Vol.3, No.5, 359-364 (2011) Natural Science
Copyright © 2011 SciRes. OPEN ACCESS
About retardation of a physicochemical processes in
overpressured sediments, South-Caspian basin,
Akper Feyzullayev
Geology Institute of the Azerbaijan National Academy of Sciences, Baku, Azerbaijan; fakper@gmail.com
Received 29 September 2010; revised 15 October 2010; accepted 22 October 2010.
In paper the role of excess pressures in cata-
genic processes of the South-Caspian basin
(SCB) is considered. The results of the carried
out researches taking into account world ex-
perience on the given problem allow to con-
clude, that SCB (mainly its deep-water part), as
well as a number of other basins of the world
with overpressures, is characterized by retarda-
tion of processes cracking of kerogen and oil,
and also reaction of transformation of clay min-
erals. Periodic intensification of these proce-
sses can provoke development of diapirs and
mud volcanoes, which are the centers of pulse
unloading of a hydrocarbon products from sys-
tem. The conclusion about high prospects of
revealing of hydrocarbon accumulations in deep
buried deposits in overpressured basins is made.
Keywords: Overpressures; Cracking; Kerogen; Oil;
Smectite; Illite; Retardation; South Caspian Basin
The conventional sedimentary theory of formation of
oil and gas from organic material has two key process
controlling factors: temperature and duration time for the
initial organic matter (ОM) [1-4].
Often the role of pressure in generation of hydrocar-
bons (HCs) is taken to be insignificant [2,5,6]. How-
ever, theoretical and experimental investigations of the
last few years dispute this statement. Thick impermeable
clay deposits represent an almost closed system [7-9]
where the processes of thermal maturation of OM, gene-
ration of hydrocarbons (HCs) and, especially, cracking
of oil to gas, produce abnormally high fluid pressure
(overpressure) [9-14].
In a closed system because of the absence of condi-
tions for HC outflow and removal from the system there
will be a continuous increase of volume as kerogen
(density around 2 g/cm3) converts to hydrocarbons (den-
sity less than 1 g/cm3), thus promoting the growth of
pore pressures in the system, which can reach and even
exceed lithostatic pressure [10,12]. As known from phy-
sical chemistry, the absence of a capability of outflow
from a closed system of products results in retardation of
the reaction rates as has been noted on the basis of ex-
perimental investigations and comparative analysis of
the processes in worldwide basins [7-9,15-19]. The re-
tardation of OM maturation in overpressure conditions is
also associated with abnormally low values of vitrinite
reflectance (Ro%) and Tmax in rock pyrolysis [7,19-21].
The termination of clay mineral transformations at
overpressured depth intervals (e.g. constancy of the sme-
ctite content vs. depth) has also been established [22].
The South Caspian Basin (SCB) is one of the most
striking examples where, due to the geological history
and recent geological structures, very favorable condi-
tions exist for the formation of overpressure. In the Plio-
cene-Quaternary avalanche sedimentation (up to 3 km/myr)
has been inferred. The total sediment thickness (up to
25 km) obtains from Jurassic time onwards. In the Ce-
nozoic section plastic terrigenous rocks prevail. More-
over, the SCB is characterized by an abnormally low
tem- perature gradient, in the central deepest part of the
basin the temperature gradients vary between 15 -
18˚C/km. As a result, overpressures are observed.
The analysis of pressure data and their gradients in the
SCB, based on well logs and actual measures of pres-
sures in wells up to depth of about 7 km, showed the
spatial variability. The intensity increases in a south-
southwest direction, coinciding precisely with changes
in the clay content in the formations and the thickness of
the clay series [23] (Table 1).
А. Feyzullayev / Natural Science 3 (2011) 359-364
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Figure 1 shows that the highest overpressures are to
be found within the limits of the Baku archipelago (zone
III), where the average pressure gradient is 18 MPа/km
(see Table 1).
Table 1 and Figure 1 reflect pressures in the Produc-
tive series (PS - lower Pliocene), a main petroleum res-
ervoir in the SCB. Underlying the PS, Oligocene-Mio-
ne deposits were drilled in the elevated flanks of the
SCB on some fields. In the central part of the basin these
deposits lie very deep and information about their ther-
modynamic conditions is not available. However, taking
into account that the formations relate to the source
rocks [22,24-28], one expects even more contrast in ex-
cess pressures than in the reservoir, reaching and even
exceeding lithostatic (geostatic) pressure. Confirmation
to this point is the widespread development in the SCB
of diapirism and mud volcanism.
On the basis of the non-uniform spatial distribution of
fluid pressures, there are unequal conditions for thermal
transformation of ОM. In the central deep-water part of
SCB (zone III on Figure 1) one anticipates the least
maturity for the thermal transformation of OM because:
1) Mello and Karner [29] indicate that deposits with
overpressure are characterized by a low thermal conduc-
tivity and play a role of insulators for heat flow. The
deep-water part of the SCB, with the largest overpres-
sure (Figure 2), is distinguished by the lowest tempera-
ture gradients (Figure 3).
2) The thickness of Oligocene-Miocene source rocks,
present mainly in clay lithofacies (up to 80 - 90% of a
section) in the deep-water part of the SCB, is largest and
exceeds 3000 m. The HC output formed there, both as a
result of transformation of OM and of cracking of oil
into gas, will be characteristic of a more closed system
in comparison to the elevated part of the basin. Accord-
ingly, lower thermochemical reactions rates of OM
transformation are to be expected in source rocks in the
SCB, related to the development of overpressure.
For verification pyrolysis parameters and measured
values of reservoir rocks (Lower Pliocene) and source
rocks (Miocene) (Table 2) were compared.
As Table 2 shows, although the Miocene deposits lie
deeper than the overlying, younger Lower Pliocene for-
mations they are, nevertheless, characterized by lower
pyrolysis values parameters (PI, Tmax and Ro), reflecting
the lower degree of OM maturation.
The change with depth of values of Ro in the SCB is
also of interest. According to Figure 4, two trends of this
parameter with depth are clearly observed. The trend
with a higher Ro gradient is characteristic for the on-
shore fields on the elevated flank part of the SCB, while
the trend with a low Ro gradient is more characteristic
Figure 1. Formation pressure distribution zones in the SCB:
I – Absheron p-la and Absheron archipelago; II – South
Absheron water area; III – Baku archipelago.
Figure 2. Distribution of excess pressures (MPa) (relative to
hydrostatic pressure) in the SCB at a depth of 6 km [30]. Blue
line is coastline.
for offshore fields distinguished by overpressure.
Note that the zone with overpressure (Baku archipel-
ago) differs from the zone with rather moderate fluid
pressure (Absheron archipelago) as also indicated by
carbon isotope composition of oils (Figure 5).
А. Feyzullayev / Natural Science 3 (2011) 359-364
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Table 1. Thicknesses of clayey series and associated pressure gradients in the SCB [23].
Average values of a clay rocks thickness (m) for
different depth intervals
1 - 2 km 2 - 3 km3 - 4 km4 - 5 km
Average values of pressure
gradients, MPa/km
I–Absheron p-la and Absheron archipelago 50 40 30 20 13.5
II–South Absheron water area 750 235 185 150 16.3
III–Baku archipelago 900 725 460 350 18.0
Table 2. Comparison parameters of pyrolysis and values of Ro (%) for a reservoir and source rocks in SCB.
Pyrolysis parameters
Age of rocks Depth interval, m
PI = S1/(S1 + S2) Tmax, ˚C
Ro, %
Lower Pliocene (reservoir) 1230 - 5688 0.23 - 0.96/0.52* 417 - 472/431 0.46 - 0.82/0.66
Miocene (source rocks) 4295 - 5775 0.03 - 0.39/0.13 419 - 439/429 0.38 - 0.48/0.44
Figure 3. Distribution of temperature gradient (˚C/100 m) in the SCB.
In the greater part of the SCB the clayey rocks consist
mineralogically mainly of smectite (40% - 50% and even
higher) [23].
The onset temperature for smektite dehydration de-
pends on the geological conditions of the basin and can
vary within the limits of 75˚C - 150˚C [31]. The critical
temperature for diagenetic transformation of smectite
varies within the limits of 86˚C - 110˚C [32].
Because the SCB has an abnormally low non-station-
ary temperature regime, the process of intensive dehy-
dration of clays (transformation of smectite to illite) is
expected to take place here at depths greater than 7 km.
А. Feyzullayev / Natural Science 3 (2011) 359-364
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Figure 4. Vitrinite reflectance (Ro, %) vs. depth in the SCB.
Figure 5. Iisotope composition of carbon in saturated and aro-
matic fractions of oils in the SCB: oil fields with normal pres-
sures (I) and overpressures (II).
However, the average value of the smectite percentage
does not change essentially in an interval of depths up to
6.2 km for both the Absheron and Baku archipelagoes
The comparison of change of the smectite content
within Absheron and Baku archipelagoes indicates a
relatively higher content of smectite in the Baku archi-
pelago in comparison with the Absheron archipelago
(Figure 6).
Because the Baku archipelago, in comparison with
Absheron water area (see Table 1), is characterized by
higher pore pressure gradients, the preservation of smec-
tite in the Baku archipelago is caused by retardation of
smectite to illite transformation under overpressure con-
The rate of thermochemical reactions in sediments
with overpressures can increase sharply if, by virtue of
any factor (tectonic, thermal and chemical convection,
effect of gravitational emersion of fluidized viscous-
unstable clay mass, etc.) and the formed products are
Figure 6. Percentage of smectite in shales vs. depth
in the SCB: 1-Absheron Archipelago; 2-Baku Ar-
Figure 7. The mud volcanoes location in the Caspian
Sea (Azerbaijan sector): I, II and III – zone with, ac-
cordingly, normal, moderate and abnormal high pres-
sures; 1-mud volcanoes; 2-deep water part of sea.
removed from the system (for example, during eruptions
of mud volcanoes or formation of mud diapirs). This
А. Feyzullayev / Natural Science 3 (2011) 359-364
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statement is confirmed by spatial change of the devel-
opment density of mud volcanoes. The more density of
their development is noted in overpressured zone (zone
III) in comparison with the zones of moderate and nor-
mal pressures (zone I and II, accordingly) (Figure 7).
The migration of HCs will then have an explosive
character, which can also be promoted by earthquakes
occurring in the SCB, the hypocenters of which are
mostly at shallow depths [25].
The results of investigations indicate that the SCB is
characterized by the following features:
unequal conditions of generations and expulsion of
HCs from source rocks;
the processes of HCs generation (mainly in
deep-buried part of SCB) are shifted to relatively
deeper depths, from where the expulsion of HCs
from source rocks is complicated. As a result in
this zone, characterizing as a relatively closed sys-
tem, overpressures and mud volcanoes are widely
there is retardation of thermal transformation of
OM and clay minerals, generation of oil and
cracking it into gas in overpressured source rocks;
mud volcanoes are focal centers of periodic explo-
sive unloading of HCs from overpressured source
Results of investigations have both scientific and ap-
plied significance.
The scientific significance is that the classical scheme
of the vertical zonality of HC generation [2,3] should be
improved with reference to the overpressured basins.
According to the latest discoveries of large accumula-
tions of oil at depths of 8.5 - 10.5 km in the Gulf of
Mexico [34], the “oil window” in basins with overpres-
sure where the retardation of OM transformation is ob-
served, have to be shifted in the classical scheme of the
vertical zonality of HC generation to greater depths in
comparison with the basins without overpressure.
This indicates an increase in liquid HC resources for
the deep-water parts of basins like the South Caspian,
which in turn challenges the necessity to increase explo-
rations in overpressured zones.
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