Page 263 - Applied Petroleum Geomechanics

254 Applied Petroleum Geomechanics

Osborne, 1998). The first two are likely to take place within the top 1 km
of burial, with only the last pulse at depths where significant amounts of
overpressure are measured. However, the volume of water released is only
about 1.4% and will not create significant overpressure unless the rock is
completely sealed. Colton-Bradley (1987) suggested that overpressure
would inhibit the dehydration reaction because the dehydration tempera-
tures are elevated with increasing pore fluid pressure. The smectite dehy-
dration reaction is therefore thought to be a secondary rather than a major
cause of overpressure but may be additive to overpressure created by
compaction disequilibrium.
In several shale-dominated basins a gradual and systematic change from
smectite to illite downwards in the stratigraphic section is observed, broadly
coincident with the transition to high amounts of overpressure (Bruce,

1984). The transition occurs over a temperature range of 70e150 C and
appears to be independent of sediment age and burial depth. By contrast, in
the highly overpressured Caspian Sea basin, there is no change in smectite

to illite ratio to a depth of 6 km and temperature of 96 C(Bredehoeft et al.,
1988). Hence the origin of overpressure by smectite diagenesis is not
conclusive. However, the coincidence of overpressure at the same strati-
graphic levels as smectite to illite transformation may be related to the
ensuing changes in the rock fabric, trapping excess fluids generated by
another mechanism, e.g., compaction disequilibrium. Another conse-
quence of the mineral transformation from smectite to illite may develop a
hydraulic seal by the growth of coalescing illite packets and reduce
permeability. This would help to retain fluids and hence contribute to the
preservation of overpressure (Freed and Peacor, 1989).

7.3.2 Smectite and illite transition identified by rock
properties

In some area, an obvious transition from smectite to illite (SeI) occurred in
shallow depths (e.g., around 1000e2000 m). This transition not only
changes rock properties (e.g., sonic velocity, bulk density, elastic modulus)
but also affects pore pressure prediction. From the cross-plot of bulk density
and sonic transit time, the smectiteeillite transition with depths can be
observed in the offset wells. Alberty (2005) found that bulk density and
sonic transit time have different empirical relations for smectite and illite in
the Gulf of Mexico:
Smectite : r ¼ 2:918 0:00517Dt (7.8)
s