Geology Reference
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t
their pervading association with commercial hydrocarbon
saturation;
t
their mandatory association with most faulted blocks of
local structures - areas (foci) hydrodynamic drainage in
combination with the appearance in a number of fields in
the same intervals of saline chlorine-calcium waters of a
typical Cretaceous stratigraphic identification.
The background water mixing with injected Cretaceous water was mod-
eled. It was found that ground waters of the productive areas on the local
structures are formation mixes with the content of a foreign component
being 65% in the Paleogene complex, 20% in the Eocene one, and on the
order of 10% each in the Oligocene and Miocene complexes. The stated
mechanism appears to be quite real. Its reality is supported by minimum
depth levels of the appearance in the alkaline medium of chlorine-calcium
and “transitional” waters on the strongest faulted structures (Los Claros,
Urdaneta and Mene-Grande) and by substantial sections' carbonate con-
tent within productive fault-blocks of the fields. The latter occurred in mix-
ing of high-alkalinity background waters with hard Cretaceous waters and
was defined by the precipitation of СаСО
3
and МgСО
3
.
Interformational cross-flows are also supported by the specifics in the
sulphate-ion spatial distribution in region's ground waters. Its largest concen-
trations in the Cenozoic water complexes are recorded only within the pro-
ductive fault-blocks and usually have the salinities typical of the Cretaceous
series. As the fraction of the injected ingredient in the Cenozoic formation
waters increases, the SO
2
−
content in them also grows and reaches its maxi-
mum in the “transitional”, chlorine-calcium and chlorine-magnesium variet-
ies. Thus, the sulphate ion is practically absent from the alkaline background
waters of the Cenozoic series; it has elevated concentration only in the waters
of a deeply-subsided Cretaceous complex and in the formation mixes of the
post-Cretaceous section with large enough content of the invaded compo-
nent. These determine the mostly Mesozoic SO
2
−
generation in the course of
a catagenic process of thermochemical filtration leaching of water-saturated
rocks (Kissin and Pakhomov, 1967; Kononov, 1965). That is a reason why the
sulphate presence in the waters of overlying complexes cannot be explained
other than as a result of the vertical hydraulic communication between the
lower and upper stages of the sedimentary section.
Based on all available data for the Maracaibo Basin we conclude that
the underground water chemistry of the local structures is to a substantial
extent defined by large-scale interformational fluid cross-flows through
the fault system.
