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Abnormal pore pressure mechanisms 259

case that smectite to illite transformation is only as an addition to the
overpressure, and primary abnormal pressure is caused by under-
compaction, as shown in Fig. 7.18. Because rock properties change greatly
from smectite to illite, it should use two different NCTs (or a composite
one); otherwise, if only illite NCT is used, the shallow pore pressure will be
overestimated, particularly if this NCT is calibrated by the measured pore
pressures in the deep formations.

7.4 Pore pressure seals and compartments

Pressure seal and compartment are two important concepts in pore pressure
prediction. A seal is formed by impermeable or extremely low permeability
lithology such as evaporite, salt, or shale. Pressure compartment has
impermeable outer seals and an internal volume (e.g., sandstone). Each
compartment exhibits effective internal hydraulic communication and has a
similar pressure gradient. Effective pressure seals are of critical importance to
maintain pore pressures in a pressure compartment. The pressure seals may
be formed by faults, salt and mud diapirs, and vertical or lateral facies
changes. The overall rate of pressure change across seals in shale has been
observed to be as great as 15 psi/ft (0.34 MPa/m) (Powley, 1990). Fig. 7.19
shows the pressure difference of more than 15 MPa across a seal. Fig. 7.20
displays that the difference of pore pressure gradients is 0.8 ppg (w0.1 SG)

Figure 7.19 Measured fluid pressures in two sandstone formations showing that there
are obvious pressure compartments, implying a seal between the two sandstones
exists.