Geology Reference
In-Depth Information
increased role in the water body of the atmospheric СО
2
-enriched fluvial
and meteoric waters (these waters drained the adjacent mountain buildups
whose rocks' mineral composition included abundant carbonate-contain-
ing associations: Agabekov, 1963; Krejci-Graf K., Hecht P., Pasler, 1957).
The salinity decrease and alkalinity increase in the Neogene syndeposi-
tional background waters were additionally affected by catagenic dehy-
dration in montmorillonite clay members of the complex in the increased
temperature intervals (in the subsided parts of the depression).
The waters of nonproductive Mesozoic-Paleogene buried erosion highs
Σ
а
+
k
-
37.4-57.2
mg.-equiv.) and the formation waters in the oil- and
gas-saturated areas of local structures in the Neogene complex (
(
Σ
а
+
k
-
23.1-105.7 mg.-equiv.) occupy an intermediate position between hard
chlorine-calcium Mesozoic brines (
Σ
а
+
k
- 455 mg.-equiv.) and the hydro-
chemical background alkaline waters of Neogene intervals (
Σ
а
+
k
- 13.2-
52.6 mg.-equiv.) (See Tables 2.10, 2.11).
The occurrence conditions, salinity and component composition in
waters of the buried highs objectively correspond with the in-formation
mixing model of different water types and different genetic origins and
stratigraphy. Modeling showed that the source ingredients of these mixes
were syndepositional hard Mesozoic brines and alkaline background
waters from the Burdigalian-Helvetian and Tortonian series draping the
said highs. These brines and background waters preliminarily under-
went desalination (by about 30
305 mg.-equiv.) due
to the paleo-infiltration of meteoric waters in the periods of post-Upper
Cretaceous and Oligocene depositional lacunas (Reznikov, 1970; Reference
book…, 1976).
Modeling showed that at mixing of both components, their ratio in
the formation mix (with the current salinity of 57.2 mg.-equiv.) should
reach (5
−
35% -
Σ
а
+
k
- 303
−
90); i.e., the fraction of the syndepositional high-salinity
Mesozoic waters not replaced in reservoirs does not exceed in total balance
of the actual hydrochemical medium 5
−
10)/(95
−
10%. In other words, unproductive
areas of the erosion highs at the current stage of region's geologic evolu-
tion is 90
−
95% saturated by genetically-foreign Neogene alkaline waters.
As noted above, commercial oil occurrences were associated with zones
of prevalent distribution (preservation) of hard brines (
−
Σ
а
+
k
- 140
−
305
mg.-equiv.,
S
2
- 15
20%-equiv.) A conclusion is that the encroachment of
relatively low-salinity hydrocarbonate-sodium agents from the adjacent
intervals of the Neogene series played negative role in the formation of
hydrocarbon accumulations within the highs.
The peculiar hydrochemical zoning within productive local structures
of the upper stage shows more than double waters salinity and lowered
−
