Geology Reference
In-Depth Information
of clay sequence compaction in initiating the fluidal flows and formation
and distribution of oil and gas accumulations.
It is important to define three terms prior to initiating the discussion.
(1)
Geodynamic
(i.e., gravity, compression, overburden) clay compac-
tion, which is a function of overburden continuously increasing in time.
(2)
Geostatic
(i.e., decompression, unloading) process, which is associated
with the relaxation of cumulative energy in the fluids compressed in pores
(energy accumulated over the previous geologic evolution). (3)
Geotectonic
compaction, which is of tectonic (stress) origin.
The first mechanism is typical of the modern geologic environment over
the areas of extended compensated deposition, subsidence, and permanent
immersion. These are mostly shelf zones and offshore areas in the internal
sea basins. The second mechanism is characteristic of the dry land areas
where the current continental deposition does not compensate for the clay
sequence subsidence so there is, to a substantial extent, preservation of
inherited abnormally high porosity (water-saturation). The last one is a
feature of belts and zones of powerful neo- and paleo-tectonics events.
In general, the infiltration basins have the direct hydrochemical zoning; the
water salinity usually increases with depth. In the same direction, the sulfate
and carbonate waters are usually replaced by the chloride ones. Basement of
these basins is usually dominated by low temperatures (rarely above 40-60°С, or
104-140
F).
Elision basins
are usually associated with young platform tectonic depres-
sions and Alpine foredeeps. Among the examples are the Terek-Kuma, Azov-
Kuban, Amu-Darya, West Turkmenistan basins, etc. They are characterized by the
domination of descending tectonic motions and the accumulation of thick sedi-
ment sections up to and in excess of 10 km. The ascending motions within these
structures were short-lived and started late, in Neogene and sometimes even in
Post-Pliocene.
As a result, on the periphery of such depressions each subsequent layer overlaps
the previous one, and there are no surface outcrops of the reservoir rocks, so the
elision rather that infiltration processes are dominant. Sandy-clayey sequences in
such regions become the source of gas-water fluids; the clays there are similar to
a porous rubber saturated with the seawater and various gas components. As they
subside, they compact and release gas-water solutions into more rigid reservoir
layers and the fault zones. Thus, overpressure emerges in the central parts of the
depressions. The fluids squeezed out of the clays usually migrate from the basin's
center to the periphery. This is facilitated by higher geothermal gradients that pro-
vide temperatures of up to 100
o
C (212
o
F) at a relatively shallow depth of 2 to 3 km
(Kholodov, http://www.scgis.ru/russian/cp1251/dgggms/1-98/khol_03.htm).
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