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264 Applied Petroleum Geomechanics

Figure 7.23 Seals and pressure compartments retaining overpressures in the Venture
field, Canada. (Based on Wheeler, 2011.)

vertical stress; therefore, in situ stress state in this region belongs to the
strike-slip faulting stress regime.
Fig. 7.23 shows that deeper than the top of overpressure at 4500 m,
several shale seals and sandstone pressure compartments are recognized in
the Venture field. These seals make the rapid transitions to high over-
pressures possible and are key factors to retain overpressures.

7.5.4 Abnormal pressures in the Central Graben, the North
Sea
The Central Graben of the North Sea is one of the world’smostprolific
oil-producing provinces. It contains major reserves of oil, gas, and
condensate presented in reservoirs ranging in age from the Devonian to
Early Eocene. These include super giant fields such as the Ekofisk. The
Upper Jurassic sandstones in the Central Graben vary from being normally
pressured, 0.01 MPa/m (0.44 psi/ft) near the graben margins, to pressure
gradients exceeding 0.02 MPa/m (0.88 psi/ft) in the center of the graben.
The Paleocene sandstones consist of sheet sandstones forming a normally
pressured regional aquifer, and the Chalk Group, where overlain by these