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within the zones of a significant subsidence of top of PS (Zyrya, Yuzhnaya,
Bakhar, Shah Deniz; Rachinsky and Durmishyan, 1970). This distribution
confirms the important role that the sealing of traps plays in the preser-
vation of gas phase and agrees with the above-described model of high-
pressure bottles presented in Figure 6.2.
The applicability of the above-described thermodynamic model to the
formation of oil and gas deposits in the South-Caspian Basin is also sup-
ported by a ballpark balance of its original hydrocarbon potential (reserves
+ resources) and the gas volumes that had gone through PS/RBS and
released to the atmosphere as a result of the mud volcano activity, carrying-
out by subsurface waters, earthquakes, and natural diffusion and degassing
(“breathing of lithosphere”).
According to our calculations, the initial in-place hydrocarbon poten-
tial in PS/RBS is 9.01 BT of oil, 1.92 BT of condensate (total of 10.93 BT),
6.16 TCM of gas, and the total hydrocarbons in oil equivalent, 17.09 BT
(Rachinsky
et al
., 2007). This estimate, however, is questioned by Chilingar
et al
. (2002, 2005) because many petroliferous structures were abandoned
on the basis of well tests, which give erroneous results in overpressured
formations.
On assuming the average content of liquids in the source gas phase of
200 g/m3, about 45 TCM of gas would be needed to dissolve, transport and
accumulate 9 BT of oil. The total loss of gas in the region during the 3 MMY
of Late Pliocene-Quaternary time (after deposition of PS/RBS) as a result
of the above-described processes was estimated at 83 TCM (Geodekian,
1968). This is twice the needed amount.
Thus, much more gas had migrated through PS/RBS than the required
minimum. This is an optimistic conclusion as it opens a possibility of an
increase of reserves in the region. On the other hand, it may be considered
as an indirect confirmation of the formation of the entire Middle Pliocene
hydrocarbon accumulations of the basin with the most intense Quaternary
neotectonics.
The above-described evidence supports the proposed thermodynamic
model for the formation of oil and gas deposits in the region through dif-
ferential entrapment of the source retrograde vapor-gas hydrocarbon mix-
ture generated within the Mesozoic-Miocene deposits.
Conclusions:
A. The geological environment in the Mesozoic and Post-
Miocene stratigraphic complexes is reasonably favorable for
the formation of large oil- and gas-accumulations within a
