Geology Reference
In-Depth Information
rapid deposition with slow and retarded drainage. See Figures 1 and 2 and
Tables 1, 2, 3, 5, 6.
A clear correlation exists between the geologic age and the total shale
porosity. At the same depths, the degree of clay compaction progressively
increases with the age of shale formation. This is especially clear for geo-
synclinal regions. For example, the porosity difference between the Central
Apsheron Pliocene shales and the Paleogene-Miocene shales of Maracaibo
Basin reaches 8%. In the platform regions, the Cretaceous-Paleogene
shales of Crimea Plain can be compared with the Weller's curve (Paleozoic
age). In this case, the difference reaches 5%. (Table 5.1).
The role of the tectono-inversion, denudation and geostructural stabi-
lization factors is mostly expressed in optimization of the conditions and
acceleration of the speed of pore water discharge. The pore water drainage
is caused by several factors:
1. Tectonic stress in “squeezing” the pore fluid.
2. Progressive improvement (mostly during regional uplifts) in
the
3. Hydraulic connection between the compacting sequences
and drainage
4. zones.
5. Decrease in overburden load over the compacting rocks.
6. Stationary fluid-mass-transfer regime, which facilitates a
more
7. complete and rapid emigration of pore fluids.
The effect of the above factors may be illustrated by the drastically
uplifted and strongly denuded (during the early Alpine tectogenesis)
Novoselkov High (Crimean Plain) and the Adygey Salient (Western Fore-
Caucasus). The shales there have low porosity, with preservation of mor-
phologic and textural characteristics. The porosity vs. depth curves for
these regions are offset markedly to the left and upward from the Weller's
curve (Figure 5.6, Table 1).
The shape and position of the curves for the areas that had undergone
a tectonic inversion is not always determined by the above stated factors.
Figure 5.6 shows the porosity values of the Oligocene-Lower Miocene
(Maikopian) shales in the Tersk-Caspian Trough. As opposed to the
Novoselkov High and the Adygey Salient, the shales preserved their mor-
phological and structural features and represent the incompetent inter-
vals. The compaction curves for this region show a close relation to the
geologic evolution. As a result of a powerful uplift (up to 2 km), which
