Page 267 - Applied Petroleum Geomechanics

258 Applied Petroleum Geomechanics

driven by temperature; and therefore, the composite normal compaction
trend will be dependent on the temperature gradient. Alberty and McLean
(2003) pointed out that in reality, it should have a compaction trend
honoring the smectite and illite characterizations, to follow the smectite
trend down to the onset of the diagenetic conversion and then cross over to
the illite trend within the diagenetic window and then follow the illite
trend thereafter.
Zhang and Yin (2017a) proposed a multisegmental NCT, which has
different compaction parameters (c s and c i ), as shown in Fig. 7.18, i.e.,
For smectites : Dt s ¼ Dt m þðDt ml Dt m Þe c s Z (7.11)
For illites : Dt i ¼ Dt m þðDt ml Dt m Þe c i Z (7.12)

ðZ Z 1 ÞDt i þ ðZ 2 ZÞDt s
For a linear S I transition : Dt t ¼ (7.13)
ðZ 2 Z 1 Þ
where Dt s , Dt i , Dt m , Dt ml are the transit time in smectite, illite, matrix and
mudline, respectively; c s and c i are the compaction parameters for smectite
and illite, respectively; Z 1 is the depth of the smectite; Z 2 is the depth of the
illite; Z 1 and Z 2 can be determined from mineral test results in offset wells
or estimated from the regional temperature profile, which is associated with
the SeI transformations.
This composite NCT can be used to predict the overpressure that is
solely caused by smectite to illite transformation. It can also be used for the

Transit time Transit time Pressure
0 240 0 240
Δt 0 Δ Δt 0
Δt s Δt s
Smectite
Hydrostatic pressure
Z 1
Δt t S-I transition Δt t
Depth Z 2 Overpressure by S-I
transformation

Illite
Under-compaction p n
Δt
Δt i Δt i p
Overpressure by S-I & under-
compaction
Figure 7.18 The NCTs in the transit time versus depth for a shale composed of
smectite and illite. Left: composite NCT; middle: composite NCT and transit time; right:
overpressure caused by smectite to illite transformation and undercompaction.