Geology Reference
In-Depth Information
The latter two mechanisms facilitate increase in geothermal gradient
1
values at shallow depths.
Under certain combinations of geologic conditions, the usually higher
faulting of the upper section may play a compensating role. This creates
the possibility for the vertical cross-flow of thermal fluids from hotter
underlying intervals. That in turn results in some leveling of formation
temperature and the corresponding decrease in the temperature gradients
in the section. This compensating role may be supplemented by micro-
biochemical processes in the weathering zone, which are accompanied by
additional heat release.
Growth in geothermal gradients up the section has low variability in
all regions and is independent of a specific geologic environment. For all
practical purposes, it is constant in the lowermost intervals. These suggest
that the major factor in defining the shape of the “geothermal gradient vs.
depth” curves is the conductive component of the total heat flow (this com-
ponent changes under the dominating influence of a difference between
the depth and surface temperature,
i.e., the cooling effect of the surface).
Comparison of the graphs (Figure 4.1) shows quantitative differences
in the general qualitative similarity of the curves (parabolic). This quan-
titative difference is expressed in broad variations of temperature val-
ues at the same depth for different regions. Examples are: in the South
Caspian Depression maximum formation temperatures reached at a depth
of 6,000 m are 110-115°
С
; in the Padan Depression and Zagros Trough,
135-140°
С
; but in the Irrawaddy-Andaman Depression they reach about
185°С; in the Indolo-Kuban Trough 19
0
-195°С; and in the Los Angeles
Basin, 215-220°С.
Figure 4.2 illustrates functional determination of this phenomenon by
the specifics of regional geotectonic evolution and of geologic structure.
As shown in Figure 4.2, minimum formation temperatures in the sections
1
Additional issue in the South Caspian depression is ubiquitous association with
this depth interval (regardless of its stratigraphic position) of the contact zone
between most sandy part of the section and more clayey overlying deposits (in the
Apsheron area, mostly sandy Balakhany Fm. of the Middle Pliocene Productive
sequence and more clayey Sabunchi and Surakhany formations; in the Lower
Kura Depression, the relatively sandy Upper Pliocene Apsheron Stage and mostly
clayey Quaternary deposits, etc.), where the section's heat conductivity and, hence,
temperature change tempo (geothermal gradients) drastically change (Mekhtiyev,
Yakubov and Rachinsky, 1968; Mekhtiyev, Geodekian and Rachinsky, 1973;
Rachinsky and Muradian, 1983; Rachinsky and Kuliyev, 1984).
