Geology Reference
In-Depth Information
The accumulation mechanism proposed above may be interpreted as
follows:
The anticlinal structures down the regional dip not only become more
tectonically “quiescent” (less deformed and faulted) but are also overlain
by a thicker sediment cover, including clay seals the thickness of which
increases down dip. A combination of these two factors results in lower
communication between the productive intervals and the surface, improv-
ing their sealing capacity and decreasing degassing. In other words, the
increase in gas saturation of PS/RBS section down the dip of folded sys-
tems is a result of better preservation due to better isolation of the hydro-
carbon-saturated horizons.
The above ideas regarding the formation of gas (gas-condensate) accu-
mulations and the oil distribution, according to their composition and
reserves, testify to a relative briefness and synchronicity of the hydrocarbon
accumulation processes in the Lower and Upper PS/RBS, mostly during the
Quaternary tectonic phase. Otherwise, such a characteristic oil distribution
according to its specific gravity in the region's section would not have taken
place. Let us assume, for example, that oil in the Lower PS appeared there
prior to the deposition of Upper PS. In such a case, due to low formation
pressure in the Lower PS at that time, the “retrograde” oil would have a
significantly lower specific gravity than that at present and would approxi-
mately match in its composition the light present-day oils of Upper PS.
The validity of our accumulation model is confirmed by the spatial dis-
tribution of 445 accumulations in the Apsheron region. The analysis shows
that all phase types (various proportions of different phases) are differenti-
ated according to depth. Gas accumulations are usually not found deeper
than 500 m. The oil and oil-gas accumulations (accounting for the bulk of
discovered accumulations) are discovered at an average depths of 1,100 m
(for the former) and 1,500 m (for the latter). The oil accumulations with gas
caps are not found deeper than 2,500 m. Gas-condensate accumulations
are associated with the deepest traps (average depth of 3,600 m). Typical
features of gas accumulations are their small size and their position above
the oil and oil-gas accumulations. This indicates their secondary nature
and formation due to degassing of underlying deposits.
As mentioned above, oil and oil-gas accumulations are associated pre-
dominantly with the structures that had undergone intense tectonic defor-
mation during Late Pliocene-Quaternary time. Deformation involved the
uplift and subsequent erosion of local highs (Balakhany-Sabunchi, Shaban
Dag.-Atashkya, Binagadi-Chakhnaglyar, etc.). The gas-condensate accu-
mulations are associated with deeply-buried weakly-deformed structures
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