. The gas cloud observed on seismic data above Valhall ma y be due to present-day leakage, whilst the gas clouds above other fie lds could be related to leakage which occurred when the fracture press ure was lower than today. Overpressuring is a dynamic process dependen t on burial, hydrocarbon generation and migration, diagenesis and seal integrity and it controls both chalk porosity and hydrocarbon trappin g.