Page 142 - Origin and Prediction of Abnormal Formation Pressures

SMECTITE-ILLITE TRANSFORMATIONS 1 19

TABLE 4-9
Relative proportions of primary and secondary montmorillonites (in %) using different magnifications (after
Buryakovsky et al., 1995, table 8, p. 215)

Photomicrograph Magnification Montmorillonite content (%) Total content Portion of
number primary secondary ( % ) secondary
montmorillonite
(%)
1 3000 14.5 7.1 21.6 32.9
2 1000 13.5 6.5 20.0 32.5
4 3000 10.5 5.4 15.9 34.0
5 1000 14.3 5.2 19.5 26.7

for montmorillonite changes from 3 to 16 km. Inasmuch as the average geothermal
gradient in the Baku Archipelago is 16~ the limiting depth may be 8-9 kin. On
the basis of the data obtained by Khitarov and Pugin (1966), Buryakovsky et al. (1995)
suggested the following equation relating the depth of montmorillonite occurrence H in
km to the geothermal gradient G in ~

H - 261G -123 (4-2)

Stratigraphic sections with abnormally high pore pressures may, however, have even
greater limiting depths. Inasmuch as there is a linear relationship between pressure and
depth, the equation for the limiting depth Hlim, can be presented as follows"

nlim- 261KaG -123 (4-3)

where Ka is a dimensionless factor to account for the pore-pressure anomaly (ratio of
the actual (or predicted) pore pressure to the hydrostatic pressure).
Predicting clay-mineral transformations in the Caspian Sea region at depths > 6.5
km is of great importance. Data derived from extrapolation and from physical and
mathematical simulation (Buryakovsky et al., 1982) indicate that conditions at depths
of 9 km or more in the South Caspian Basin do not favor catagenesis of argillaceous
rocks. As shown by Buryakovsky et al. (1982), the porosity of shales at depths of more
than 9 km can be as high as 10%, which means that there are abnormally high pore
pressures of gravity-filtration origin. Using Eq. 4-3, at G -- 16~ and Ka = 1.8, the
limiting depths are found to be 15-17 km in the center of the basin. This indicates that
the shales retain their sealing properties, whereas the reservoir rocks have rather high
porosity (the predicted porosity at a depth of 9 km is on the average 7-9%, whereas the
permeability is 1.5 to 11 mD. The presence of abnormally high pressures and relatively
low temperatures indicate that hydrocarbons may be present; thus, it is likely that the
South Caspian Basin may have commercial accumulations of oil and gas at depths of 9
km or deeper.