Page 247 - Applied Petroleum Geomechanics

Abnormal pore pressure mechanisms 241

Converting a pressure into the EMW at a given depth can use the
following equation:
pressure ðpsiÞ
EMWðppgÞ¼ (7.4)
0:051948 TVD ðftÞ
where TVD is the true vertical depth in ft.

7.2 Origins of abnormal pore pressures

Overpressures can be generated by many mechanisms, such as compaction
disequilibrium (undercompaction); hydrocarbon generation and gas
cracking; aquathermal expansion; tectonic compression (lateral stress);
mineral transformations (e.g., illitization); and osmosis, hydraulic head, and
hydrocarbon buoyancy (Swarbrick and Osborne, 1998). The following
mechanisms will be examined in the following sections:
• Compaction disequilibrium;
• Hydrocarbon generation;
• Formation uplift and unloading;
• Smectite diagenesis.

7.2.1 Overpressures by compaction disequilibrium
In many cases, compaction disequilibrium or undercompaction has been
determined to be the primary cause of overpressure, particularly in
geologically young rocks (such as formations in the Tertiary, i.e. Neogene
and Paleogene periods) and fast subsided basins (sedimentation
rate > 500 ft/m.y. or 152 m/m.y.). Examples of areas where compaction
disequilibrium is cited as the primary reason of abnormal pressure include
the US Gulf Coast, Alaska Cook Inlet; Beaufort Sea; Mackenzie Delta;
North Sea; Adriatic Sea; Niger Delta; Mahakam Delta; Nile Delta; Malay
Basin; Eastern Venezuelan Basin (Trinidad); and Potwar Plateau of Pakistan
(Law and Spencer, 1998; Burrus, 1998; Heppard et al., 1998; Powley,
1990; Nelson and Bird, 2005). In these areas, the abnormally pressured
rocks are mainly located in the Tertiary and late Mesozoic sedimentary
formations, the depositional setting is dominantly deltaic, and the lithology
is dominantly shale.
When sediments compact normally, formation porosity is reduced at the
same time as pore fluid is expelled. During burial, formation overburden
stress increase is the prime cause of fluid expulsion. If the sedimentation rate
is low, normal compaction occurs, i.e., equilibrium between increasing