Page 345 - Petroleum Geology

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The depth of generation of abnormal pressures is also important in under-
standing abnormal pressures because, as we shall see in the next chapter, the
relative incompetence of abnormally pressured mudstones can lead to defor-
mation of the regressive sequence due to mechanical instability, and this de-
formation can lead to the creation of petroleum traps. The mechanical hypo-
thesis requires a shallow depth of initiation of abnormal pressures, and this
depth is estimated by the effective compaction depth, ze (= 6z). The effec-
tive compaction depth is therefore given a real meaning: it indicates the maxi-
mum depth at which compaction equilibrium was lost, and the overburden
at that time is represented by the sequence of sediments of thickness ze above
the mudstone being considered. This is found to be typically about 600 m or
2000 ft near the bottom of the transition zone, and to extend upwards into
the sandy part of the overburden.
This interpretation is supported by occurrences of abnormal pressures at
such depths. In their classic study of Pedernales in the Orinoco delta, Kidwell
and Hunt (1958, p. 805, fig. 8) found that mudstones above 45 m (140 ft),
less than 10,000 yrs old from radio-carbon dating, had pore pressures above
normal hydrostatic. In the East Coast basin of North Island, New Zealand,
the well Rotokautuku 1 encountered abnormal pressures at 356 m (1168 ft)
and the well had to be abandoned at 627 m (2057 ft) because of the difficul-
ties they caused (Katz, 1974, p. 469). Spinks (1970) reported abnormal pres-
sures at 640 m (2057 ft) in the Gulf of Papua. Figure 14-8 shows the measured

PRESSURE ''
MPa 20 500rn.

I
+
a
w
a

1

m ft r103 A=A, 0.5 0.6 0.7 0.8 1.0 0.9 0.8 0.7
Fig. 14-8. Pressure-depth plot of shallow oil reservoirs in Trinidad, with their correspond-
ing equilibrium depths. (Data courtesy of Royal Dutch/Shell Group.)