Page 136 - Origin and Prediction of Abnormal Formation Pressures

SMECTITE-ILLITE TRANSFORMATIONS 1 13

in shales may reduce the montmorillonite dehydration rate and release of water. The
result will be similar to that from a low geothermal gradient, i.e., reduction in the
rate of hydromica formation with depth. Under favorable conditions, the hydromicas
may also be hydrated; this is accompanied by a release of heat and leads to their
transformation to a secondary montmorillonite. The relative magnitudes of desorption
of water and hydromica hydration may determine the rate of development of anomalous
pressure.
The sedimentation rate and sediment sources do not remain constant with time; some
zones may differ in the dehydration rate because of changes in the sedimentation rate
or type of sedimentary material. Transitions from a zone with normal pressures and
normal dehydration rate to an AHFP zone may indicate either diagenetic and catagenetic
processes, or a lag in the development of diagenetic and catagenetic processes. The
montmorillonite content may remain the same or even increase with depth, but this does
not necessarily mean that the process of dehydration of montmorillonite to hydromica
is replaced by the hydromica hydration, although this is possible. Instead, it could mean
that dehydration process in the AHFP zones is slow; therefore, these zones may be
characterized by higher montmorillonite contents than those in younger zones of normal
shale pore pressure.

EFFECT OF HYDROCHEMICAL FACTORS

The hydrochemical environment in a basin of sedimentation has a significant
influence on the intensity of postsedimentary transformation. First it is important,
therefore, to ascertain the nature of the hydrochemical regime observed in the Cenozoic
complex of the South Caspian Basin, namely: whether it is a consequence of diagenetic
and catagenetic processes in shales and the transformation of clay minerals, or it is
formed predominantly as a result of the action of other factors. In this connection,
the problem presenting the greatest interest is the origin of the inverted hydrochemical
profile in the section of the South Caspian Basin, i.e., replacement of deep calcium
chloride waters by little-mineralized sodium bicarbonate waters. Numerous data from
laboratory analyses and field observations indicate a decrease in the mineralization
of pore waters in sands with depth. Replacement of calcium chloride water by alkali
sodium bicarbonate water is characteristic for the AHFP zones in the South Caspian
Basin areas (Buryakovsky, 1974). Analogous data on the decrease of formation water
salinity with increasing pressure have also been noted in the Gulf of Mexico (Fertl,
1976).
According to Chilingar (1957) the relationship between the chemical composition of
the Apsheron Peninsula waters and the stratigraphic depth is subject to the following
rules. (1) The mineralization of water decreases with stratigraphic depth (also see Rieke
and Chilingarian, 1974, pp. 265-269; Samedov and Buryakovsky, 1966). (2) CI-, Ca 2+,
and Mg 2+ ions decrease with depth. (3) (Na + + K +) and (HCO~- + CO 2- + H. K-) ions
gradually increase with depth. (4) The transition from hard to alkaline waters occurs at
maximum concentration, not exceeding 0.1 g-equ per 100 g of water (5-6.5~ As
a rule, the waters are hard at concentrations above 0.1 g-equ. (5) The HCO~- content