Geology Reference
In-Depth Information
A logical conclusion of the aforementioned is that the formation of
accumulations in the first group highs occurred as follows:
the
primary
accumulation
(hydrocarbons enter the traps together with alkaline waters)
and the secondary accumulation (influx
of additional liquid oil due to its
cross-flow
from aggregations in the underlying section intervals).
Conceptual possibility for the accumulations to form under this scheme
is supported by the aforementioned presence in the first structure group of
the NKP accumulations from the second group of structures formed to a
certain extent due to cross-flow of the liquid oil from the overlying hori-
zons and formations. A convincing example to illustrate this position is
Table 8.1. It includes the data on the variations in the net to gross sand ratio
from various Productive Sequence's stratigraphic intervals as a function of
the distance from its base and up the section (for fields Buzovny-Mashtagi
and Kala typical of the subject region).
As Table 8.1 shows, moving upward from the selected comparison
level, the relationships between saturated and non-productive reservoirs
in the section (i.e., the degree of the reservoir hydrocarbon saturation) as
a whole consistently declines. This reflects the dominant role in the for-
mation of Productive Sequence accumulations of vertical fluid influx from
the overlying sediments. There are insubstantial deflections from this gen-
eral pattern in the sandiest intervals of the Kala Prospect (Sabunchi and
PK formations). These deflections in no way negate its universal nature.
They only confirm again the aforestated position of a substantial role of
lithological and hydraulic factors in the processes of hydrocarbon wash-
out from traps in the lower section intervals and its subsequent accumula-
tion upside.
Main ground water and, hence, hydrocarbon influx paths into reservoir
beds of the Middle Pliocene sediments in the subject region are: faults, mud
volcano eruption apparatuses, diapir plug contact zones with the enclosing
rocks, elevated fracturing areas, etc. They connect the said stratigraphic
complex with deeper deposits. That is why within the described cause and
effect connection of oil and gas occurrences with hydrochemistry is also
defined the hydrogeological (hydrodynamical) connection between the
Pliocene and the underlying intervals.
Different saturation extent of local structure sections with alkaline
waters and hydrocarbons is controlled in any individual case by geologic
conditions and tectonic particulars of a specific area. They include, first and
foremost, different accumulating capacities of traps. These are determined
by their geometric volume, morphology and number of faults, intensity
of the migration flow, conductive and capacitative properties of enclosing
rocks, spatial position of traps in relation to oil and gas source zones, etc.
