Geology Reference
In-Depth Information
mg-equiv. (20-22.9 g/l); at the same time, the primary alkalinity values
decline to 3-7%-equiv. In high-productivity fault-blocks of the Bolivar,
Lamar and La Concepcion fields (where the underlying Cretaceous and
Paleogene sediments contain oil and gas accumulations), “transitional”
chlorine-magnesium and sulphate-sodium waters with the secondary
salinity factor no greater than 3%-equiv. are sporadically present.
In the
Oligocene complex
(the Icotea and La Rosa formations) the
main ground water type is hydrocarbonate-sodium with a total salinity
of 45-50.2 mg-equiv. (12.9-14.3 g/l), primary alkalinity factor is 21.15-
31.6%-equiv. At the commercial scale hydrocarbon saturation the salinity
increases to 58-72 mg-equiv. and the primary alkalinity factor declines to
10-15%-equiv.; “transitional” varieties are common (Lama and Los Claros
fields).
The hydrochemical background in the
Miocene complex
(Lagunillas,
Isnotu and La Villa formations) is created by alkaline waters with salin-
ity of 36.3-41.4 mg-equiv. (10.4-11.8 g/l) and primary alkalinity factor of
41.9-46.2%-equiv. (fields in the Bolivar, Lama and Mene-Grande zone).
The waters in contact with hydrocarbon accumulations have higher salin-
ity, to 50-60 mg-equiv., and lower alkalinity (the primary alkalinity factor
up to 20-25%-equiv.)
A peculiar group in the region is low-salinity (up to 5 g/l), with ele-
vated secondary alkalinity factor (up to 40%-equiv.), hydrocarbonate-
sodium water of the infiltration origin. As a rule, this type is encountered
on the eroded, intensely uplifted highs and is associated with reservoir
surface exposures. It covers relatively small areas and relatively thin sec-
tion intervals. The distinctive feature of this type is practical composition
and salinity constancy within the entire interval of its manifestation (the
Eocene-Miocene) (see Table 2.14). The effect of the present-day meteoric
waters on the formation of hydrochemical environment in the water com-
plexes is believed to be limited. This is supported by the domination of
the reservoirs in all section's intervals of different background waters with
totally different component composition and salinity.
The following feature is recorded for the region as a whole. In the back-
ground waters of all stratigraphic complexes (starting at a certain depth,
about 1,000 and temperature 65
С) is observed the appearance and subse-
quent regular increase of the sulphate-ion content with depth from 0.1
°
−
0.3
mg-equiv. in the upper intervals to 3
4 mg-equiv. in the most subsided
(heated) intervals (see Figures 2.22, 2.23). The “
sulphate-ion vs. depth
” and
“
sulphate-ion vs. formation temperature
” correlations are: SO
2
−
= 0.94
−
⋅
10
-6
Н
1.782
, SO
2
−
=
0.34
⋅
⋅
10
-6
⋅
t
3.217
.
