On 29 May 2006, an eruption of steam,
water, and, subsequently, mud occurred in eastern Java in a location where none
had been previously documented.
This “pioneer” mud eruption (the first to
occur at this site) appears to have been triggered by drilling of over-pressured
porous and permeable limestones at depths of ~2830 m below the surface.
We propose that the borehole provided a
pressure connection between the aquifers in the limestones and over-pressured
mud in overlying units.
As this was not protected by steel casing,
the pressure induced hydraulic fracturing, and fractures propagated to the
surface, where pore fluid and some entrained sediment started to erupt.
Flow rates remain high (7000–150,000 m3 per
day) after 173 days of continuous eruption (at the time of this writing),
indicating that the aquifer volume is probably significant.
A continued jet of fluid, driven by this
aquifer pressure, has caused erosion and entrainment of the over-pressured mud.
As a result, we predict a caldera will form around the main vent with gentle
sag-like subsidence of the region covered by the mud flow and surrounding areas.
The eruption demonstrates that mud
volcanoes can be initiated by fracture propagation through significant
thicknesses of overburden and shows that the mud and fluid need not have
previously coexisted, but can be “mixed” within unlithified sedimentary strata.
Understanding how Earth recycles elements,
compounds, minerals, or even sediment is a major scientific quest, which
transcends several disciplines, including chemistry, biology, and earth science.
In sedimentary geologic systems, the cycle
time can be particularly significant. For instance, the burial of sediment (and
pore fluid) to depths in excess of 5 km, and their remobilization and transport
back to Earth’s surface, can take millions to tens of millions of years (e.g.,
Kopf et al., 2003).
One prerequisite for this long-term
recycling process is the development of elevated pore fluid pressure
The excess fluid provides the required
energy for the breach of seals and for the transport of a fluid-sediment mix
back to the surface, where it is redeposited as sediment (e.g., Stewart and
Davies, 2006; Deville et al., 2006).
Mud volcano systems are one of the many
expressions of this process, and many have been documented globally (Kopf, 2002;
Significant eruptive edifices can develop,
which are often grossly similar in form to their more intensively studied
igneous counterparts (Stewart and Davies, 2006), although substantially smaller.
However, many of the fundamental processes
involved in the recycling of buried fluid and sediment through mud volcano
systems are poorly understood, and studies are still in their infancy.
Elementary questions remain; for instance:
(a) Do the fluid and mud come from the same
beds, or is the fluid transported from deeper levels into mud source beds where
mud is entrained?
(b) How is the plumbing system that feeds
mud and fluid to the surface initiated and sustained? and
(c) What is the three-dimensional
architecture of the feeder systems and how do they evolve through time?
On 29 May 2006, a mud eruption was observed
in the Porong subdistrict of Sidoarjo in eastern Java (Fig. 1).
At the time of this writing, the erupted
mud pool (a) has a volume of ~0.012 km3, (b) covers an area of ~3.6 km2 and is
up to ~10 m thick, (c) has buried 4 villages and 25 factories, and (d) displaced
There have been 13 fatalities as a result
of the rupture of a natural gas pipeline that lay underneath one of the holding
dams built to retain the mud.
The eruption has unofficially been named
“Lusi” (Lumpur “mud” Sidoarjo), and this name is adopted here. It occurred
during the drilling of a nearby exploration borehole (Banjar Panji-1);
therefore, in this case several factors (e.g., pressure, depth, stratigraphy)
that are normally not constrained in natural mud volcano systems are calibrated.
Although we propose that Lusi is man-made,
it does offer a unique opportunity to address the mechanisms of initiation and
maintenance of a mud volcano.
The aims of this paper are to consider why
the eruption occurred, compare it to other natural examples, and evaluate what
we can learn about how mud volcano systems work.
MUD VOLCANO SYSTEMS
Mud volcanoes are common on Earth (Milkov,
2000), but particularly so in compressional tectonic belts (e.g., Azerbaijan:
Planke et al., 2003; Indonesia: Ware and Ichram, 1997), within deltas (e.g.,
Mississippi: Neurauter and Bryant, 1990), and submarine slopes undergoing
gravitationally driven detachment (e.g., Niger delta: Graue, 2000).
The volcanoes can be long lived features,
composed of a series of mud “cones,” which indicate a pulsed eruptive history
(Evans et al., 2007) that can occur over 104–106 yr time spans.
The term “mud volcano system” was coined by
Stewart and Davies (2006) to describe the set of structures associated with a
constructional edifice (mud volcano) and feeder complex that connects the
volcano to its source stratigraphic unit (Fig. 2A).
The system is driven by pressure and a
source of fluid, which may or may not coexist with mud source beds (see Deville
et al., 2003). Above the fluid source is a feeder conduit (Fig. 2B), the
detailed structure of which is largely unknown.
It probably consists of a complex system of
fractures and mud-filled dykes (Fig. 2C) that feed a fluid-sediment mix to
Earth’s surface (e.g., Morley, 2003). The fluid-sediment mix then erupts to form
the “mud volcano”—a term we only use to describe the edifice (Fig. 2D).
The plumbing of mud volcano systems is
For instance, the mud and fluid could
coexist at the time of initiation, analogous to magma (e.g., Davies and Stewart,
2005), or the fluid could be transported from a deeper source, remobilizing mud
at shallower stratigraphic levels (Deville et al., 2003; Kopf et al., 2003; You
et al., 2004).
Some mud volcano systems are thought to
comprise multiple mud chambers at different stratigraphic levels (Deville et
al., 2003; Planke et al., 2003) whereas other models propose that mud volcano
systems comprise significant masses of mud, in the form of bulbous-shaped
diapirs (Brown, 1990; Milkov, 2000).
A “pioneer mud volcano” (e.g., Fig. 2A) is a term used by Davies and Stewart
(2005) to describe the first mud volcano that erupts in a location where no mud
volcano system previously existed.
They envisage that if a substantial mud
volcano develops, a positive feedback loop can become established where
subsidence of the overburden due to loading, conduit wall-rock erosion, and
volume loss at depth causes new fractures and faults to form in the overburden
These structural apertures provide new
pathways for a fluid-mud mix.
Figure 2. Components of a mud volcano
system revealed by three-dimensional seismic data and outcrop.
(A) Schematic illustration of the main components of a mud volcano system. Mud
volcano systems can be divided into intrusive and extrusive structural domains.
Fluid may either coexist with the mud source or enter from a deeper source (blue
arrows) causing remobilization of shallower mud and entrainment of other
The mud-fluid mix is transported through fractures and faults to the surface,
where stacked cones form due to episodic eruptive and quiescent periods.
(B) Seismic coherency cube (see Bahorich and Farmer, 1995) across the Gunashli
mud volcano (South Caspian Sea, from Davies and Stewart, 2005), showing feeder
conduits, the detailed internal structure of which is unknown.
(C) Mud-filled dykes from the Jerudong anticline in Brunei (see Morley, 2003).
These types of mud-filled fractures are potentially what allow for the transport
of the mud-fluid mix to the surface.
(D) Photograph of mud volcano terrain from Azerbaijan comprising several
gryphons from which small mud flows emanate.
The East Java basin is an inverted
extensional basin (Matthews and Bransden, 1995). It comprises a series of
east-west–striking half-graben that were active in extension during the
Paleogene and reactivated in compression during the early Miocene to Recent.
The Oligo-Miocene to Recent basin was
filled with shallow marine carbonates and marine muds, some of which are known
to be “over-pressured” (see Osborne and Swarbrick, 1997).
As a result of the compressional inversion,
these strata are gently folded with normal and reverse faults cutting the
inversion anticline crests (see Matthews and Bransden, 1995).
A small section of one of these
east-west–trending anticlines was targeted by the Banjar Panji-1 exploration
Mud volcanoes have been documented before
in East Java.
For example, they are found within the
crest of the Sangiran Dome (part of one of the east-west–trending Neogene folds:
Watanabe and Kadar, 1985) and near Purwodadi, which is 200 km west of Lusi (Fig.
over-pressured lower Miocene clays probably
equivalent to the Tuban or Tawun Formations (similar age to the Kujung
limestone—see Matthews and Bransden, 1995) and the Upper Kalibeng Formation are
considered to be the source of the mud (Watanabe and Kadar, 1985).
Volumes, Rates, and Dimensions
The typical eruption volume, duration,
rate, spatial extent, and aspect ratio of selected naturally occurring mud
volcanoes can be compared to Lusi (Tables 1 and 2, respectively).
These comparisons show that the Lusi
eruption has a significant volume, duration, and spatial extent.
The average eruption rate is not
particularly high. Lusi has an anomalously high aspect ratio (Table 2). It is
also worth noting that long-lived mud volcanoes that consist of several cones
that develop as a result of multiple eruptive and non-eruptive developmental
stages (Evans et al., 2007) are known to have volumes of up to ~22.5
km3—dwarfing the current but still highly active Lusi edifice (Stewart and
Key Events and Subsurface Data
Banjar Panji-1 was an exploration well that
was targeting gas within Oligo-Miocene age Kujung Formation carbonates within
the East Java Basin.
The well reached a depth of 2834 m, after
which an eruption of steam, water, and a minor amount of gas was observed at
5:00 a.m. on 29 May 2006, 200 m southwest of the well.
On the second and third of June 2006, two
further eruptions started 800–1000 m to the northeast of the well, but both of
these stopped on 5 June 2006 (United Nations Final Technical Report, 2006).
It is reported by local villagers that the
water-mud mix at the surface had a temperature of 70–100 °C; a continuous plume
of steam seen on early to recent photographs of the eruption supports such high
An earthquake of magnitude 6.3 occurred at
5:54 a.m. local time on 27 May 2006, with an epicenter 280 km west-southwest of
the Lusi eruption, near Yogyakarta (U.S. Geological Survey, 2006).
The eruption of a dilute mud-water mix has
persisted from the site of the initial eruption, and mud now covers an area of
~3.6 km2 (Fig. 3).
Satellite images of the Lusi eruption taken ~100 days after the eruption
started. (A) Entire area of eruption. (B) Close-up of the main vent
(marked by clouds of steam [white]), which appeared 200 m southwest of
the exploration well. Both images taken September 2006, courtesy
National University of Singapore Centre for Remote Imaging, Sensing and
Schematic three-dimensional representations of the Lusi mud volcano
showing four main developmental stages. The first three diagrams depict
the evolution between May 2006 and Dec. 2006 (A–C), and the fourth
diagram (D) shows the predicted next phase of evolution.
(A) March to May 2006: Banjar Panji-1 well drills toward Kujung
Formation, through over-pressured mud (Kalibeng Formation) and
interbedded sands and muds.
(B) May 2006: Kujung Formation carbonates are penetrated, which leads to
a “kick” (influx of fluid into the well bore). The kick causes
hydrofracturing of overlying strata (probably initiated within the
Kalibeng Formation). Drilling mud and pore-fluid enter the well bore,
driven by the excess pressure upward, through porous and permeable
strata and the fracture system. Entrainment of over-pressured Kalibeng
Formation muds occurred.
(C) May to December 2006: entrainment of Kalibeng Formation muds causes
a subterranean conduit to form, the walls of which undergo period
(D) Post-2006: caldera forms around the vent, and gentle sag-like
subsidence of the region where the flow extends. Smaller mud cones may
be erupted as a result of conduit establishment due to foundering of the
Unreleased geologic data (lithological log,
biostratigraphic determinations, gamma ray, sonic, density logs) indicate that
the well drilled the following (shallowest first):
(a) the Pleistocene age Pucangan and Kabuh
(b) then ~1000 m of over-pressured muds
with some sand interbeds (the Upper Kalibeng Formation [Pleistocene age]),
(c) ~1300 m of interbedded sands and muds,
(d) the well penetrated a limestone
(presumed to be the Kujung Formation), which was also over-pressured. There was
no casing set between the bottom of the hole (the Kujung Formation) and ~1743 m
of the overburden, including part of the 1000 m of over-pressured Upper Kalibeng
Formation mud and the entire 1300 m of interbedded muds and sands (Fig. 4A).
We know that (a) in the Banjar Panji-1, the
pore pressures at 2130 m (700 m above the Kujung limestone) are 38 MPa (5500
psi); and (b) that in a well 5 km away called Porong-1, the pressure within the
Kujung limestone aquifer was 48 MPa (6970 psi) at 2597 m.
Given the pore pressure of 38 MPa (5500
psi) at 2130 m in the Banjar Panji-1 well, we calculate an overpressure of 16
MPa (2300 psi) at this depth.
In the Porong-1 well, we use the pressure
of 48 MPa (6970 psi) at 2597 m to calculate an overpressure of 21 MPa within the
On the assumption that the Kujung limestone
is a regional aquifer (which seems likely given the high continuous flow rates
at Lusi), we predict the overpressure was ~21 MPa at the base of the Banjar
Panji-1 at 2830 m.
We propose that the drilling of the
over-pressured Kujung limestone caused an influx of pore fluid into the well
bore (known as a “kick”).
The well bore itself provided the pressure
connection from the limestone to any shallower aquifers as well as the
over-pressured muds of the Upper Kalibeng Formation.
The eruption started with steam and water,
and this did not come to the surface through the well bore, but instead took
place 200–1000 m away (Fig. 4B).
Therefore, the transport route for the
steam and mud was not through the wellbore but through the surrounding
High pore-pressure causes natural hydraulic
fracturing of the sedimentary overburden (see Engelder, 1993) when pore
pressures exceed the fracture strength.
These conditions for the creation of
hydraulic fractures are most likely to form in the shallowest strata not
protected by steel casing.
We propose that the fractures probably
formed within the Upper Kalibeng Formation and propagated from 1–2 km depth to
the surface over a period of hours.
The depth is backed by the temperature of
the erupted mud-water mix, which is 70–100 °C, indicative of rapid transport
from 1.5 to 3 km depth, assuming a geothermal gradient of 25 °C/km and a surface
temperature of 28 °C.
Such drilling-induced fracture and fluid
flow processes, where the well bore provides the necessary initial pressure
communication, has been witnessed elsewhere; for example, in subsurface blowouts
that occurred in Brunei in 1974 and 1979 (see Tingay et al., 2005).
At Lusi, the influx of pore water into the
well bore may have initially come from the Kujung limestones, but once the heavy
drilling mud had been displaced into the new fractures, fluid would have also
started to flow from porous and permeable formations in the overburden.
The passage of fluid into over pressured
(and therefore undercompacted) mud would lead to entrainment of the unlithified
sediment (Fig. 4C), which would also contribute its pore water to the mix.
Mud is cohesive, and in a similar way to
the entrainment of mud in sedimentary settings, the shear stress imposed by the
adjacent moving water has to overcome the sediment’s cohesive yield strength
(e.g., Dade et al., 1992; Kranenburg and Winterwerp, 1997) for it to be
Such an entrainment process has been
proposed for mud volcanoes in the UK, for instance, where water from an
underlying aquifer passes through mud-rich overburden, causing the formation of
a subterranean cavern system (Bristow et al., 2000).
The same general process has also been
proposed by Deville et al. (2003) for mud volcanoes in Trinidad.
We envisage that collapse of the Upper
Kalibeng strata will contribute to the mixing process.
It is also conceivable that the hot water
in large caverns will allow convection cells to develop, which will contribute
to the mixing process (e.g., Deville et al., 2003). The resultant dilute
water-mud mix is moving up fractures to the surface as a fluidized sediment flow
with the mud in suspension.
The mix started to erupt at the surface,
driven by the pressure of the pore fluids in the Kujung limestones.
Erosion of the walls of the fractures is
also likely (it occurs in other mud volcanoes) and therefore a major conduit
would grow upward and laterally, periodically collapsing inward.
This particular mixing mechanism for mud
volcanism has probably led to the very dilute composition of the mud-water mix
and the high aspect ratio of the edifice.
If a continuous 2830 m column of an
erupting mud-water mix has a density of 1.3 gcm−3, based on an assumed water:mud
ratio of 80:20, the mud column would exert a pressure of 36 MPa (5225 psi) at
the bottom of the Banjar Panji-1 exploration hole.
This pressure is 12 MPa less than our
estimate of the pressure within the Kujung limestone (48 MPa); therefore, it is
most likely that the flow that is being witnessed is driven by this pressure
Gas exsolution and expansion (Brown, 1990)
are not considered important lift mechanisms at present.
NEXT DEVELOPMENTAL STAGES
Maintenance of flow depends upon one of two
factors. If there is a continuous pathway to the surface due to the subsurface
erosion of the conduit walls, the influx of the pore fluid and eruption will
continue until the aquifer pressure equals the pressure due to the vertical
column of erupting mud-water mix (i.e., 12 MPa).
Alternatively, if mud gains access to the
surface through fractures that remain open against the minimum stress, flow will
reduce substantially only when the fracture closure pressure is reached; this
pressure will depend on the depth at which the fracture(s) occur.
Once the pressure drive abates, the
compaction of the extruded and intruded mud can cause low levels of mud-water
eruption, potentially for years or decades to come, as noted in other mud
volcanoes such as Piparo in Trinidad and many mud volcanoes in Azerbaijan
between violent (active) eruptive phases.
If our model of entrainment of the mud
within the Upper Kalibeng Formation is correct, then unless the pore pressure
drops to allow flow to stop, the subterranean caverns will undergo collapse
We predict that the region around the vent
will form a caldera and that the area of the mud flow will undergo less
significant sag-like subsidence. This subsidence pattern is consistent with the
behavior of other mud volcanoes (Stewart and Davies, 2006).
The subsidence that caused the fracture of
a gas pipeline buried by the mud volcano and dam system indicates that collapse
may have already started.
Induced by Drilling or Earthquake?
We propose that Lusi is the direct result of connection of a high-pressure fluid
at depth with shallow sediments at a depth at which fractures can be initiated.
Once initiated, the fractures would have
propagated to the surface, driven by the deep pressure.
Drilling activity has allowed this
connection, and our preferred model is that the earthquake that occurred two
days earlier is coincidental.
The primary reasons for not considering an
earthquake to be the trigger or contributing factor are
(a) no other mud volcano eruptions were
reported in Java at the same time;
(b) the earthquake preceded the eruption by
two days; seismogenic liquefaction usually occurs during earthquake-induced
shaking of sediment (e.g., Ambraseys, 1988);
(c) there are no reports of a “kick” during
the earthquake or immediately afterward; and (d) sand, rather than mud, is more
conducive to liquefaction due to earthquake shaking because it is a
non-cohesive, granular sediment.
An earthquake could have generated new
fractures and weakened the uncased section of the well, but it would be highly
coincidental for an earthquake-induced fracture to form 200 m away from this
well and provide the entire fracture network required for an eruption on the
Initiation and Subterranean Mixing
A fundamental question in mud volcano
system studies is how they are initiated.
The model proposed by Brown (1990), Van
Rensbergen et al. (1999), Davies and Stewart (2005), and Stewart and Davies
(2006) is that hydrofractures can penetrate several kilometers of the crust and
transport a fluid-sediment mix that erupts to form a pioneer volcano.
Because in this case we know that the
mud-water mix has been transported ~2 km through the overburden, through new or
reactivated fractures, the Lusi eruption supports the models proposed by these
The Lusi eruption also strengthens the
concept that rather than the source water and the source mud coexisting in the
same stratigraphic unit (mudrocks at 2.0 km depth have strength and not the
porosity of 70%–80% required for the Lusi sediment composition), the fluid has a
deeper source, and mud is entrained from within the overburden (e.g., Bristow et
al., 2000; Deville et al., 2003; Kopf et al., 2003; You et al., 2004).
This subterranean mixing model differs from
the concept of mud and fluid coexisting (Davies and Stewart, 2005; Stewart and
Davies, 2006) and contrasts with models for the subsurface remobilization of
sands where coexistence of sand and fluid is the general assumption.
The mud is particularly susceptible to
entrainment due to overpressure, which does not allow normal compaction (Osborne
and Swarbrick, 1997). The aquifer pressure provides the pressure drive.
Subsurface blowouts are not uncommon events
(e.g., Tingay et al., 2005) and can involve sediment entrainment, but this scale
of sediment mobilization, triggered by drilling activity, has not been
A combination of factors account for this
being so rare:
(1) the penetration of an over-pressured
mud that is susceptible to erosion followed by
(2) the penetration of an aquifer that
releases large volumes of pore water and
(3) the man-made pressure linkage provided
by 1.7 km of open hole section.
It is very likely that Lusi was initiated
as a result of access by a high-pressure aquifer at depths in the region of
2.5–2.8 km through an open-hole section of the Bajar Panji-1 well to depths at
which fractures could be initiated.
Lusi indicates that mud volcanoes can be
initiated by fracture propagation from multi-kilometer depths, which triggers
fluid flow and the rapid establishment of a subterranean mixing system, into
which water is transported from deeper successions.
Prediction of the next developmental stages
is fraught with difficulty, but the unabated 173 days of very active eruption
indicate a large aquifer has been penetrated, and we can be confident that some
sort of eruptive activity (perhaps lower-level) will continue for many months
and possibly years to come.
A region several kilometers wide should
undergo sag-like subsidence over the coming months with more dramatic collapse
surrounding the main vent.
Modeling and direct measurement of the
inevitable land subsidence will help to predict what the future impact the Lusi
mud volcano has on the local population.
R. Davies is grateful to Anthony Mallon for
discussion. Mark Tingay is thanked for the photograph in Figure 2C. We are
extremely grateful to the National University of Singapore Centre for Remote
Imaging, Sensing and Processing (CRISP) for permission to use the satellite
images. We thank Achim Kopf, Peter Van Rensbergen, and an anonymous reviewer for
thoughtful and prompt reviews and Gerry Ross for guidance during the preparation
of this paper.
Richard J. Davies, Centre for
Research into Earth Energy Systems (CeREES), Department of Earth Sciences,
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Manuscript received 1 November 2006; accepted
10 December 2006.
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