Geology Reference
In-Depth Information
of the stratigraphy and depth (and some physicochemical processes caused
by these factors), the waters of a similar geochemical profile would be
found in the same stratigraphy and at the same depths. As indicated above
and shown in Figure 2.16, this is not what is observed.
Processing of the same data for all fields resulted in the following: the
correlation coefficient equals -0.56, the correlation ratio is 0.62, the cor-
relation coefficient error with the probability 0.997 (three-sigma interval)
is ±0.174. Taking a possible error into account, the correlation coefficient
can be about -0.39, which is tantamount to the absence of correlation.
Relatively low correlation coefficient and ratio values together with rela-
tively high possible error do not make the correlation between the ground
water salinity and depth convincing.
A different picture is observed in correlation of ground water salinity
against its depth as counted up the section from the Productive Sequence
base. In this case the correlation coefficient is -0.86, the correlation ratio
is 0.95, and the correlation coefficient error with the probability 0.997 is
±0.065. The empiric regression curve of the ground water salinity in depth
is shown in Figure 2.17 (Rachinsky, 1970).
The curve appearance and high values of the calculated statistical
parameters indicate quite credible proportional correlation between the
formation water salinity and the distance of
water-saturated intervals from
the base of the Middle Pliocene sequence. This correlation, as Figure 2.17
shows, may be approximated by a polynomial of higher that the second
power.
Apparently, such correlation form is a result of the water salinity change
in the Middle Pliocene section and in the overlying intervals being the
function of many variables. The major variable is depth but they also
include the per-unit oil-saturation of intervals, reservoir permeability, fault
conductivity and hydraulic resistivity on the way of flowing waters.
The conducted statistical processing and comparison of numerical cor-
relation parameters between the water salinity, their depth and the dis-
tance from the Base of the Middle Pliocene suggest that the ground water
salinity in the Pliocene section of the Apsheron oil and gas region does
not depend on the water stratigraphy but is mostly a function (with all
other conditions equal) of the water interval distance from the base of the
Middle Pliocene sequence.
A special note: the water salinity decrease in the section of all fields in
the region without exception always begins from the uppermost oil- and/
or gas-saturated interval. It does not depend on its stratigraphy: it may
be the Middle Pliocene Productive Sequence or its overlying Akchagyl
and Apsheron Stages; i.e., the salinity inversion includes not only Middle
