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58 TEMPERATURE AND PRESSURE IN THE SUBSURFACE

3.2.4. Effect of pressure and temperature

The abnormally low temperatures may be responsible for the absence of clear-cut
clay–mineral transformation. Khitarov and Pugin (1966) and Magara (1982) have
indicated that temperature is a major factor influencing montmorillonite degrada-
tion. Also of interest is the effect of illite degradation on the geothermal gradient.
Inasmuch as hydration of clays is an exothermic reaction, there may be elevated
gradients at depth ranges where the illite is transformed to montmorillonite, all other
conditions being equal.
In the areas of South Caspian Basin, the average geothermal gradient is
approximately 161C/km, and the temperature at a depth of about 6 km does not
exceed 1101C. A characteristic feature is that the geothermal gradient becomes lower
at a depth of approximately 4 km (Table 3.3). Table 3.3 includes also the data on
pore-pressure gradient in these areas.
The increased geothermal gradient at a depth of about of 4 km may be related to
the illite-to-montmorillonite transformation, which releases heat. At a depth of
about of 4 km, the transformation rate exceeds some limit, which causes hydration to
predominate over dehydration. One should, therefore, consider the effects of
temperature on diagenetic and catagenetic processes.
An increase in temperature may accelerate the process of montmorillonite
transformation into non-swelling minerals (illite and chlorite). Consequently,
sections with high geothermal gradient should be characterized by a small
montmorillonite content. On the other hand, inasmuch as a temperature decrease
retards the process of montmorillonite transformation, sections with low geothermal
gradient should be characterized by a high montmorillonite content.
Fig. 3.14a shows the dependence of montmorillonite content on the geothermal
gradient in shales of the South Caspian Basin. The highest montmorillonite contents
are found in the shales of the Baku Archipelago and Lower Kura Depression, which
are characterized by a low geothermal gradient (161C/km). The Absheron Peninsula

TABLE 3.3

Variation of geothermal gradient and pore pressure gradient with depth (average values are presented in
the denominator)
Depth range (m) Geothermal gradient (1C/km) Pore-pressure gradient (MPa/m)
1000–2000 10215 0:01220:020
12 0:016
2000–3000 10212 0:01320:021
11 0:017
3000–4000 8211 0:01420:022
10 0:018
4000–5000 15219 0:01520:023
17 0:019
5000–6000 21223 0:01520:023
22 0:019
46000 15225 0:01620:024
20 0:020