Geology Reference
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t The absence in the first group of structures (as opposed to
the second one) of a clear correlation between an interval
depth and oil density. Under specific conditions of hydro-
carbon accumulation in the Productive Sequence this is
only possible with additional inflow into the earlier formed
accumulations or a cross-flow of the liquid oil into “empty”
traps.
t The position in the second group of structures of the NKP
accumulations, formed predominantly by liquid oil cross-
flows from basal intervals of the Productive Sequence,
among the accumulations of the first group (see above).
t Geologic specifics of the eroded structures stimulating fluids
cross-flows (local discharge areas).
Differentiation of the accumulations by their formation schemes is a
manifestation of various fluid vertical migration mechanisms operating in
the subject region.
When accumulations formed after the first scheme, a dominant role
belonged to the inter-formational vertical migration of alkaline waters and
single-phase-retrograde hydrocarbons from the sedimentary complexes
overlying the Productive Sequence. The hydrocarbon influx into the traps
was accompanied by a simultaneous influx together with them of low-
salinity alkaline waters (Mekhtiyev and Rachinsky, 1967). Understandably,
if the gas phase in a trap is preserved, the total amount of the alkaline
waters invading the trap is substantially smaller than if they aggregated
in place of the liquid phase, which formed after a lengthy and multi-stage
running through the trap of the source single-phase mix. Correspondingly,
the composition of the formation waters in contact with gas and conden-
sate aggregations on one hand and with oil on the other will differ. A
smaller amount of the invaded alkaline waters results in smaller desalina-
tion and modification of high-salinity section-syngenetic hard waters, and
vice versa. This mechanism is responsible for a well-known fact that, in the
Productive Sequence on the west flank of the South Caspian Depression,
the waters in contact with gas-condensate aggregations, compared with
those in contact with oil accumulations, always have higher salinity and
lower alkalinity (Mekhtiyev, Rachinsky and Polaudin, 1970; Mekhtiyev,
Akhundov and Rachinsky, 1970).
The second mechanism associated predominantly with intra-forma-
tional liquid oil cross-flows into earlier formed accumulations appears to
be subordinated and operating mostly in the accumulations re-formation
processes.
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