Page 84 - Geology and Geochemistry of Oil and Gas

60 TEMPERATURE AND PRESSURE IN THE SUBSURFACE

Fig. 3.15. Pore fluid pressure gradient (Z) in shales (small circles, 1) and in reservoir rocks (large circles, 2)
in the Baku Archipelago, South Caspian Basin (after Buryakovsky et al., 1995, Fig. 2, p. 205).

On the other hand, AHFP can lead to transformation of illite to secondary
montmorillonite by the absorption of water. At AHFP, the smaller particle size of
clay minerals favors the transformation of illite, as shown by the relationship between
the pore size (determined from SEM data) and depth (Table 3.2 and Fig. 3.16).
The writers propose the following scheme for the relationship between the
clay–mineral transformation and the formation temperature and pressure.
In a basin where the subsidence rate is equal to the rate of accumulation of
sediments, the depth at which diagenetic (and catagenetic) transformation
(desorption of water) begins remains more or less the same and is largely determined