Geology Reference
In-Depth Information
zones, hydrogeological “windows”, etc., which provide interformational
and intraformational vertical-horizontal fluid cross-flow up to its dis-
charge onto the surface. Thus, the reservoirs in less favorable structure-
tectonic and lithological conditions for the ground water outflow have
lower practically realizable accumulative capabilities. A symptomatic illus-
tration is the world oil reserves distribution by large geotectonic elements.
As published (Bakirov, Varentsov and Bakirov, 1970; Beka, Vysotsky, 1976;
Lindtrop, Anfilatova and Dmitriyeva, 1970), over 70% of the world's oil
and gas reserves are associated with the platform and mobile belt merger
zones, i.e., with the most mobile areas of the Earth's crust with active water-
exchange, neotectonics and the combination of optimal conditions for the
geo-fluid-dynamic system discharge.
The major discharge paths for the subsurface reservoir are various
disruptions of rock integrity, faults, lithological unconformities, areas of
elevated fracturing, mud volcano eruption apparatuses, contact zones of
diapir and salt bodies with enclosing rocks, etc. Thus, the role of these dis-
locations in the formation of oil and gas accumulations is believed to be
definitive in two equisignificant aspects.
First, the stated dislocations serve as the communications channels
for the oil-gas-producing zones with the accumulation volumes and pro-
vide the hydrocarbon and the accompanying water transport into traps.
Second, they are emigration paths for the ground water that earlier satu-
rated the reservoirs. This determines the possibility of new fluid volumes
entering them. These positions result in a conclusion that tectonic and/or
lithological unconformities in the section are a necessary attribute for the
formation of oil and gas occurrences; without them significant hydrocar-
bon accumulation in the traps cannot occur.
The validity of this concept is supported by clear association in all
world regions of major hydrocarbon reserves (about 85%) down to a
2,000-2,500 m depth range (Bakirov, Varentsov and Bakirov, 1970; Beka,
Vysotsky, 1976; Lindtrop, Anfilatova and Dmitriyeva, 1970). Deeper, the
oil reserves drastically decline with a relative increase in the fraction of
gas in the total balance. The specificity of such distribution may only be
associated with changes down the section in the manifestation nature of
the fluid-dynamical regime of the subsurface. In particular, it is associ-
ated with drastically different at different depths mobility, conditions and
circumstances of ground water discharge. The upper intervals of the sedi-
ment cover have more favorable drainage conditions as the water emi-
gration from reservoirs and the creation of accumulating pore-fracture
volumes is easier there compared to deeply buried beds.
