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In thick shale-rich sedimentary basins, characterized by deep focused fluid migration from overpressured levels toward the surface, sediments can be remobilized in subsurface and produce different types of geological structures. The most well-known phenomenon is mud volcanism, which corresponds to superficial vents expelling mud and gas flows. Mud volcanoes occur in different geologic settings around the world, including compressional areas, deltaic settings, and hydrothermal areas. They develop in greater numbers offshore than on land and their global number in deep seas is still unknown. Understanding mud volcanism is a matter of concern because the natural degassing of high fluxes of methane and/or carbon dioxide associated to mud volcanoes has never been properly globally quantified, though it contributes significantly to the budget of greenhouse gases in the atmosphere. Mud volcano eruptions constitute also poorly understood natural hazards to local communities and navigation in shallow water areas. Mud volcanism is also prone to initiate submarine slope instability and so to cause damages to offshore infrastructures or tsunamis. Drilling in context of mud volcanism is exposed to high risk because of the occurrence at depth of high overpressure conditions susceptible to induce seal fracturing and massive blow-out of fluids or mud. A very regrettable example started in Java the 29 may 2006 and is still occurring today. Also, mud volcano studies provide precious information for petroleum exploration about the nature and age of the geological formations and of the various fluids present at depth. Mud volcano systems result from a reaction chain of several processes developed from depth to the surface. Commonly, the fluids (water and gas) and the solid fraction are issued from different geological formations and not from a unique source. Water comes for the compaction of shale and from deep overpressured reservoirs. Most of the times, the gas is mainly thermogenic methane associated with hydrocarbon generation at depth, but carbon dioxide is dominant in hydrothermal and volcanic contexts. Fluids migrate through hydraulic fracture systems which are initiated by high overpressure condition at depth. The rise of the fluids produces the remobilization of deep sediments and inducestheir transport toward the surface. The mobilized sediments are mostly issued from argillaceous seals but also from deep sandy reservoirs. Indeed, in most of the cases, the mud is rich in thin, angular, and mechanically damaged quartz grains (comparable to crushed glass) and which are probably cataclastic flows issued from sheared and collapsed deep sandy reservoirs. Relatively large rock fragments (exotic clasts and breccias) resulting from fracturing processes along the mud volcano conduits are transported by the rising mud during eruptions. The mud volcano edifices observed at the surface are commonly associated with the development of concentric collapses and associated calderas. The expulsion of the fluids varies according to cyclic phases punctuated for some mud volcanoes by catastrophic eruptions. These eruptions could occur when high excess pore pressure develops at depth, which induces the opening of a fracture system favoring cyclic successive fluid releases and pressure decreases. A threshold effect when fluids are oversaturated in dissolved gas generating successive massive gas discharges can also be invoked.
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In: Volcanoes: Formation, Eruptions and Modelling ISBN: 978-1-60692-916-2
Editors: N. Lewis, A. Moretti, pp. 95-126 © 2009 Nova Science Publishers, Inc.
Chapter 5
MUD VOLCANO SYSTEMS
Éric Deville*
IFP, 1-4, av. de Bois-Préau, 92 852 Rueil-Malmaison, France
ABSTRACT
In thick shale-rich sedimentary basins, characterized by deep focused fluid migration
from overpressured levels toward the surface, sediments can be remobilized in subsurface
and produce different types of geological structures. The most well-known phenomenon
is mud volcanism, which corresponds to superficial vents expelling mud and gas flows.
Mud volcanoes occur in different geologic settings around the world, including
compressional areas, deltaic settings, and hydrothermal areas. They develop in greater
numbers offshore than on land and their global number in deep seas is still unknown.
Understanding mud volcanism is a matter of concern because the natural degassing of
high fluxes of methane and/or carbon dioxide associated to mud volcanoes has never
been properly globally quantified, though it contributes significantly to the budget of
greenhouse gases in the atmosphere. Mud volcano eruptions constitute also poorly
understood natural hazards to local communities and navigation in shallow water areas.
Mud volcanism is also prone to initiate submarine slope instability and so to cause
damages to offshore infrastructures or tsunamis. Drilling in context of mud volcanism is
exposed to high risk because of the occurrence at depth of high overpressure conditions
susceptible to induce seal fracturing and massive blow-out of fluids or mud. A very
regrettable example started in Java the 29 may 2006 and is still occurring today. Also,
mud volcano studies provide precious information for petroleum exploration about the
nature and age of the geological formations and of the various fluids present at depth.
Mud volcano systems result from a reaction chain of several processes developed from
depth to the surface. Commonly, the fluids (water and gas) and the solid fraction are
issued from different geological formations and not from a unique source. Water comes
for the compaction of shale and from deep overpressured reservoirs. Most of the times,
the gas is mainly thermogenic methane associated with hydrocarbon generation at depth,
but carbon dioxide is dominant in hydrothermal and volcanic contexts. Fluids migrate
through hydraulic fracture systems which are initiated by high overpressure condition at
depth. The rise of the fluids produces the remobilization of deep sediments and induces
* eric.deville@ifp.fr
Éric Deville 2
their transport toward the surface. The mobilized sediments are mostly issued from
argillaceous seals but also from deep sandy reservoirs. Indeed, in most of the cases, the
mud is rich in thin, angular, and mechanically damaged quartz grains (comparable to
crushed glass) and which are probably cataclastic flows issued from sheared and
collapsed deep sandy reservoirs. Relatively large rock fragments (exotic clasts and
breccias) resulting from fracturing processes along the mud volcano conduits are
transported by the rising mud during eruptions. The mud volcano edifices observed at the
surface are commonly associated with the development of concentric collapses and
associated calderas. The expulsion of the fluids varies according to cyclic phases
punctuated for some mud volcanoes by catastrophic eruptions. These eruptions could
occur when high excess pore pressure develops at depth, which induces the opening of a
fracture system favoring cyclic successive fluid releases and pressure decreases. A
threshold effect when fluids are oversaturated in dissolved gas generating successive
massive gas discharges can also be invoked.
INTRODUCTION
In sedimentary basins characterized by thick shale-rich sediment deposition (notably
deltaic systems and accretionary wedges), subsurface sediment remobilization can be
widespread and produce a wide diversity of structures. These phenomena are commonly
designed by different terms (mud volcanoes, mobile shale, shale diapirs, clay diapirs, mud
diapirs, argillokinetic structures…). The most famous sedimentary remobilization
phenomenon is mud volcanism, which has been described since antiquity, which corresponds
to surface vents expelling mainly mud (water and thin solid particles) but also rock clasts and
gas flows. Temperatures associated to mud volcanism are much cooler than those in igneous
processes. Thousands of mud volcanoes occur globally (figure 1) and they develop in greater
numbers in offshore regions than on land (see Higgins and Saunders, 1974; Guliyiev and
Feizullayev, 1998; Milkov, 2000; Dimitrov, 2002; Kopf, 2002, for some inventories and
references). Indeed, most of them have been identified on land and in shallow water, but we
estimate that well over 10,000 may exist on continental slopes (figure 2) and abyssal plains,
where many of them are probably still to discover. They are found in different geologic
contexts (figure 1), notably in convergent orogens such as the Barbados prism (Biju-Duval et
al., 1982; Westbrook and Smith, 1983; Brown and Westbrook, 1987; 1988; Brown, 1990;
Langseth et al., 1988; Le Pichon et al., 1990; Henry et al., 1990, 1996; Griboulard et al.,
1991; Lance et al., 1998; Sumner and Westbrook, 2001), Trinidad (Higgins and Saunders,
1974; Dia et al., 1999; Castrec-Rouelle et al., 2002; Deville et al., 2003), North Venezuela
(Jacome et al., 2003; Duerto et al., in press), Colombia (Vernette et al., 1992), Panama (Breen
et al., 1988; Reed et al., 1990), Costa Rica (Shipley et al., 1990; Kahn et al., 1996), Peru-
Chile trench, Western Offshore USA (Orange et al., 1999) and Canada, Beaufort sea
(Hovland and Judd, 1988), the Aleutian trench (Von Huene, 1972), Alboran sea (Perrez-
Belzuz et al., 1997), the Mediterranean ridge (Camerlenghi et al., 1992, Kopf and Behrmann,
2000; Kopf et al., 2001), Black Sea (Ivanov et al., 1996; Konyukhov et al., 1990; Woodside et
al., 1997), Ukraine in the Kerch penninsula (Shnukov et al., 1992), the Appennines and Sicily
(Martinelli, 1999), Azerbaijan and the South Caspian basin (Hovland et al., 1997; Cooper,
2001; Planke et al., 2003; Stewart and Davies, 2006), Andaman island, Borneo (Tongkul,
1989), the Makran accretionary prism (Delisle et al., 2001; Wiedicke et al., 2001; Ellouz et
Mud Volcano Systems 3
al., 2007a and b), the Xinjiang province of China (Xie et al., 2001), New Zeland (Stoneley,
1965; Ridd, 1970), Indonesia (Barber et al., 1986), Papua-New Guinea (Bayliss et al., 1997),
Taiwan (Shih, 1967; Yassir, 1987), Japan (Kobayashi, 1992; Ogawa and Kobayashi, 1993);
Sakaline, Bering Sea (Geodekyan et al., 1985) and very probably many other places,
especially in deep seas. They are also very widely present in passive margins in deltaic
settings, such as the Niger (Damuth, 1994; Cohen and McClay, 1996; Graue, 2000), the Nile
(Loncke et al., 2004; Dupré et al., 2007) the Gulf of Mexico and the Mississipi (Bernard et
al., 1976; Neurauter and Roberts, 1994; Sassen et al., 2003), Indus fan (Collier and White,
1990), Bengal fan, or also along classical passive margins, like the Norvegian sea at the
Aakon Mosby site (Vogt et al., 1997; Bogdanov et al., 1999; Hjelstuen et al., 1999), Storegga
areas, eastern offshore USA (Schmuck and Paull, 1993), Offshore Baffin Island (Woodworth-
Lynas, 1983), etc. They also develop in active hydrothermal areas, such the Yellowstone Park
(Pitt and Hutchinson, 1982), the Etna area in Sicily (Etiope et al., 2002), Salton Sea
hydrothermal area in California, Wrangel Mountains in Alaska (Sorey et al., 2000),.... Mud
volcanoes are always associated with gas seepage mostly hydrocarbon gases, especially
methane (CH4) but when mud volcanoes are associated with magmatic volcanoes they also
emit incombustible gases, mainly carbon dioxide (CO2)(table 1). Understanding mud
volcanism processes is a matter of concern for a wide range of disciplines in earth sciences.
Indeed, the natural degassing of high fluxes of methane and/or carbon dioxide associated to
mud volcanism has never been properly globally quantified, though it contributes
significantly to the budget of greenhouse gases in the atmosphere and the global carbon cycle
(Deville and Prinzhofer, 2003; Judd, 2005; Kvenvolden and Rogers, 2005). Depending of the
authors, first order estimates of mud volcano emissions vary between several tens thousands
to more than 100,000 Tg/yr of gases (mainly methane and CO2) which may escape from mud
volcanoes to the atmosphere and the ocean (Dimitrov 2002; Etiope, 2003; Milkov, 2003;
Kopf, 2003). This huge field of uncertainty reflect the fact that actually it is very difficult to
define the exact gas fluxes because in a single mud volcano site the flux is difficult to
estimate (high dispersion and very time variable flux which is almost impossible to quantify
during the massive eruptions, probably several millions of m3 of gas/day), and also of course
because many mud volcanoes are still to be found, notably in deep seas. Mud volcano
eruptions are also poorly understood natural hazards for local communities and navigation in
shallow water. Drilling in context of mud volcanism is also exposed to high risk because of
the occurrence at depth of high overpressure conditions susceptible to easily induce seal
fracturing and massive blow-out of fluids or mud. Such an eruption occurred notably during
oil and gas exploration in offshore Brunei in 1979 (Tinglay et al., 2003). Another possible
regrettable example happened in Java the 29 may 2006 (Davies et al., 2007, Mazzini et al.,
2007), and is still occurring at the time we publish this paper. In continental margins, mud
volcanism activity is also very prone to initiate slope instability and so to cause potential
damages to offshore facilities (cables, production units,...), or even to cause tsunamis
(historical cases are suspected notably in the offshore Norway and the Mediterranean offshore
of Egypt). Also, mud volcanoes studies provide precious information for the exploration of
subsurface fluids (notably hydrocarbons), especially in the areas where no wells are available
(by giving indication about the nature and age of the geological formations and of the various
fluids present at depth).
Éric Deville 4
Table 1. Few examples of gas analyses taken from different mud
volcanoes of the world (from Deville et al., 2003; Ferrand, 2007)
C1 C2 C3 iC4 nC4 CO2 N2
Trinidad (Piparo) 92.44 0.45 0.02 0.03 0.01 06.63 00.42
Nile delta (Isis) 97.76 1.24 0.10 0.02 0.03 00.85
Mediterranean ridge
(Napoli) 90.30 5.07 04.63
Ukraine (Androsov) 83.68 1.66 0.46 0.06 01.53 12.61
Azerbaijan (Bozdag) 96.28 0.89 0.14 0.02 0.02 02.65
Pakistan (Chandragrup) 98.42 0.21 0.03 01.34
Sicily (Aragona) 97.75 00.65 0.07 01.21 00.32
Sicily (Paterno) 05.88 94.06 00.06
Figure 1. Global distribution of the main known mud volcanoes sites in the world, and location of the
figures shown in this chapter.
Figure 2. Field of mud volcanoes in the eastern continental slope of the offshore of Trinidad.
Mud Volcano Systems 5
Few years ago mud volcanism remained a very poorly understood phenomenon. Notably,
in most of cases, geologists did not know what was the origin of the various mobilized phases
and if the liquid phases (water, hydrocarbons), gas phase (dissolved or free gas), and the solid
particles of the mud were issued from the same or different levels. The deep architecture of
these systems and how do they evolve in time was very poorly constrained. The dynamics of
the mud volcanoes was also something poorly understood. Several works made during the
last decade in different parts of the world have significantly improved our knowledge about
mud volcano systems.
SURFACE FEATURES
Mud volcanoes are the superficial consequence of the transfer of liquefied sediments
toward the surface. The largest structures are several kilometers large, up to 10 km in
diameter, with several hundred meters in height (figure 3). On the surface, they can present
very different aspects. Some mud volcanoes show regular shape of cones up to kilometer-long
and up to hundred of meters high for some of them (figures 3 and 4). They are constituted by
the progressive stacking of superficial mudflows, and they have a general aspect very similar
to magmatic stratovolcanoes (figures 5 and 6). Other mud volcanoes correspond to smooth
domes or mud shields which are resulting from massive eruption of mud that can spread out
and flow a great distance from a vent. Some mud volcano sites correspond to fields of steep-
sided cones shorter than 10 meters and which are generally called gryphons, or to mud pools
(or salse when water dominant; figures 7, 8, 9, 10). In these cases, the vents can be randomly
distributed, or located along linear or circular fractures systems. Certain mud volcanoes
correspond to pools or lakes of mud in which mud and gas is continuously expelled. The
bigger pools are characterized by convective cells with an upward displacement of mud above
the deep outlet, and ring-shaped rolls associated with the burial of the mud on the flanks of
the pools (figure 11, 12, 13). During the phases of relative quiescence during two eruptions
(but during which the flow expelled can vary significantly through time), the expulsion is
active either in griffons, or in mud pools. At some sites, the gas flows are sometimes higher
than 10 m3 of gas per minute, and generally, the mudflows are much more moderate
compared to the gas flows (commonly 10 times lower). Also, many mud volcanoes are
surrounded by circular fault systems generating circular depression comparable to the
magmatic calderas. Circular depressions develop sometimes inside a cone. When the
depression at the top of the cone is filled by mud, this generate a flat circular topography at
the top (with one are several vents) and bounded by steep flanks ("mud-pies"). Offshore, the
material found when we core the mud volcanoes corresponds to a staking of mud flows or to
gravity destabilization on the slopes of the mud volcanoes. In some cases, these deposits are
also rich in carbonated diagenetic crusts (Aloisi et al., 2000; Deville et al., 2006; figure 14).
Also, offshore, when the mud volcanism activity interacts with high recent sedimentation
rate, as observed in eastern offshore Trinidad, subsurface mud volcanoes show a Christmas
tree geometry related to the stacking of successive edifices (figure 15).
Éric Deville 6
Figure 3. Cone-shaped mud volcano in Chandragrup, Pakistani Makran. This mud volcano edifice is
more than 100 m high.
Figure 4. Cone-shaped mud volcano with a crater on the top, in Chandragrup, Pakistani Makran.
Figure 5. Mud flow in the Dashgill mud volcano in Azerbaijan.
Mud Volcano Systems 7
Figure 6. Active mud flow in the Aragona mud volcano in southern Sicily.
Figure 7. Field of small cones (gryphons) of mud volcanoes, Jahu pass, west on Bella, in the Pakistani
Makran.
Figure 8. Field of small eruptive cones (gryphons) in Dashgill, Azerbaidjan.
Éric Deville 8
Figure 9. A small vent erupting mud (gryphon) in Moruga Bouffe, Trinidad.
Figure 10. Methane bubble and mud eruption from a gryphon in Dashgill, Azerbaijan.
Figure 11. Mud lake in Lagon Bouffe, Trinidad.
Mud Volcano Systems 9
Figure 12. Temperature section within the deepest mud pool of the Lagon Bouffe mud volcano in
January 2002. A, B, C, and D correspond to the different temperature profiles acquired within the pool
the 23rd January 2002, between 1:50 and 3:52 pm local time. Note the shape of the isotherm envelopes
associated to convective processes.
Figure 13. A brine pool found at the Mediterranean sea bottom in the deep offshore of Nile Delta in a
mud volcano site called Menes caldera (- 3000 m below sea level). This picture has been taken during a
dive of the NAUTILE submarine (© IFREMER). The central part corresponds to brine saturated in
dissolved methane. The white material around the pool is a bacterial mat. The pool developed on
ancient mud flows at the sea bottom.
Éric Deville 10
Figure 14. An example of core collected on a mud volcano in the Barbados accretionary wedge.
Figure 15. Satellite view of the mud volcanoes of Chandragrup Pakistani Makran. Note the circular
fault system around the mud volcanoes and the associated caldera.
Mud Volcano Systems 11
MUD VOLCANOES: ORIGIN OF THE EXPELLED PRODUCTS
Gas
From a global geochemical point of view, mud volcanism processes were until recently
very poorly known, and notably in many cases, we did not know the origin of the gas, and in
the case of hydrocarbons commonly we did not know if the gas was of thermal cracking of
organic matter or of bacterial origin. In purely sedimentary contexts, in most of the cases
studied (see notably Deville et al., 2003; Ferrand, 2007), the gas is primarily methane (CH4)
associated with moderate concentrations of heavier hydrocarbon gas (C2+), nitrogen (N2) and
carbon dioxide (CO2). Isotopic studies of the gas (notably δ13C of methane) show that the gas
is in most of the cases coming from the thermal cracking of organic matter at depth, and in
some cases only from a bacterial generation (Ferrand, 2007). On the example of southern
Trinidad, the analysis of noble gas radiogenic isotopes shows that the gas expelled from the
mud volcanoes has a shorter residence time in subsurface than the gas associated with the oil
fields (Battani et al., 2001 and in press). And so, the gas of the mud volcanoes can not be
issued from a direct leakage from the hydrocarbon fields, but must come directly from deeper
source areas. The available data suggest that fast gas migration occur from depth to the
surface cross-cutting hydrocarbon (HC) reservoirs in which the gas has been retained since a
longer time. In case of volcanic/hydrothermal context, like the mud volcanoes close to the
Mount Etna, the dominant gas is actually CO2 (see table 1).
Water
Mud volcanoes are generally located in context of recent thick sedimentation associated
to the occurrence of zones of fluid overpressure at depth. The overpressure is related to the
difficulties of the fluids contained in sediments to escape towards the surface, when sediments
are especially clay-rich, relatively impermeable, and when they are quickly buried. When
impermeable layers are crossed by migration pathways, the water raises and springs on
surface. Mud volcanoes would correspond to the surface vents of these deep fluids. The
studies published on the chemistry of the water of mud volcanoes confirmed that the expelled
water is issued from sediments (see notably Dia et al., 1999). We recognize this water by
specific concentrations in cations and anions, as well as by isotopic concentration of
characteristic elements. The water of the mud volcanoes is generally poorer in chlorides than
the sea water, except in certain cases where the conduits of mud volcanoes cross-cut salt
layers, for example in the Mediterranean area in Sicily, the Mediterranean ridge and the Nile
delta, where mud volcanism is associated to the development of Brine pools or salse (Dupré
et al., 2007; figure 13). The geoscientists also showed that the water can also results from
aquifers crossed by the mud during its ascent, which mixes with the water of deep sediments.
The study of the composition of this water also allows calculating its temperature when the
elements which it contains were dissolved. These temperatures are sometimes very high
compared to the mud temperature in surface. For instance, on some mud volcanoes of the
island of Trinidad, the temperature of dissolution reached 150°C (Dia et al., 1999). These
high temperatures indicate that mud volcanoes take their roots at several kilometers deep. For
Éric Deville 12
some vents of the gulf of Mexico, some authors have even invoked the possibility that water
suffered supercritical conditions, meaning that temperature reached at least 400°C (Hovland
et al., 2006). In many cases also the mud contains significant amounts of liquid hydrocarbons.
In some vents, the hydrocarbons can even be the dominant constituent of the liquid phase.
Solid Fraction
Ejected materials by mud volcanoes are slurry of fine solid particles (figure 18)
suspended in liquids which may include water (with variable salinity) and hydrocarbon fluids.
During the eruptions the mud flow also drives rock fragments (clasts and breccias; figure 19,
20). In a recent past, and at a worldwide scale, it was commonly admitted that the parent beds
of the mud were issued from a unique source formation (see for instance Dimitrov, 2002;
Kopf, 2002 with many references). Actually, in all the places we studied, the material found
in the mud is systematically polygenic and issued from various levels and not from a single
stratigraphic horizon. This is notably well-illustrated by detailed dating studies using
nannofossils which show a systematic mixing of species (Deville et al., 2003a). This suggests
that the mud consists of a mixture of microscopic elements of various origins issued from all
the sedimentary formations pierced by the mud conduits. These polygenic particles of the
mud are driven by the water phase, as well as various clasts expelled during mud volcano
eruptions. The mobilized sediments in the mud are mostly various clay minerals and thin
quartz particles (Deville et al., 2003a; Deville et al., 2006). Microscopic studies combined
with X-ray diffraction and SEM studies of the mud have shown that the solid particles within
the mud are largely composed of clays (smectite, illite, kaolinite, vermiculite), chlorite, and
muscovite, as well as abundant grains of quartz, feldspar (albite, K-feldspar), carbonates
(calcite, dolomite, siderite), titanium oxides (rutile, anatase), apatite, barite, and pyrite (figure
18). Grain size varies from less than 0.1 μm, to more than 200 μm. The grains are supported
within a very thin matrix consisting of a mixture of various clays, micas sheets, and small
fragments (< 3 μm) of quartz and albite with clear angular shapes and internal microfractures,
especially in the quartz. The internal cracks (mechanical damage) probably results from
shearing during compaction at great depth. Such quartz grains can represent more than 90%
of the solid fraction within the mud. In the mud volcanoes having recent eruptive activity
exotic clasts and breccias (mainly centimetric to multi-metric) are exhumed during the
eruptions (Higgins and Saunders, 1974; Kopf et al., 2000; Deville et al., 2003a, and many
others). They are mainly fragments issued from various formations of the sedimentary pile
present at depth. The nature of the clasts is polygenic (carbonates, sandstones, shales, calcite,
sulphur nodules, etc.). Some clasts are obviously rounded older pebbles initially interbedded
within sedimentary formations and mobilized during eruptions, but in most of the cases, clasts
show angular shapes resulting from intense fracturing. Fractures are commonly filled with
carbonate cements (Ca and Ca-Mg). Frequently, real breccias made up of angular and initial
joined elements are included within calcite crystallizations. We interpret most of the angular
clasts and the breccias to be the result of hydraulic fracturing processes (figure 20). On
several sites, using notably nannofossils, it has been possible to date most of the individual
clasts expelled by the mud volcanoes. According to the ages obtained, in all the cases studied,
Mud Volcano Systems 13
these belong to very different formation pierced by the deep conduits of the mud volcanoes
(mixing of different formations).
Figure 16. An example of mud volcano offshore Trinidad, showing the subsurface geometry of the
edifice and of the feeding pipe below. Above: Depth migrated 3.5 kHz profile. Below: Time migrated
seismic line. Location in figure 2.
Figure 17. An example of seismic profile across a mud volcano in the eastern offshore of Trinidad
(courtesy Shell, Agip, Petrotrin). Note the stacking of different volcanic edifices. This "Christmas tree"
structure is probably the result of the cyclic development of volcanic edifices during the Pleistocene
periods of high stand (low sedimentation rates, clay-rich well stratified sediments) and of the draping of
the sedimentary volcanoes during the periods of low stand (high deposition rates, notably mass-flows,
Brami et al., 2000), but the mean expulsion rate of mud is not necessary directly related with
sedimentation rate (see location in figure 1; modified after Deville et al., 2006).
Éric Deville 14
Figure 18. Microscopic nature of the mud, an example from a vent of the Erin mud volcano in south
Trinidad (SEM). A: A general view showing the different minerals within the mud; B: Thin-grained
angular and damaged quartz grain.
Figure 19. Chaotic zone with clasts and breccias zone resulting probably from a mud volcano eruption,
offshore Nile Delta, Isis mud volcano, Sea water depth 950 m.
Mud Volcano Systems 15
Figure 20. An example of carbonate clast expelled by the mud volcano of Anglais Point in southern
Trinidad. Open fractures filled by carbonate cement are interpreted as the result of hydraulic fracturing
processes.
RISKS ASSOCIATED TO MUD VOLCANOES
Catastrophic Eruptions
As evidenced by historical data, the mud volcanoes have different phases of activity,
including for some mud volcanoes catastrophic events followed by periods of relative
quiescence characterized by moderate activity (Kugler, 1965; Higgins and Saunders, 1964,
1974; Gulliyiev and Feizullayev, 1998). We have seen that during catastrophic events, the
material expelled consists of high flows of gas, mud, polygenic clasts, and breccias issued
from various formations of the sedimentary pile. Spectacular eruptions occur frequently in
Azerbaijan (Aliyev et al., 2002), where the most active onshore mud volcano is probably
Lokbatan (eruption every 2-6 years; Aliyev et al., 2002). In several cases these eruptions in
Azerbaijan are associated with gas explosions (figure 21). Several kilometers-long mud flows
have been observed also in Pakistan (figures 22, 23). Also, for instance, in the Trinidad
Island, a catastrophic eruption occurred in Piparo on February 22, 1997, destroying several
houses. Another eruption occurred recently in Devil’s Woodyard on May 8, 1995. Short-lived
islands associated with catastrophic eruptions appeared several times in the Columbus
channel, offshore southern Trinidad (Arnold and McReady, 1956; Higgins and Saunders,
1967). The most famous site corresponds to the Chatham ephemeral island whose last
eruptions occurred on November 15, 2002 and May 10, 2001. Previous eruptions occurred on
August 1, 1964, December 21, 1928, and November 3 and 4, 1911, and so were
chronologically spaced out of 17, 36 and 35 years, the two last ones can be regarded as
related twin eruptions. Mud volcano eruptions are also known in deep seas. Figure 19 shows
chaotic products on the sea floor after an eruption in the northern offshore of Egypt, in the
deep Nile delta.
Also, a regrettable example occurred since 29 May 2006 in Java (Indonesia) where a mud
volcano started spewing out hot, viscous and foul-smelling slurry in the district of Sidoarjo
figures 24, 25). This mud volcano has been nicknamed Lusi, a combination of "Lumpur," the
Indonesian word for mud, and Sidoarjo, from the name of the city. It is still debated if the
eruption was related or not to the drilling operation which took place at the same time few
hundred of meters from the eruption site (Davis et al., 2007; Mazzini et al., 2007). The mud
Éric Deville 16
has covered about 500 hectares, has damaged four villages, roads, rice fields, factories, rivers
and sea coast and displaced about 25,000 people. Between 5,000 m3 and 150,000 m3 of mud
is erupting from the volcano every day. The mud is coming from pressurized hot water 2,700
meters below the surface. As it rises, the water mixes with sediments to form viscous slurry of
mud that is spreading around the volcano crater and forming a peak. Many experts believe
that the volcano could continue spewing mud for years or decades to come. As a reference, a
drilling accident offshore of Brunei in 1979 caused an eruption which took nearly 30 years to
stop.
Figure 21. Gas explosion on the Lokbatan mud volcano in Azerbaijan in May 2001.
Figure 22. Satellite view of the Kandewari mud volcano, in south Pakistan (Makran). Note the
extension (about 5 km) of the mud flow of January 2001 (dark grey).
Mud Volcano Systems 17
Figure 23. Giant mud flows on the Kandawari mud volcano, in the Pakistani Makran.
Figure 24. Mud and vapor eruption on the Lusi Mud Volcano, in Java. (source: Gallo-Getty,
http://english.aljazeera.net/NR/exeres/BE5EF1A2-5B9A-4329-ACC9-7054CB62AD2F.htm).
Figure 25. Mud flooding caused to the town of Sidoarjo by the eruption of the Lusi Mud Volcano, in
Java. (source: Gallo-Getty, http://english.aljazeera.net/NR/exeres/BE5EF1A2-5B9A-4329-ACC9-
7054CB62AD2F.htm).
Éric Deville 18
Mud Volcano Eruptions and Earthquakes
Several authors have evoked a possible link between the frequencies of eruption of
certain mud volcanoes and the seismic activity (Chigira and Tanaka, 1997; Guliyev and
Feizullayiev, 1998; Aliyev et al., 2002; Martinelli and Panahi, 2003; Nakamukae et al., 2004;
Baciu and Etiope, 2005; Mellors et al., 2007). For the case of the eruption of the Lusi mud
volcano in Java, a relationship this earthquake activity has also been suggested (Mazzini et
al., 2007). Also many scientists suggest monitoring gas emissions and activity of mud
volcanoes because they can be suitable to predict strong earthquakes. Some correlations are
indeed rather evident. A beautiful example is provided by a mud flow of more than 5 km in
length emitted by the mud volcano of Kandewari in the South of Pakistan, in the Makran
prism, which followed upon the violent earthquake (7.7 Mw) which affected the NW of India
and the South of Pakistan on January 26th, 2001 (Delisle et al., 2001; Deville and Prinzhofer,
2003; Ellouz et al., 2007a; figures 22, 23). Another example correspond to a mud volcano on
the island of Baratang, in the Middle Andaman islands, which erupted throwing mud above
the height of surrounding trees just several minutes after the 2004 great Sumatra-Andaman
Islands 9 Mw earthquake. Thus, punctually, a link exists obviously between eruptions of mud
volcanoes and earthquakes. However, in most of the cases, there is no direct obvious
correlation between eruptions and violent seismic events (figures 26, 27). From the available
historic data it appears that each eruptive mud volcano has its own period of catastrophic
activity (periods of several years), and this period is highly variable from one volcano to
another (figure 26). The frequency of activity of mud volcanoes seems essentially controlled
by local pressure regime within the sedimentary pile. At the most, earthquakes can, in certain
cases, activate an eruption close to its term.
Figure 26. The timing of the main mud volcano eruptions in Trinidad and Azerbaijan between 1900 and
Present. Mud volcano eruptions are often cyclic. The time interval between two eruptions varies
significantly from one mud volcano to another but for a same volcano this time interval is relatively
regular. This regularity would come from a threshold effect associated to the pressure conditions at
depth.
Mud Volcano Systems 19
Figure 27. Comparison between the timing of the main last mud eruption and the seismicity in Trinidad
between 1990 and 2005. We do not observe a direct correlation between eruptions and earthquakes. In
this case the periodicity of the eruptions is independent of the seismicity.
PROCESS MODEL FOR MUD VOLCANO SYSTEMS
It is now commonly admitted for many years, notably following the interpretations of
Edberg (1974) and Higgins and Saunders (1974) that mud volcanism is associated to the
generation of overpressure at depth but the process model of genesis of the mud volcanoes
remains still debated. Until a recent past the most common interpretation was that mud
volcanoes derive from the rise of a unique shale rich-source horizon by a process of balloon-
like mud diapirism mirroring salt diapir emplacement (see notably Milkov, 2000; Kopf,
2002). Several recent studies in different geological framework support the conclusion that
such processes model does not fully account for mud volcano system geometry (Cooper,
2001a, b; Deville et al., 2003a and b; Deville and Prinzhofer, 2003; Stewart and Davies,
2006). Indeed, overpressurred shale can be deformed according to ductile processes, but this
type of deformation is assumed to develop preferentially at depth. Despite of what scientists
thought again few years ago, it has never been possible to show real piercing shale diapiric
activity (Van Rensbergen et al., 2003; Deville et al., 2006). At a global scale, no real secant
intrusive mobile shale bodies have ever been identified as it occurs commonly in salt
tectonics. A key part of the process of mud volcanism, absent from salt diapirism, involves
fluid being transported upward through a conduit system initiating a reaction chain of
subsurface sediment remobilization processes associated to the development of several
distinctive structural elements and to sediment remobilization at several levels (Deville et al.,
2003a; Davies and Stewart, 2005; Stewart and Davies, 2006). This system feeds the extrusive
mud edifices developed in surface.
Éric Deville 20
Overpressure Generation
Subsurface sedimentation remobilization processes result from several dynamic
phenomena controlled by the development of overpressure at depth, which provides the
energy for breaching the seals and for the transport of the mud (fluid-sediment mixing)
toward the surface. Overpressure contributes also to sediment remobilization by reducing the
strength within the overpressured layer. Overpressure generation is favored by the
conjunction of fast sedimentation rates of relatively impermeable sediments leading to
compaction disequilibrium (sedimentary loading), and in context of compressive stress
regimes by layer-parallel shortening and tectonic overloading. If fluids cannot escape fast
enough relative to the pore space reduction, then the fluid generating the overpressure
supports the load and lateral stresses. This is notably favored by the high deformation rates in
accretionary prisms. This has an important role in the dynamic development of overpressure,
which is typically a non-static phenomenon. Moreover, the increase in temperature at depth
induces the cracking of hydrocarbons, which is an additional factor for overpressure
generation. Although the gas expelled by the mud volcanoes in deep water is most likely
dissolved, the occurrence of free gas bubbles, especially in the shallowest areas, is also likely
to reduce the density of the sediments, providing an additional factor favoring mobile
sediment rise.
Hydraulic Fracturing
As suggested by several authors (Brown and Westbrook, 1988; Brown, 1990; Deville et
al., 2003 a and b), the progressive deformation within the sedimentary pile is susceptible to
generate high pore pressure in the center of geological depressions which is susceptible to be
transmitted laterally by permeable horizons towards highs where sedimentary thickness and
load are smaller. When pore pressure approaches the vertical load, this leads to hydraulic
fracturing. Indeed, in the clasts expelled by the mud volcanoes, we have seen above that the
common occurrence of angular blocks including none preferentially oriented fractures
reflecting isotrope cracking has been interpreted as the result of hydraulic fracturing.
Resulting hydraulic fractures propagate hundreds of meters above the overpressure source
and establish a pathway for the rose of overpressured, fluidized material. The fractures of the
hydraulic wall-rock clasts and breccias extruded during eruptions are filled by carbonate
cements (figure 20) and on every mud volcanoes showing evidences of recent eruptions,
abundant large fragments of calcite veins have been observed. This suggests that water (with
dissolved carbonates) but not mud, is responsible for the initial hydraulic fractures, and that
water migration and carbonate cementations predated mud migration. It is worth to note that
it is what happened in the case of the birth of the Lusi mud volcano in Java, where water
came first and was followed later by mud expulsion. We have seen above that mud volcano
sites show often scattered arrays of meter-scale mud edifices (gryphons), suggesting that there
can be several conduits in a small area (Hovland et al., 1997; Planke et al., 2003). Although
exposed gryphon swarms are at high structural levels relative to the mud source several
authors have extrapolate this model to the subsurface. Notably Stewart and Davies (2006)
suggested that numerous pipes repeatedly intrude the overburden at approximately the same
location, forming a steep, cylindrical zone of heavily intruded country rock or amalgamated
Mud Volcano Systems 21
mud pipes. With a high net volume of intruded mud, this cylindrical zone would have low
mechanical strength relative to the surrounding in-situ strata. This steep cylindrical or conic
zone of intrusions then undergoes differential compaction, resulting in a downward-tapering
conical collapse opening upward into sag. Faults and fractures within and on the margins of
the cone provide inherent weaknesses that are exploited by later fluidized flows (Kurszlaukis
and Barnett, 2003; Morley, 2003).
Reservoir Collapse
Studying the solid fraction expelled by mud volcanoes, we have seen that the mud
includes very thin, angular and damaged quartz grain. This powder of quartz (comparable to
"crushed glass"; figure 18) results from brittle deformation of sandstone levels at depth,
probably associated to reservoir collapse during deformation. Deformation of quartz at high
stresses is commonly accommodated by granulation and cataclastic flow and so the resulting
thin particles are prone to be transported by fluids and to be incorporated within the rising
mud. Indeed, increasing effective pressure conditions is susceptible to produce compactive
strain associated with considerable particle-size reduction (Karner et al., 2005). If it is the
case, the origin of the water phase of the mud expelled by the mud volcanoes is not
necessarily to be found directly in shale horizons but the water flow might be issued largely
from deep sandstone reservoirs.
Fluid Migration
Whereas shale-rich sedimentary environments are relatively impermeable for fluid
migration, the development of hydraulic fractures and the remobilization of overpressured
fluids generate focused fluid escape from depth toward the surface. At depth, the
overpressured fluids are focusing toward these localized fracture outlets allowing the fluids to
escape upward, whereas the expulsion of the mud in the upper parts of the conduits creates a
pressure gradient between the mud column and the surrounding shallow zones of normal
pressure which is prone to induce fluid flows diverging from the main conduits. As outlined
above, the fluid expulsion regime varies according to cyclic phases. This cyclic activity of the
mud volcano occurs at different frequencies ranging from cycles of several years for the most
catastrophic events, to cycles of several minutes for gas and mud flows during the quiescent
phases. The very high frequencies (short periods) could be simply related to the dynamics of
two-phase flows (gas and mud) through the mud volcano conduits (Deville et al., 2004; figure
28). However, low frequency cycles, notably the catastrophic events, are very probably
controlled by the dynamic development of overpressure at depth. These could be related to
the fact that when high excess pore pressure occurs at depth, hydraulic fracturing is
responsible for opening the fracture network favoring successive fluid release and cyclic
pressure decrease (figure 29). Such processes could be enhanced by a threshold effect when
fluids are over-saturated with gas. In that case, massive degassing of large volumes of
dissolved gas is possible at depth that could suddenly raise the fluid pressure and damage the
sealing properties of the sediments located above the gas-charged deep mud plugs.
Geochemical results suggest that fast gas migration occur from depth to the surface (low
Éric Deville 22
residence time deduced from radiogenic isotopes of the noble gas; Battani et al., in press). In
some cases these fluid migrations cross-cut HC reservoirs in which the gas has been retained
since a longer time. That means that the migration pathways of the mud volcanoes and the
HC accumulations are disconnected (Deville et al., 2003a). This is in good agreement with
the fact that the important pressure depletion in the producing HC reservoirs (notably in
Azerbaijan and Trinidad) since decades has never disturbed the mud volcanoes activity. We
suppose that the disconnection between both systems is achieved by a solidified mud cake
around the mud conduits isolating these conduits from the surrounding reservoirs.
Figure 28. Cyclic temperature changes within the Palo Seco mud volcano conduit in Trinidad;
measurements were done at a depth of 34 m within the conduit (January 25th 2002). Note the cold
pulses followed by thermal re-equilibrium tendency. Cold pulses are related to cooling effect of
expanding gas during gas discharges.
Figure 29. Conceptual sketch of the reaction chain of the remobilization processes.
Mud Volcano Systems 23
Solid Fraction Remobilization
We have seen that the solid grains contained within the mud are originating from several
levels, as well as the exotic clasts and breccias extruded during eruptive phases which are
considered mostly as detached and hydraulic fractured fragments issued from the edges of the
conduits. This suggests a progressive incorporation of solid particles issued from various
formations during the mud ascent, involving deep horizons, and the incorporation of various
materials (thin grains and clasts) from the stratigraphic pile crossed by the mud conduits.
Consequently, the edges of the feeder pipes are potential zones of material withdrawal.
Indeed, seismic data interpretation in different areas of the world shown that mud volcano
systems are often centered above a thinned region of the underlying formations. We interpret
this thin region as resulting from withdrawal of solid particles from the conduits into the mud
and evacuation to the overlying mud volcano. Depending of the fluid dynamics, the depletion
zones might preferentially localize in certain levels. Notably, the argillaceous seals at fluid
retention depth are prone to provide most of the clay-rich fraction reworked within the mud.
Mud Intrusions
Mud volcanoes are the proofs that mud can migrate in subsurface and can intrude
fractures network (Morley, 2003). But the occurrence in subsurface of large volumes of
liquefied sediments accumulations remains more questionable. Though, ancient mud plugs
(presently solidified) have been drilled several times by oil industry, notably onshore
Trinidad, it is actually very difficult to define if these are the result of a stack of ancient mud
volcanoes or really ancient intrusive mud chambers. In the second hypothesis, we can
suppose that the initially intrusive material was liquefied, and that progressively the mud has
solidified while the water phase was progressively expelled at the surface, or into the
surrounding strata. Seismic anomalies (low velocities) have been evidenced previously below
mud volcanoes in different parts of the world where they been interpreted as kilometric sub-
circular deep chambers containing gas charged liquefied sediments (Cooper, 2001; Duerto
and McClay, 2002; Stewart and Davies, 2006).
Stack of Mud Volcano Edifices
Stacks of mud volcanoes edifice occur when mud volcanism develop in a high
sedimentation rate context, as observed in eastern offshore Trinidad associated to the vicinity
of the sedimentary input of the Orinoco delta. In that case, subsurface structure of the mud
volcanoes shows a “Christmas tree” geometry related to the stacking of successive edifices
(figure 17). We mentioned earlier that this is probably due to the fact that these volcanoes
developed on flat surface mostly during phases of low sedimentation (high stands), and they
were progressively covered (onlaps) during phases of high sedimentation (low stands) by
turbidites or mass-flows (figure 17). The general structure in this case is controlled by the
cyclicity of turbidite sedimentation rate but not necessarily by changes in the mud expulsion
rate.
Éric Deville 24
Collapse
In many cases, zones of sagged strata occur directly above the area of depleted mud
source. Locally, the collapse is commonly associated with the development of circular fault
systems which are observed around the larger mud volcanoes. An example of such caldera
can be seen around the mud volcano of Chadragrup in southern Pakistan (figure 15). These
circular fault systems form downward tapering cones. Very nice 3D seismic illustrations of
these types of the ring faults have been evidenced in the south Caspian area (Stewart and
Davies, 2006). Several processes can generate these collapse structures. The progressive
withdrawal of fragments from the borders of mud conduits and the transfer of this solid
material by the mud toward the surface is prone to generate a collapse rooted in the depletion
zone. Also the sudden discharge of deep mud plugs is susceptible to generate a collapse
rooted in the deep mud accumulation. But also the release of pore pressure in overpressured
and undercompacted sediments is susceptible to produce localized compaction at depth close
to the conduits of the mud volcanoes. These phenomena can contribute together to initiate a
collapse of the overlying strata.
CONCLUSION
Mud volcanism is generally not associated to magmatic volcanism, and it is mainly
associated to the dynamics of expulsion of the fluids in thick and high deposition rate
sedimentary basins. In that case, the dominant gas is CH4. In some particular cases, some mud
volcanism processes are associated to the expulsion of fluids in magmatic volcanism
provinces. In that case the dominant gas is CO2. The model of a unique source of mud
volcanism in a particular formation of the mud is not longer supported, and clearly there are
different origins for the fluids and the solid fraction. Mud volcanism origin is associated to a
displacement of dewatering fluids related to the compaction of the sediments. The reaction
chain of the sediment remobilization includes different processes as hydrofracturing, possibly
collapse of sandstone reservoir, focusing fluid escapes, sediment withdrawal from the fluid
conduits, mud intrusions, in some cases collapses in the normal pressure zone, and stack of
mud volcanoes when interfering with high recent sedimentation rates. These phenomena are
controlled by the development of strong overpressure documented at depth, which is
necessary for mud extrusion. Overpressure generation is notably favored by the conjunction
of fast sedimentation rates of relatively impermeable sediments and high deformation rates
leading to compaction disequilibrium. Also, the increase in temperature, at depth, induces the
cracking of hydrocarbons, which is a classical additional factor for overpressure generation
notably during gas cracking. The regime of the expulsion of the fluids varies according to
cyclic phases. Low frequency cycles could be related to the fact that when high excess pore
pressure occurs at depth, hydraulic fracturing is responsible for the opening of fracture
network favoring successive fluid release and cyclic pressure decrease. Such processes could
be enhanced by a threshold effect when fluids are over-saturated in gas. In that case massive
degassing of large volume of dissolved gas is possible at depth that could suddenly rose the
fluid pressure and damage the sealing properties of the overlying sediments.
Mud Volcano Systems 25
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... Although mud volcano (MV) surface morphologies have been described in detail and worldwide, both onshore and offshore, little is known about their evolution and formation processes (e.g. Kopf, 2002;Deville, 2009;Mazzini & Etiope, 2017). On the other hand, earlier extensive work on fluid emissions at the surface has improved understanding of subsurface processes and the multiple sourcing of the different elements forming the material ejected by mud volcanoes (Deville, 2009;Kopf, 2002;Mazzini & Etiope, 2017). ...
... Kopf, 2002;Deville, 2009;Mazzini & Etiope, 2017). On the other hand, earlier extensive work on fluid emissions at the surface has improved understanding of subsurface processes and the multiple sourcing of the different elements forming the material ejected by mud volcanoes (Deville, 2009;Kopf, 2002;Mazzini & Etiope, 2017). ...
... These structures are mainly located along the crests of anticlines where oil and gas have accumulated in reservoirs (E. Deville, 2009;Javanshir et al., 2015;A. Mazzini & Etiope, 2017). ...
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... D, modèle 3D montrant la connexion entre la zone source de boue en profondeur et la surface, les terminaisons en biseaux de l'extrusion et l'effondrement du système de transfert, d'après Stewart & Davies (2006). Deville (2009) est le premier à faire une distinction claire entre source de fluide et source de boue (Figure 1-18). Il examine des systèmes volcaniques en domaine onshore et en domaine sous-marin. ...
... Cette zone devient la zone de formation de la boue, ce qui change des « réservoirs de boue » précédemment définis. Cette zone source de boue se trouve quelque part entre la source de fluides en profondeur et le volcan, édifice de surface (Deville, 2009). Cette zone de formation de boue correspond à une « chambre » où les fluides, qui migrent depuis une plus grande profondeur, entrent en interaction avec le sédiment afin de former de la boue qui sera ensuite extrudée en surface. ...
... Les volcans de boue sont des édifices montrant des phases actives provoquant des éruptions de gaz et de matériel argileux, ces phases actives sont courtes et espacées dans le temps (Deville, 2009). Les phases calmes sont également marquées par des sorties de matériel (boue, gaz et huile) mais impliquant des volumes plus faibles, permettant la mise en place des « manifestations de surface », soit des édifices à plus petites échelles (conversation personnelle avec A. Aliyev, Geological Institut of Azerbaijan). ...
... Requirements for mud volcanism are (1) subaqueous deposition of thick sequences of fine-grained sediments, (2) development of overpressure (Kopf 2002, Deville et al. 2003, Van Rensbergen et al. 2003, Deville 2009), and (3) subsequent triggering events to initiate sediment mobilization. These requirements are likely to have been met in southern Acidalia. ...
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The morphology of mud volcanoes (MV) has been extensively studied over the last few decades. Although recent research has begun to focus on deep processes and structures, yet little known about mud generation mechanisms. This study aims to investigate the feeding system and the formation of an active kilometer-scale MV by relying on a 3-D seismic survey and in-situ dataset on the Absheron anticline (South Caspian Basin). Seismic data show a depleted area in the Anhydritic Surakhany Formation (ASF), whose mineralogical composition fits with surface mud. Well data show that the ASF is a succession of evaporitic beds and low temperature shales near its fracture pressure. Biostratigraphic analysis confirms a Pliocene origin for the mud, suggesting that the ASF may be the source. Numerical modeling of sedimentation coupled with laboratory test results and well sonic logs fairly reproduces the observed in-situ overpressure trend. Two-dimensional methane diffusion coupled with overpressure caused by rapid sedimentation highlights the superposition of critical fracturing conditions with methane-saturated sediments at the base of the studied MV. The present study demonstrates the predominant role of fluid overpressure due to sedimentation and gas saturation in the formation of the Absheron MV, and this is shown to occur as follows: (1) methane migration through the thrust-related faults reaching the ASF, accompanied by (2) a lateral overpressure caused by rapid sedimentation flowing through the ASF leading to (3) hydro-fracturing of overpressured and methane-saturated sediments resulting in an important decrease in overpressure, causing (4) gas exsolution and expansion triggering sediment remobilization.
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A 3D seismic, AUV and geotechnical data integration has been conducted in order to assess the potential impact of a large Mud Volcano (MV) activity and its related mudflows for subsea field development. The studied area is located within the South Caspian Basin (SCB). The tectonic compression of the SCB led to the development of horizontal stresses within the sedimentary sequence and to the formation of anticlinal folds throughout the basin, pierced by large mud volcanoes affecting up to the seabed. The potential future material expulsion and associated seabed mudflows may represent a significant threat for any subsea installation (flowlines, wellhead, templates…) of E&P activities. The 3D seismic dataset has therefore been used to identify and describe the MV complex interaction with large regional Mass Transport Complexes (MTC). AUV data were essential to provide high resolution informations regarding the geometry and succession of the different mud flows through time, associated with the MV activity. A large pseudo-circular topographic high forming the crater of the MV at seabed is located into a wider low circular relief. In more details, within the crater, smaller steep side cones associated to seabed high seismic amplitudes are identified as gryphons (secondary small vents), from where recent seabed mud flows have been expulsed. The surrounding depression around the MV has canalized the recently expulsed mudflows southward thanks to lower relief and the present bathymetry. Five recent seabed mudflows have been clearly discriminated, indicating that the MV has been active during the last 5000 years B. P. As a consequence and since the latest catastrophic event (dated 735 years BP) exceeds the estimated mud expulsion recurrence time (500 years), the mud flow zone is considered as a hazardous area for the time being leading to a "No Go" classification for any drilling activity and subsea installation. This study demonstrates the added value of HR geophysical (AUV) data and geotechnical ground truthing integration with the regional shallow geological understanding derived from 3D seismic data to assess geohazards in a complex and active shallow geological context.
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Mud volcanoes represent the largest expression of natural methane release into the atmosphere; however, the gas flux has never been investigated in detail. Methane output from vents and diffuse soil degassing is herewith reported for the first time. Measurements were carried out at 5 mud volcano fields around Sicily (Italy). Each mud volcano is characterized by tens of vents and bubbling pools. In the quiescent phase, methane emission from single vents ranges between 0.01 and 6.8 kg/day. Diffuse soil leakage around the vents is in the order of 102-104 mg m-2 d-1. An exceptional flux of 106 mg m-2 d-1 was recorded close to an everlasting fire. Soil CH4 flux is positive even at large distances from the mud volcano fields suggesting a diffuse microseepage over wider areas. A total of at least 400 tons CH4 per year can be estimated over the area investigated alone (∼1.5 km2).
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New regional interpretation of approximately 2000 km of seismic profiles constrained with more than 20 wells evenly located in the Maturín Foreland Basin in Eastern Venezuela show an extremely thick foreland sediment accumulation, varying from 7 km in the west to 10 km in the east. The interpretation also demonstrates that the total shortening in the seismically imaged portion of the Monagas Foothills and Foreland Thrust Belt decreases from the west (50 km) to the east (35 km), showing no direct relationship between shortening and sediment accumulation. Depth-converted isopach maps show large thicknesses of middle Miocene, Pliocene, and Pleistocene sediments, which is indicative of different episodes of tectonically controlled subsidence. Maximum tectonic-subsidence rates, calculated from decompacted isopach maps, are higher during the Pleistocene (2875 m/Ma) than during the middle Miocene (1260 m/Ma) and Pliocene (1243 m/Ma). Three large depocenters were identified from west (thinnest) to east (thickest), which migrated from northwest (adjacent to the Serranía Thrust Belt) in the middle Miocene to southeast in the present. The thickest Pliocene and Pleistocene depocenters, located in the eastern part of the basin, are not related to thrust-sheet loading, as evidenced by the lack of major active thrust in this area during this time. This shows that the continental lithosphere has subsided by a greater magnitude in the eastern part of the basin than in adjacent areas. Subduction loading associated with the subduction of the South American Continental Plate under the Caribbean could have generated additional subsidence in the Maturín Basin. This is supported by gravity anomaly evidence. Free-air gravity anomalies for the southeastern Caribbean offshore and Bouguer anomalies for Eastern Venezuela show a continuous negative-gravity anomaly extending from the Barbados Accretionary Prism to Eastern Venezuela, suggesting that the Lesser Antilles Subduction Zone may extend southwestward and affect the Maturín Basin.
Article
The purpose of present paper is the description of particular geological structures conventionally named mud volcanoes or maccalube, occurring on land frequence with particular in Italy. Mud volcanic structures are located along many compressive margins and are characterized by eruptive activity involving deep seated fluids constituted by a mixture of water, gases and clays. The intense bubbling seen in the brackish waters which fill the cones of mud volcanoes has attracted interest during oil prospecting. Methane is the most abundant of the expelled gases. The extrusion of mud mixed with brackish water characterizes the liquid phase. This is strictly linked to the local geodynamic environment. Tectonic stresses strongly influence the fluid emissions from mud volcanoes.
Article
This paper focuses on tectonic, sedimentary, and hydrogeologic processes presently occurring at the eastern leading edge of the Barbados accretionary prism. Thrusts and folds develop over a décollement hosted at the deformation front in weak and probably undercompacted sedimentary layers of Miocene age. The volume of the anticlinal potential traps is changing from north to south and is related to the thickness of accreted sediments, i.e., of the Neogene-Quaternary section. This thickness is increasing drastically from north to south as it approaches the South American continent and the Orinoco deep-sea fan. The middle and distal parts of this fan are now being incorporated into the prism, supplying the piggyback basins developing between the growing folds and the immediate foreland in front of the prism with deep-water clastic sediments. These potential reservoirs in the Neogene-Quaternary section are possible targets for future exploration, subject to their geophysical identification, in such areas where the seismic resolution may be altered seriously by the complexity of structures. The distribution of potential source rocks is poorly known, but marine middle-Late Cretaceous source rocks are widespread on the northern South American continent, and also may have been deposited in the near-deep marine environment. Efficient paths for the migration of hydrocarbons originating and expelled from such source rocks are provided by deep-water clastic layers, active faults, and widespread mud volcanoes. The success of efficient entrapment of hydrocarbons could, in some cases, be hampered seriously by the efficiency of these vertical conduits connecting deep stratigraphic layers and the sea bottom.
Article
This paper describes a seabed feature interpreted as a mud volcano which occurs on the southeast Baffin Island Shelf in about 230 m of water. The morphology of the mud volcano is described from four independent sets of remotely sensed geophysical data and its location above a major geological contact is shown. The mechanism of formulation may be by the extrusion of a pressurized mixture of unconsolidated sediments, oil, gas and conate water escaping to the seafloor by upward movement along a contact between Tertiary and Precambrian rocks. This may occur during periods of earthquake activity. The age of the volcano is not known but evidence suggests it may be a recent feature.