Page 137 - Origin and Prediction of Abnormal Formation Pressures

114 L.A. BURYAKOVSKY, R.D. DJEVANSHIR, G.V. CHILINGAR, H.H. RIEKE III AND J.O. ROBERTSON, JR.

(in g-equ) does not exceed the C1- content. (6) Usually, the waters do not contain the
SO 2- anion (A/ < $1). If present, however, its concentration does not exceed 0.0004
g-equ per 100 g of water. Mekhtiev (1956, in Rieke and Chilingarian, 1974, p. 265)
also showed that in the Azerbaijan oil fields mineralization of waters is decreasing with
stratigraphic depth and calcium chloride waters
rC1- - rNa +
rMg 2+ >1
where r is percent equivalent, are gradually replaced by bicarbonate waters
rNa + - rC1-
>1
rSO z-
For magnesium chloride type of water
rC1- - rNa +
rMg 2+ <1
For details on classification of waters, see Chilingar (1956, 1957, 1958), Samedov
and Buryakovsky (1966), and Buryakovsky (1974).
The appearance of hydrochemical inversion in this section of the South Caspian
Basin sediments may be explained by a genetic connection of the hydrochemical regime
with the development of abnormally high pore pressures in shales. As shown by data
presented in Table 4-5, the most intense AHFP development is characteristic for the
Baku Archipelago and Lower Kura regions, where the water is primarily of the sodium
bicarbonate type.
Pore water chemistry is determined by the compaction processes in argillaceous
rocks and the squeezing out of pore water (see Chilingarian et al., 1994). In turn, hydro-
chemical factors influence the processes of diagenetic and catagenetic transformation of
clay minerals. Fig. 4-10 shows the dependence of montmorillonite content on the total
salinity of the formation water for the two types of pore solutions in sands which are
characteristic for the South Caspian Basin: calcium chloride and sodium bicarbonate.
As shown, a direct relation exists for the sodium bicarbonate water, i.e., with decreasing
water salinity, the conditions for preservation of montmorillonite are improved and its
content in the clays is increased. Increase in the total salinity of water is caused by the
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 region,
as well as in the rocks from the lower division of the Productive Unit of the Apsheron
Peninsula and adjacent offshore area, i.e., sections in which the argillaceous rocks are
characterized by increased montmorillonite content. There is an inverse relationship
between the montmorillonite content and the presence of calcium chloride type waters.
The chloride content, in particular sylvite (KC1), increases with increasing water salinity.
As shown, the alkali medium is favorable for the formation and preservation of
montmorillonite. This was also confirmed by the results of computer geochemical
simulation (Buryakovsky et al., 1990).
A program RAMIN in PL-1 language was utilized, which is similar to the geochem-
ical model proposed by Kharaka and Barnes (1973). The program RAMIN makes it