Page 78 - Geology and Geochemistry of Oil and Gas

54 TEMPERATURE AND PRESSURE IN THE SUBSURFACE

Fig. 3.12. Experimentally determined relationship between the pore pressure (p p ) and the total stress (s)
for an argillaceous rock core (after Buryakovsky et al., 1995, Fig. 6, p. 207). Arrows show increasing and
decreasing total overburden stress (confining pressure).

In studying the abnormally-high formation pressure, of special interest are the
young sedimentary basins, which are characterized by the presence of thick, rapidly
accumulated sand/shale sequences. A vivid example is the South Caspian Basin,
which is distinguished by a diverse and rather unique association of the following
features:
an exceptionally high rate of sedimentation (up to 1.3 km/my) (my ¼ million
years);
a very thick (up to 25 km) sedimentary column; thickness of the Quaternary –
Pliocene sand–silt–shale sediments up to 10 km;
argillaceous rocks make up 50–95% of the section and play a key role in
determining the mineralogic, lithologic, geochemical, and thermodynamic
characteristics of the basin;
5
abnormally-high pore pressure in shales (average factor of abnormality up to
K a ¼ 1:8);
low heat flow and low formation temperature (at depths around 6 km, the
temperature is approximately 105–1101C);
an inverted character of the water chemistry and total salinity profile (the
chemistry of water changes with depth from calcium chloride and magnesium
chloride to sodium bicarbonate type, and water salinity decreases with depth);
wide development of mud volcanism.

5
Abnormality factor K a ¼ p a =p n , where p a is abnormally-high formation pressure and p n is the normal
hydrostatic pressure.