David Shilston C. Geol., FGS, FRSA
Past President of The Geological Society of London
April 2022
1. Introduction
David Shilston has worked as a geologist in many places in the world. One particular place that piqued his geologist’s interest was Sidoarjo in East Java, Indonesia, where a mud volcano made an unexpected appearance. David’s involvement with LUSI was to discretely monitor the volcano’s activity for clients considering investing in the area.
Life in the quiet backwater of Sidoarjo was disrupted on 29th May, 2006 by the sudden eruption of a mud volcano. What seemed an unusual and bizarre occurrence soon turned into a nightmare for the community with 40,000 residents losing their homes, 25 factories destroyed, an expressway rendered unusable, and 13 killed due to the rupture of a gas pipeline. The consequence of the eruption were the abandonment of 3.6 square kilometres of economically productive land surface and the continuing need to protect against the threat of severe contamination of adjacent rivers. Eighteen years after the initial eruption, mud still exudes to the surface and is up to 10m thick (see Fig. 1). High initial eruption rate gradually slowed, following a decreasing, exponential curve (see Fig. 2).
Fig. 1 LUSI, a 'mud volcano' eruption that began in 2006 flooded houses in Sidoarjo, in Indonesia's East Java province, seen here in April 2007. A mud geyser erupts (in the background) in the middle of a 100-metre crater.
Fig. 2 Discharge measured at LUSI from December 2006-2011. Vertical bars- range of reported values. Horizontal bars- approximate time period in which each discharge measurement was made. Red solid line- best fit to the discharge measurements. Dashed lines- 95% confidence interval.
The eruption became known LUSI, short for ‘Lumpur Sidoarjo’, ‘lumpur’ being the Indonesian word for mud. Whilst teams coped with the practical realities of the continued spillage, the question for geologists was, ‘what caused this eruption?’. There were two possible competing hypotheses, namely, inadequate drilling precautions whilst prospecting for gas, or disruption of surface rocks by a distant, moderate earthquake during early drilling. Either option involved a thorough consideration of the geology of the area and scientific approach to reviewing the evidence.
2. Mud Volcanoes
Fig 3: Akpatlawuk mud volcano, western Turkmenistan
Whilst we are all familiar with rock-based volcanoes (e.g. Mt Etna, Mount St Helens), mud volcanoes are a rarer beast but by no means uncommon (see Fig. 3). There are thought to be ~700 mud volcanoes globally of which 350 are active (see Fig. 4). They come in different sizes and exhibit, by virtue of size, different behaviours. The biggest is Baku in Azerbaijan. Larger ones can emit methane, which may ignite producing flames. Rather than occurring randomly, they appear where key factors are found, such as deltas or continental shelves or subduction zones. In effect, places where mud can accumulate, and the stratigraphic layers are prone to disruption. In fact, LUSI occurred in a very active subduction, otherwise known as the Pacific Ring of Fire, where the 2004 massive Indonesian earthquake and tsunami occurred.
Fig. 4: World geographical distribution of mud volcanoes
The other key feature of LUSI, besides being a volcano, is mud, a fine grade material which becomes soft and gooey if wet, and very hard and rigid, if dry.
3. A local diversion
Mud volcanoes tend to exist on a continuum, from the very large to the very small. An example of the latter can be found, intriguingly, at Templars Firs, Royal Wootton Bassett, in Berkshire. Other such springs occur in the neighbourhood. There is much speculation about the origin and local and national significance of Templar Firs. There are 5 vents, each several metres in across and around 1 metre high, mud blisters essentially, with a liquid mud core and a skin of vegetation. Mud flow is several litres per day. Mud departs the site into a local stream and much collectible fossils are washed out of the mud. Examination of the fossils suggest an origin within the Ampthill Clay.
At most locations, including Templar Firs, springs occur in valley bottoms cut into Ampthill Clay along synclinal (downward folding strata) axes (see Fig.5). The springs overlie a chamber of mud, 20m deep. It is the hydrostatic head of water in the Corallian limestone aquifer, beneath the Ampthill Clay that drives water outflow to the surface.
Fig. 5: Cross section showing mud springs in the Wootton Bassett area. Water is contained within the Corral Rag limestone. At its deepest, water is under high pressure. Fractures in the rock encourage its escape into the overlying mud layer- see Fig. 6.
Fig. 6: Schematic cross-section of the mud springs at Templars Firs showing the movement of groundwater and mud. Groundwater in the Coral Rag aquifer travels from the outcrop area around Wootton Bassett and moves up through the Ampthill Clay carrying the mud in colloidal suspension to the surface.
Templar Firs provides a simplistic and possibly useful analogue for LUSI and other mud volcanos.
4. Determining the cause of LUSI
Prior to the eruption, drilling had commenced locally to determine if economically extractable gas was available for exploitation.
Fig. 7: 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 post-2006 evolutionary phase.
A possible, if not probable, development of LUSI is highlighted in Fig.7.
A) The Banjar-Panji-1 well was targeting gas at 2,834m and drilling directed towards the Kujung formation through the overpressured Kalibeng muds and interbedded sands and muds. A minor amount of steam and water was noted nearby. Later eruptions of mud and water started 800-1000m away then ceased. Later, a distant, 6.3 magnitude earthquake occurred, the epicentre being 280km away.
B) Kujung carbonates were breached. This led to a sudden influx of fluid into the bore. This kick caused hydrofracturing of overlying strata. Notably, there was no casing set between the base of the bore and 2000m of overburden, including Upper Kalibeng muds. Drilling, it is thought, of overpressured Kujung limestone (a regional aquifer) caused an influx of pore fluid into the well bore, this providing the initial connection between the limestone and mud layers uncontained by the absence of drill casing. Kalibeng muds were mobilised.
Hydrofractures are a recognised risk in drilling and can penetrate several kilometres of crust. Resulting subsurface blowouts are not uncommon.
C) Entrainment of overpressured muds caused subterranean conduits to form, the walls of which underwent periodic collapse. The pressurised muds sought the surface through fractured overlying strata. Collapse of the Upper Kalibeng strata contributed to the mixing process. Erosion of fracture walls caused a major conduit(s) to grow both upwards and outwards before collapsing inwards providing a mixing mechanism.
D) In the post-eruptional period, a caldera formed around the vent with some sag-like subsidence of the region. Smaller mud cones occasionally erupted as new conduits were formed as older ones closed off due to overlying subsidence.
Although David’s experience suggested that drilling was the cause of LUSI, an alternative, later, hypothesis suggested the earthquake as the causative event. For most scientists, a magnitude 6.3 earthquake, 250km away, 47 hours before the onset of the mud volcano eruption, would not be sufficient to cause the disruption seen. Seismic waves emanating from the epicentre are not powerful enough to trigger such a response. Later investigation of the site identified a hard, dense rock, shaped like a parabolic reflector sitting on top of the mud reservoir. Computer modelling suggested that both the nature of the rock and its geometry could have acted as a reflector for seismic waves, with seismic energy being sufficiently focussed to liquefy the mud source, triggering an eruption. Liquified mud was then injected into an adjacent fault, causing it to slip and linking it to a hydrothermal volcanic system deeper down. That previously trapped system underwent alteration and became connected (fatally) to the free surface. Evidence suggested that pressure had been building for 30 years prior to 2006 and an eruption of some sort was inevitable. The earthquake was, in effect, the final trigger
Whilst controversy makes for interesting and intellectual discussion, a definitive cause has prevented a clear-cut resolution of liability and full recompense for affected locals. This is the view taken by the Humanitus Sidoarjo Fund, an Australian charity that works for the Indonesian government in scientific and social issues. Although there is a rejection of responsibility by the drilling company, they have contributed US$570 million in disaster aid to the affected communities.