Page 86 - Geology and Geochemistry of Oil and Gas

62 TEMPERATURE AND PRESSURE IN THE SUBSURFACE

by the geothermal gradient. Inasmuch as the desorbed (interlayer) water is added to
the interstitial water, abnormally high pore pressures may develop if the water
cannot escape. Under some conditions, the rising pore pressure in shales may reduce
the montmorillonite dehydration rate and release of water. The result will be similar
to that arising from a low geothermal gradient, i.e., reduction in the rate of illite
formation. Under favorable conditions, the illite may be hydrated, which is
accompanied by a release of heat and its transformation to secondary montmor-
illonite. The relative intensity of dehydration (illite formation) and the illite
hydration (formation of secondary montmorillonite) may determine the pore
pressure.
The sedimentation rate and the sediment sources do not remain constant
with time. Thus, different 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 the effect of diagenetic and catagenetic processes or a lag in the
development of these processes. The montmorillonite content may remain the same
or even increase with depth. This, however, does not mean that the process
of dehydration of montmorillonite to illite is replaced by the illite 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 (or equal) montmor-
illonite contents than those in the younger zones with normal shale pore pressure.

3.2.5. Effect of formation water chemistry

The formation water chemistry has a significant influence on the intensity of
postsedimentary transformations. Thus, it is important to ascertain the nature of
hydrochemical scenario observed in the Cenozoic sequence 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 (e.g., compaction). In this connection, the
problem presenting the greatest interest is the origin of inverted hydrochemical
profile in the section of the South Caspian Basin, i.e., with depth, calcium chloride
waters are replaced by less saline sodium bicarbonate waters. The writers obtained
numerous data from the laboratory analyses and field observations, indicating a
decrease in the salinity of pore waters in sands with depth. Replacement of calcium
chloride water by alkaline 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 were also noted in the
Gulf of Mexico (Fertl, 1976).
The appearance of hydrochemical inversion in the stratigraphic section of
the South Caspian Basin may be explained by the genetic relationship between
the hydrochemical environment and the development of abnormally high pore
pressures in shales. The water of primarily sodium bicarbonate type characterizes the