Page 87 - Geology and Geochemistry of Oil and Gas

ABNORMALLY-HIGH FORMATION PRESSURE 63
most pronounced AHFP zones in the Baku Archipelago and Lower Kura
Depression.
According to Chilingar (1957), the relationship between the chemical composition
of the Absheron Peninsula waters and the stratigraphic depth is subject to the
following rules:
(1) The total salinity of formation waters decreases with stratigraphic depth (also see
Samedov and Buryakovsky, 1966; Rieke and Chilingarian, 1974, pp. 265–269).

(2) The Cl , Ca 2+ and Mg 2+ contents decrease with depth.
+
+
+

2
+
(3) (Na +K ) and (HCO 3 +CO 3 +H +K ) contents gradually increase with
depth.
(4) The transition from hard-to-alkaline waters occurs at maximum concentration,
not exceeding 0.1 g-eq. per 100 g of water (5–6.51Be). As a rule, the waters are
hard at concentration above 0.1 g-eq.

(5) The HCO 3 content (in g-eq.) does not exceed the Cl content.
2
(6) Usually, the waters do not contain SO 4 anion. If present, however, its
concentration does not exceed 0.0004 g-eq. per 100 g of water.
Mekhtiev (1956) (in: Rieke and Chilingarian, 1974, pp. 265) also showed that in the
Azerbaijan oilfields water salinity is decreasing with stratigraphic depth and calcium

+
chloride waters (rCl rNa )/rMg 2+ 41, where r is percent-equivalent, are gradually

+
2
replaced by bicarbonate waters: (rNa rCl )/rSO 4 41. For magnesium chloride

+
water, (rCl rNa )/rMg 2+ o1. For details on classification of waters, see Chilingar
(1956, 1957, 1958), Samedov and Buryakovsky (1966), and Buryakovsky (1974).
According to Chilingarian et al. (1994), the chemistry of pore water is determined
mainly by the compaction of argillaceous rocks and squeezing-out of pore water.
The formation water chemistry, in turn, influences the diagenetic and catagenetic
transformation (of clay minerals) processes.
Fig. 3.17 shows the dependence of montmorillonite content on the total salinity of
formation water for the calcium chloride and sodium bicarbonate types of pore
waters in sands of the South Caspian Basin. As shown, a direct relationship exists for
the sodium bicarbonate type of water, i.e., with increasing water salinity, the
conditions for preservation of montmorillonite are improved and its content in
the clays increases. Increase in the total salinity of water is caused by an increase in
the content of carbonate and bicarbonate salts of alkali-earth metals. Sodium
bicarbonate-type waters are present in the Baku Archipelago and the Lower Kura
Depression, as well as in the rocks from the Lower Productive Series of Absheron
Peninsula and adjacent offshore area, i.e., sections in which the argillaceous rocks
are characterized by higher montmorillonite content. There is an inverse relationship
between the montmorillonite content and the presence of calcium chloride type of
waters. The chloride content [in particular sylvite (KCl)] increases with increasing
water salinity. Thus, alkaline medium is apparently favorable for the formation and
preservation of montmorillonite.
Thus, the geochemical environment at great depths in the South Caspian Basin
deposits not only conducive to the preservation of allothigenic montmorillonite, but
possibly allows the transformation of illite into secondary montmorillonite.