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64 TEMPERATURE AND PRESSURE IN THE SUBSURFACE

Fig. 3.17. Relationship between the montmorillonite content and total salinity of formation water (after
Buryakovsky et al., 1995, Fig.9, p. 213). Water type: 1 – sodium bicarbonate, 2 – calcium chloride.

3.2.6. Secondary montmorillonite

According to the data cited above, a rather close relation exists between the
various clay mineral contents and the formation temperature, pressure, and water
chemistry in the stratigraphic section of the South Caspian Basin. The stability of
montmorillonite at great depths depends on many parameters. In the section of Baku
Archipelago at depths greater than 4–5 km, formation of secondary montmorillonite
from illite was observed using SEM. This is explained by the relatively low
temperatures, abnormally high pore pressures in shales, and the alkaline pore water
+
+

enriched in K , Na and HCO 3 , ions (see Buryakovsky et al., 2001).
The postsedimentary (diagenetic and catagenetic) transformation of Middle
Pliocene shales of the South Caspian Basin is characterized by the retardation of the
process of transformation of montmorillonite into illite or chlorite at great depths,
and the substitution of this process by the process of transformation of illite into
highly swelling minerals of the montmorillonite (smectite) group. These processes are
closely related to the low geothermal gradient and increasing pressure at depth and
inverted hydrochemical profile of these deposits.

3.3. ORIGIN OF ABNORMAL FORMATION PRESSURES

Various origins of abnormally-high formation pressure (AHFP) and abnormally-
low formation pressure (ALFP) are discussed in detail by Chilingar et al. (2002). The