Geology Reference
In-Depth Information
and formation of zones of dominant gas accumulation as a
function and measure of faulting in the natural reservoirs
(Durmishyan, Muradian and Rachinsky, 1974; Mekhtiyev,
Tsaturyants and Rachinsky, 1968).
Fluids movement (including hydrocarbons) from clays into reservoir
beds and their subsequent migration is possible only under pressure gra-
dients, which are the effective difference between AHPP of the generation
zones and formation pressure of the accumulation zones.
When reservoirs are available within a mostly clayey sequence with
AHPP, their oil and gas potential is defined, first and foremost, by the thick-
ness and regional continuity of permeable intervals. In thicker reservoirs, it
is possible for relatively large accumulations to form. In lens-shaped vari-
able sandstone members, hydrocarbon saturation is usually sporadic and
is mostly the liquid phase (oil).
Indeed, the syngenetic nature of AHPP in thick substantially clayey
deposits does not allow for the existence in them of large and high gas
accumulations with substantial and mandatory, in such cases, excess pres-
sure. Should such accumulations be present in the sequences with synge-
netic AHPP, it would result in the formation in their upper zones of high
pressure (regional AHPP plus excess pressure caused by the difference in
gas and water specific gravity). That in turn would unavoidably result in
fracturing the overlying rock integrity and in the destruction of the accu-
mulations. In some cases, pressure gradients would exceed the lithostatic
one. It is obvious that large gradients cannot facilitate the preservation of
already formed accumulations. Still, such an environment in a general
case is favorable for the formation of oil and gas accumulations. Thus, it is
possible that commercial accumulations do form but are destroyed in the
process of re-formation due to the stated reasons. Therefore, it is unlikely
to discover large hydrocarbon accumulations in the reservoir members
within thick uncompacted clayey sequences with lithostatic level synge-
netic AHPP.
Favorable for the formation of oil and gas accumulation zones is the
geologic environment where reservoirs are overlain by a thick clayey
series with AHPP. In such a case, oil and gas occurrences within perme-
able intervals may be partially caused by the descending fluid migration
from an oil and gas clayey source sequence. An important factor there
is the availability of highly favorable conditions for the preservation of
already formed hydrocarbon accumulations. Under this mechanism, the
decisive role belongs not only to the presence of a clayey seal but also
to the development in its section of syngenetic AHPP. Syngenetic AHPP
