Page 83 - Geology and Geochemistry of Oil and Gas

ABNORMALLY-HIGH FORMATION PRESSURE 59

Fig. 3.14. Dependence of the montmorillonite content on the geothermal gradient (a) and on the pore-
pressure gradient (b) in shales (after Buryakovsky et al., 1995, Fig. 7, p. 211).

and the adjacent offshore areas, which have a higher geothermal gradient
(24.0–28.51C/km), are characterized by lower montmorillonite contents.
Low temperature apparently does not favor the transformation of montmor-
illonite to illite. Under otherwise equal conditions, this transformation increases with
depth, which means that some additional factors must be influencing the
transformation. One of these factors, discussed by Serebryakov et al. (1995), is the
lack of potassium ion in interstitial water.
Inasmuch as the transformation of montmorillonite into illite proceeds with the
removal of interlayer water from montmorillonite, conditions at which the desorbed
water can enter the pore space without hindrance will be favorable for the
development of this process. Factors opposing the withdrawal of interlayer water
from clays, may lead to slowing down or cessation of the transformation of
montmorillonite into illite or chlorite. The writers believe that such a factor is the
abnormally high pore pressure. The fluid pressure gradients in shales at
1000–6000 m, which are based on more than 2000 determinations, range from
0.012 to 0.024 MPa/m, with a mean of 0.018 MPa/m (see Fig. 3.15 and Table 3.3).
The dependence of montmorillonite content on the pore-pressure gradient in
shales is shown in Fig. 3.14b. There is a close correlation between these two
parameters. In regions of the Baku Archipelago and Lower Kura Depression,
characterized by intense development of AHFP (pore pressure gradients in shales of
0.018–0.019 MPa/m), the montmorillonite content in shales reaches an average of
53%. In regions with moderate development of AHFP (Absheron Archipelago and
the South Absheron Offshore Zone), the montmorillonite content decreases to 17%.
There is no adequate discussion in the literature on the effect of pore pressure in
shales on the clay–mineral diagenesis and catagenesis. Theoretically, the rising
pressures reduce the dehydration rates. The production of illite in shales involves an
increase in the volume of free water as a result of the release of bound water, which is
denser than the free water. Factors opposing this increase in volume (such as high
pore pressure in shales) will reduce the dehydration rate.