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Abnormal pore pressure mechanisms 243

of overpressure. In the formation with undercompaction, porosity and pore
pressure are higher than those in the normally compacted formation.
To predict abnormal pore pressure generated by compaction disequi-
librium, one needs to obtain the porosity under normal compaction
condition. It is commonly accepted that porosity decreases exponentially as
depth increases in normally compacted formations (e.g., Athy, 1930):

f ¼ f e cZ (7.5)
n
0
where f n is the porosity in normally compacted formation; f 0 is the
porosity of the mudline; Z is the true vertical depth below the mudline;
c is the compaction constant in 1/m or 1/ft.
This relation has been widely applied because in many shales and some
sandstones, normal porosity profiles generally show a concave downward
curvature (Fig. 2.5), as described in Eq. (7.5). A similar relationship
(Eq. 2.17) exists between porosity and effective stress (e.g., Dutta, 2002;
Flemings et al., 2002; Zhang, 2011). Using porosity, effective stress, and
pore pressure relationship, the pore pressure can be obtained from porosity
data based on compaction equilibrium theory (Zhang, 2011; Zhang and
Wieseneck, 2011). Fig. 7.7 presents an example on how to use compaction
disequilibrium to calculate pore pressure. Firstly, bulk density or sonic log in
offset wells are analyzed to obtain the porosity in normally compacted
shales, including to pick shale formations from the gamma ray log and
calculate porosity from bulk density log in the picked shales (e.g., Eq. 2.6).
By doing so, the porosity points of shales at different depths can be obtained
(the black points as shown in Fig. 7.7). The normal compaction trendline
(NCT) can be obtained (f n ¼ 0.46e 0.001118Z , Z is in meters) from
Eq. (7.5) based on the shallow porosity data. Fig. 7.7 plots the porosity
variations with depth compared to the NCT in both linear and logarithmic
scales. It shows that the shale is in normal compaction condition when
depth is less than 3200 m. From 3200 to 3600 m, the formations are slightly
undercompacted with a higher porosity than the normal compaction trend.
This implies that the pore pressure gradient starts to increase. The porosity
at depth of >3600 m is significantly higher than the normal compaction
trend, and the pore pressure is highly overpressured (Fig. 7.7) (refer to
Chapter 8 for more details in pore pressure calculations).

7.2.2 Overpressures from hydrocarbon generation
Generation of liquid and gaseous hydrocarbons from kerogen maturation is
kinetically controlled and dependent on a combination of time and