Geology Reference
In-Depth Information
The geodynamic and geotectonic mechanisms eventually result in the
formation of shales, lamination, tabulation and fracturing. The geostatic
compaction does not significantly change the pore space morphology of
clays, so that the clays mostly preserve their structure.
Regardless of the mechanism involved, there is a clay drainage and emi-
gration of the compressed pore waters from their pore space. The following
factors control the nature and speed of this process in a specific geologic
environment:
1. Rate and depth of clay subsidence.
2. Thickness of clay sequences.
3. The sign (plus or minus) of regional tectonic movements
and intensity of the tectonic stress.
4. The presence and duration of interformational depositional
interruptions.
5. Extent of the filtration, fracture, diffusion and film perme-
ability of clay varieties.
6. Spatial variability of the clay mineralogical composition and
lithology.
7. Contents of sand-silt, carbonate, and siliceous material.
8. Degree of diagenetic and catagenetic alterations of clays.
9. Clay mineral composition and efficiency of dehydration
mechanisms.
10. Ratio of tight to reservoir intervals, which serve as drainage
(discharge) avenues, in the compacted sequence.
11. Intercommunication of permeable intervals with the near-
surface intervals and the surface (along faults, for example).
12. Geologic age of compacted intervals and duration of their
residence at a certain depth and temperature.
13. The directions (vectors) of stress.
The above factors complexly interact. As a result, under different nat-
ural environments, the quantitative parameters of the compaction pro-
cesses may significantly differ despite a similar qualitative direction. It is
possible to identify the end-members of these series (“sample functions”).
On one hand, the function describes the compaction of old, slowly and
intermittently subsiding, with major interformational hiatuses, thin, dras-
tically variable, sandy (calcareous, siliceous) clays alternating with thick
and regionally continuous reservoirs. On the other hand, the function
describes the compaction of young, continuously and intensely subsiding,
thick, lithologically homogenous monomineral clay sequences hydrauli-
cally isolated from the reservoir intervals.
