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International Geology Review
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Geochemistry of fluids discharged from mud
volcanoes in SE Caspian Sea (Gorgan Plain, Iran)
Mahin Farhadian Babadi, Behzad Mehrabi, Franco Tassi, Jacopo Cabassi,
Elena Pecchioni, Ata Shakeri & Orlando Vaselli
To cite this article: Mahin Farhadian Babadi, Behzad Mehrabi, Franco Tassi, Jacopo Cabassi,
Elena Pecchioni, Ata Shakeri & Orlando Vaselli (2020): Geochemistry of fluids discharged from
mud volcanoes in SE Caspian Sea (Gorgan Plain, Iran), International Geology Review, DOI:
10.1080/00206814.2020.1716400
To link to this article: https://doi.org/10.1080/00206814.2020.1716400
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Published online: 27 Jan 2020.
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ARTICLE
Geochemistry of fluids discharged from mud volcanoes in SE Caspian Sea
(Gorgan Plain, Iran)
Mahin Farhadian Babadi
a
, Behzad Mehrabi
a
, Franco Tassi
b,c
, Jacopo Cabassi
c,b
, Elena Pecchioni
b
,
Ata Shakeri
a
and Orlando Vaselli
b,c
a
Department of Geochemistry, Faculty of Earth Sciences, Kharazmi University, Tehran, Iran;
b
Department of Earth Sciences, University of
Florence, Florence, Italy;
c
CNR-IGG Institute of Geosciences & Earth Resources, Florence, Italy
ABSTRACT
A geochemical study was carried out on gas, water and mud samples from four mud volcanoes in
Gorgan Plain, SE Caspian Sea (Iran) in order to investigate fluid primary sources and secondary
processes controlling fluid chemistry. The chemical composition of light alkanes and the isotopic
feature of methane indicated an origin related to a thermogenic source. Gases discharged from
Neftlijeh evidenced anaerobic biodegradation processes with addition of secondary microbial
methane. Chemical composition of discharged waters revealed two main groups i) brine-type
Na
+
–Cl
−
waters from Gharenyaregh and Neftlijeh mud volcanoes, which were marked by relatively
high Na
+
/Cl
−
, B/Cl
−
and Li/Cl
−
ratios and low Ca
2+
/Cl
−
,Mg
2+
/Cl
−
and K
+
/Cl
−
ratios, ii) waters from
Sofikam and Inche, characterized by relatively low Na
+
/Cl
−
, B/Cl
−
and Li/Cl
−
ratios and relatively
high Ca
2+
/Cl
−
,Mg
2+
/Cl
−
and K
+
/Cl
−
ratios. The chemical and isotopic characteristics of the dis-
charged waters suggest that evaporated Caspian seawater trapped in sediments is likely repre-
senting a reliable water source. The maximum formation depth at Gharenyaregh and Neftlijeh mud
volcanoes were estimated at about 6 km depth whereas the generation depth of the rest was
significantly shallower. Thus, the observed compositional differences can be related to the differ-
ent depths of the fluid source feeding them.
ARTICLE HISTORY
Received 3 September 2019
Accepted 12 January 2020
KEYWORDS
Mud volcanoes; fluid
geochemistry; secondary
geochemical processes;
Gorgan Plain; Caspian Sea;
Iran
Introduction
Mud volcanoes (MVs) are produced by the outflow of
mud, water and gas phases, mostly consisting of
methane, with significant concentrations of higher
hydrocarbons compounds, CO
2
,N
2
and H
2
S, released
by overpressured organic-rich sediments rapidly buried
in sedimentary basins (Dia et al. 1999; Milkov 2000;
Dimitrov 2002; Etiope et al. 2007). Fluids uprising
through deep-rooted feeding channels may also be
stored in mud chambers located at intermediate-to-
shallow depth, giving rise to buried MVs that can be
recognized with geophysical surveys (Deville et al.
2003; Mazzini et al. 2009). MVs occur in many sedimen-
tary basins related to convergent plate margins, accre-
tionary wedges, passive margins within deltaic systems
(Kopf et al. 2001), and active hydrothermal areas (Etiope
et al. 2002). They are known to be associated with faults
and faulted anticlines in active tectonic settings (Kopf
2002), where the tectonic stress, mainly compressional,
acts as the main driving force. MVs are investigated for
hydrocarbon exploration, as a possible evidence of sub-
surface petroleum accumulations (Deville et al. 2003;
Milkov 2005; Etiope et al. 2009a), and pose environmen-
tal concerns, since methane is a potent greenhouse gas
(Milkov 2005; Etiope et al. 2008).
The origin of methane and light hydrocarbons dis-
charged from MVs is commonly ascribed to (i) thermal
degradation of pre-existing organic material (thermo-
genesis) and/or (ii) microbial activity (Bernard et al.
1978; Schoell 1980, 1983; Chung et al. 1988; Seewald
et al. 1998; Whiticar 1999; Seewald 2003; Takai et al.
2008), the latter not being necessary related to any
potential source rock (Schoell 1983). Gases from these
two sources can be distinguished by using the carbon
isotopic composition of methane (Milkov and Etiope
2018): δ
13
C
1
values lighter than −50‰ vs. V-PDB are
typical of microbial activity, i.e. a process proceeding
through either fermentation or carbon dioxide reduc-
tion, whereas δ
13
C
1
values from −15‰ to −75‰ vs.
V-PDB are commonly shown by thermogenic methane,
i.e. thermal breakdown of pre-existing organic matter,
typically associated with relatively high concentrations
of heavier hydrocarbons (Whiticar 1994). Carbon and
hydrogen isotopes of methane coupled with the relative
abundances of light alkanes are used to discriminate
CONTACT Mahin Farhadian Babadi mahin.farhadian@gmail.com Department of Geochemistry, Faculty of Earth Sciences, Kharazmi University, Tehran, Iran
Supplemental data for this article can be accessed here.
INTERNATIONAL GEOLOGY REVIEW
https://doi.org/10.1080/00206814.2020.1716400
© 2020 Informa UK Limited, trading as Taylor & Francis Group
between these two-genetic processes (Bernard et al.
1978; Schoell 1980, 1983; Chung et al. 1988; Whiticar
1999), since thermogenic gases have relatively high con-
centrations of ethane, propane, butane and pentane
than those recorded in microbial gases (Schoell 1980;
Hinrichs et al. 2006). However, the original chemical and
isotopic composition of deep-originated gas may be
affected by post-genetic processes, such as anaerobic
biodegradation of petroleum coupled with an addition
of secondary microbial methane occurring at relatively
shallow depth (Pallasser 2000; Etiope et al. 2009b; Milkov
2011, 2018), which may mask the pristine chemical and
isotopic features of the primary gases.
Consequently, C
1
-C
4
hydrocarbons were widely inves-
tigated to reveal the origin of gas discharged from fluid
escape in sedimentary structures associated with various
geologic settings (e.g. Delisle et al. 2002; Deville et al.
2003; You et al. 2004; Guliyev et al. 2004; Etiope et al.
2006, 2007; Tassi et al. 2012; Prinzhofer and Deville 2013;
Ray et al. 2013). A data-set including chemical and iso-
topic signatures of light hydrocarbons discharged from
worldwide onshore MVs indicated that they are domi-
nantly related to thermogenic process (Etiope et al. 2008,
2009a), suggesting the presence of a potential source
rock at catagenetic stage, i.e. occurring at temperatures
typically >60°C (Hunt 1984; Milkov 2005). Chemical and
isotopic compositions of the water phase discharged
from MVs are good tracer to identify type and possible
source of fluid. Additionally, useful insights can be
obtained on depositional environments (presence of
marine or non-marine evaporites), diagenetic processes,
such as dissolution and precipitation of minerals, ion
exchanges, organic matter degradation and clay mineral
dehydration, which can be affected by temperature,
depth and mixing processes (Rittenhouse 1967; You
et al. 1993; Hanor 1994; Worden 1996; Dia et al. 1999;
Kopf and Deyhle 2002; Kharaka and Hanor 2004; Chung
et al. 2015).
Solid phases associated with the discharged fluids,
called mud breccia, generally consist of clay-rich mud
matrix and heterogenic rock fragments extruded from
subsurface plumbing systems of MVs (Dimitrov 2002;
Kopf and Deyhle 2002). Clay mineral alteration (e.g. illi-
tization of smectite) is commonly assumed to be related
to mud volcanism (Kopf and Deyhle 2002
; Lavrushin
et
al. 2005). Since MVs mainly consist of smectite-rich
mud (Fitts and Brown 1999), boron can be adsorbed by
this clay mineral and then released to pore fluids
through temperature-driven smectite-illite transforma-
tion during burial or tectonic processes (Colten-Bradley
1987; You et al. 1996). Consequently, insights into tem-
peratures at depth in the various compressional tectonic
settings can be gathered by this element, although
boron can also be enriched by degradation of organic
matter in buried sediments (Williams et al. 2001; Kharaka
and Hanor 2004).
The South Caspian Basin, which includes the Southern
Caspian Sea and the coastal zones of Iran, Azerbaijan and
Turkmenistan, is one of the oldest gas- and oil-bearing
provinces in the world (Smith-Rouch 2006). This basin is
well known for a large number of small-to-huge MVs
(Figure 1) that occasionally produce impressive eruptions
(Planke et al. 2003). These structures predominantly
release thermogenic gas and are often found in associa-
tion with petroleum fields (Fowler et al. 2000).
This paper reports the chemical and isotopic features of
hydrocarbon-rich gases and waters, as well as a qualitative
estimation of the mud-forming minerals, emitted from four
onshore MVs, namely Sofikam, Inche, Gharenyaregh and
Neftlijeh (Omrani and Raghimi 2018) which are located in
the Gorgan Plain (SE Caspian Sea, Iran), to provide informa-
tion abou t the origin and the main geochemical processes
controlling flui d geochemistry.
Geological setting
The South Caspian Basin, one of the deepest sedimen-
tary basins in the world (Devlin et al. 1999), is bounded
by Caucasus, Talesh, Alborz and Kopeh Dagh mountains.
Sediments derived by erosion and dismantling of these
mountain belts fill up the subsiding South Caspian Basin
and led, in the Pliocene-Quaternary, to the deposition of
thickest sedimentary series at a rate of up to 3 km/Ma
(Brunet et al. 2003). A thick clay-dominated sedimentary
cover (up to 25–30 km) and a low geothermal gradient
(15–18°C/km) characterize the South Caspian Basin. The
high sedimentation rate was likely responsible of under-
compacted sedimentary sequences, pore water over-
pressure, maturation of organic material and formation
of structural traps, which are typical of mud volcanism
areas (Abrams and Narimanov 1997; Tagiyev et al. 1997).
More than 400-active onshore and offshore MVs are
present in this region. They are mostly associated with
hydrocarbon fields within hydrocarbon-bearing faulted
anticlines (Guliyev and Feizullayev 1996;Fowleret al.
2000). The geochemistry of fluids discharged from the
South Caspian Basin MVs, including those exposed in
Cheleken peninsula (western Turkmenistan) and
Azerbaijan were extensively studied (Planke et al. 2003;
Davies and Stewart 2005; Mazzini et al. 2009; Oppo et al.
2014;Lavrushinet al. 2015). The Oligocene-Early Miocene
Maykop Formation, including layers of anoxic fine-grained
and organic-rich sediments (Abrams and Narimanov 1997;
Feyzullayev et al. 2001;Hudsonet al. 2008), is to be
regarded as the main fluid source (Figure 2;Inanet al.
1997;Fowleret al. 2000). However, some clasts found in
2
M. FARHADIAN B ABADI ET AL.
the mud breccias suggested possible contributions from
deeper sedimentary sequences (i.e. Middle-late Miocene
Diatom Series and Mesozoic deposits; Guliyev and
Feizullayev 1996;Inanet al. 1997; Feyzullayev et al. 2001).
The Gorgan Plain, where MVs of current work pre-
sented, is located at 36° 40´ to 37° 30´ N and 53° 38´ to
55° 38´ E (Figure 3) in the Golestan province (NE Iran).
The sedimentary successions of this Plain are part of
those of the South Caspian and Kopeh Dagh Basins.
The Kopeh Dagh Basin, as an elongated E-W trending
basin located to the northeast of Iran and east of the
South Caspian Basin. The boundary between the thick
sedimentary series of South Caspian (Pliocene to
Pleistocene) and the faulted and eroded formations of
Kopeh Dagh (Jurassic to Eocene) beneath the eastern
Gorgan is marked by a major Eocene-Oligocene angular
unconformity. Although, above this unconformity, the
faulted shale ridge structures on seismic section has
been observed which are probably Maykop Formation
(Robert et al. 2014). Ongoing geochemical investigations
on a newly dug petroleum exploration well located near
Sofikam MV also revealed the occurrence of a sequence
similar to that characterizing the Maykop formation
(Dr M. Mosavi Rohbakhsh; personal communication),
the latter being possibly the source rock of gas seepages.
Figure 1. Schematic map showing the location of onshore mud volcanoes in the South Caspian Basin including those of Azerbaijan
(mud volcanic regions: (I) Caspian, (II) Absheron, (III) Shemakha–Gobustan and (IV) Kura; Lavrushin et al. 2015; Jakubov et al. 1971),
Turkmenistan (Oppo et al. 2014) and Iran.
Figure 2. Generalized schematic stratigraphic column of South
Caspian Basin (adopted from Green et al. 2009).
INTERNATIONAL GEOLOGY REVIEW
3
The shale ridge structures are overlain by the Late
Miocene yellow horizon and the Pliocene Cheleken for-
mation. In the South Caspian Basin, the main hydrocar-
bon reservoirs are hosted within the Cheleken
formation, which shows an increasing thickness from
the Eastern Gorgan Plain towards the Caspian Sea and
mainly consists of green or dark red sand-containing
marls and thick layers of sandstone and conglomerate
deposits (Guliyev and Feizullayev 1996; Abrams and
Narimanov 1997; Fowler et al. 2000; Planke et al. 2003;
Stewart and Davies 2006; Torres 2007). An overpressured
gas-dominated layer was found in the Cheleken forma-
tion during the drilling of two exploration wells located
near Gharenyaregh MV in Gorgan Plain. The Cheleken
formation is overlain by Late Pliocene claystone and
marls with minor interbedded sandstones (Akchagyl
Formation), followed by Pleistocene (Apsheron forma-
tion) and late Pleistocene-Holocene strata of the Baku,
Khazarian, Khvalynian and Neocaspian stages (Mosavi
Rohbakhsh 2001). Thin layers of volcanic ashes were
Figure 3. Schematic geological map of southeastern of Caspian Sea, Gorgan Plain, Iran showing the location of investigated MVs
(modified after Saeedi and Andalibi 1993) controlled by faults/folds of Kopeh Dagh Basin (adapted from Omrani and Raghimi 2018).
4
M. FARHADIAN B ABADI ET AL.