Page 274 - Applied Petroleum Geomechanics

Abnormal pore pressure mechanisms 265

Figure 7.24 Pressureedepth profile for a well drilled in the center of the Central
Graben (Holm, 1998). Notice that the chalk in the Late Cretaceous has a lower pore
pressure, slightly higher than the hydrostatic pressure. The hard overpressure occurs
below the chalk.

sandstones, is similarly normally pressured. Fig. 7.24 displays a pressuree
depth profile for a well drilled in the center of the Central Graben
(Holm, 1998). It has normal pore pressure in the chalk of the Late
Cretaceous, but the hard overpressure occurs in the Jurassic rocks below
the chalk.
Compaction disequilibrium and hydrocarbon generation are important
processes in the development of overpressures within rocks of the Chalk
Group and pre-Cretaceous of the Central Graben, with hydrocarbon
generation considered as the dominant cause of overpressure in rocks below
depths of 4500 m (Holm, 1998). In a study of the Norwegian Central
Graben, Leonard (1993) subdivided the observed pressure regime into three
vertically stacked compartments, the Tertiary, the Chalk Group, and the
pre-Cretaceous. The lowermost compartment is highly overpressured in