Geology Reference
In-Depth Information
SW flank of Koturdag, etc. It is noteworthy that the said intervals in these
fields show high apparent resistivities indicating presence of the residual
oil. In the Apsheron oil and gas area, a similar picture is observed in the
Productive Sequence's Lower Group on SW flanks in the structures of
Fatmai-Zykh anticlinal zone. High apparent resistivities there go together
with the elevated formation water alkalinity (which is at about the same
level as in the productive intervals). Although waters there practically do
not contain sulphates, this may be due to the absence of conditions for
their preservation (mostly relatively shallow depths and for this reason
intense biogenic sulphate reduction).
The same Figure 2.12 shows that sometimes the sulphate content of
the formation waters in nonproductive intervals and fault-blocks may
be high and may reach the sulphate level in waters contacting both oil
aggregations and gas and condensate accumulations. In the former case,
high SO
2
−
content goes with elevated alkalinity and low salinity of the
formation waters. This is typical of hydrocarbon aggregations destroyed
by hydraulic factor. In the latter case, the elevated water sulphate content
in combination with their low alkalinity (hardness) and relatively high
salinity may be explained by a low influx of foreign sulphate-containing
alkaline waters and, respectively, hydrocarbons and by the incomplete
for this reason process of accumulation formation. In this case the tested
intervals show low (noncommercial) gas-saturation causing, as was
shown above (under all other conditions equal), SO
2
−
ion conservation
in the water medium.
Consequently, it is necessary to reconsider the earlier concept of no
sulphates in formation waters as a favorable attribute in exploration and
appraisal and consider the appearance of SO
4
ion in the water medium
being a positive exploration criterion. With all other conditions equal
maximum
SO
−
ion content is typical of predominantly gas accumulation
zones.
The following conclusions are based on studies of formation waters in
the Productive Sequence:
t
Hydrocarbon saturation in the Productive Sequence is
always associated with hydrocarbonate-sodium waters in
pure form or in mixes with hard waters.
t
Formation waters of gas-condensate accumulations com-
pared with waters of oil accumulations usually have higher
salinity, lower alkalinity (or higher secondary salinity in the
hard water medium), high content of soluble organic acid
salts and sulphates.
