Many years ago, I ran across something called the LUSI (LUmpur mud-SIdoarjo) Mud Volcano, an eruption of hot water, mud and natural gas from the vicinity of an exploratory natural gas / oil well in Java. The system started erupting in May 2006 following a blowout of the well. To date, it has covered just under 16 km2 with mud, killed 13, buried villages with as much as 30 m of mud, and generally disrupted life in its vicinity.
As with any disaster with even the hint of human involvement, finger pointing, Placing of the Blame, and lawsuits flew early and often. A lot of lawyers made a lot of money. A lot will continue to do so for years. A lot of politicians and political activists got to pontificate on the awfulness of it all. They still are.
The original placing of the blame centered on the drilling company whose well blew out. They were accused of penetrating what amounts to a cap layer on an extensive pressurized hydrothermal system underground. The corporate explanation (not unexpectedly) centered on reactivation of existing faults in the hydrothermal system by a recent earthquake and aftershocks that fractured that cap, allowing the pressurized hot water, natural gas, mud mix to escape to the surface. They pointed to the neighboring Arjuno-Welirang volcanic system as the source of heat for the underground hydrothermal system, a highly problematic claim.
A volcanic connection with what looks to be a manmade disaster? That might be interesting to write about, so I took a look.
As it would turn out, the immediate region is incredibly volcanic with a cluster of at least three other currently active stratovolcanoes including a active caldera system within 60 km (mostly S) of Arjuno-Welirang. This region includes the Kelud / Kelut volcano 38 km to the SW. It underwent a VEI 4 eruption Feb 2014. Bromo / Tengger Caldera 47 km to the SE. Semeru is 12 km south of Bromo, 56 km SE of Arjuno. Mt Kawi (Kawi-Butak) is 32 km mostly S of Arjuno and W of Kelud. Mount Penanggungan is immediately north of Arjuno. Finally, there are the heavily eroded Anjasmoro volcanic complex which includes remains of a caldera 10 km to the W of Arjuno-Welirang.
Of the volcanoes in this cluster, Kelud, Bromo and Semeru are all currently active. Arjuno-Welirang last erupted in 1952. Penanggungan last erupted perhaps 1,800 years ago and still has an active hydrothermal system. There are no known historic eruptions from Kawi-Butak or Anjasmoro.
Clearly, there is enough available magma to power any conceivable hydrothermal system. Additionally, there is also natural gas and oil present in significant quantity. Exploration for those products may or may not play well with existing volcanically powered hydrothermal systems.
LUSI Mud Volcano
Mud volcano systems are relatively common worldwide, with over 1,100 known both onshore and offshore. They are particularly common in East Java. There is a half-graben filled with pressurized carbonates and marine muds. It is part of an extensional basin filled with sediments parallel to the long axis of Java active for millions of years. Some of the pressurized material escapes from time to time creating mud volcanoes. They are typically located at the top of anticlines or along faults in the area. Two prominent active mud volcanoes are found near Surakarta in central Java and Purwodati City 200 km W of LUSI. . There is a neighboring mud volcano some 15 NE of LUSI. At least one 500-year-old temple was situated on its northern edge. It was constructed from material products of that mud volcano.
The East Java Basin has significant oil and natural gas, with at least three major fields in a 7,250 km2 area. This would seem to exacerbate the problem, as the impermeable cap on the pressurized subsurface system is penetrated from time to time by drilling. The regional strike-slip Watukosek fault crosses the LUSI area. There are a number of extinct mud volcanoes aligned with this fault from Java NE to Madura Island. There is a fault escarpment along the fault line.
The LUSI mud volcano is located around 10 km NE of Penanggungan volcano, 15 km N of Arjuni-Welirang. Shales in this area were laid down rapidly and are as such undercompacted and overpressurized. Local geologic conditions are similar to other areas with mud volcanoes. There is a collapse structure around 7 km from LUSI that forms a depression around the crater. It is likely an extinct mud volcano that as it stopped its activity, collapsed around its vertical feeder channel.
Drilling on an exploratory natural gas well started late March 2006. The well progressed through a thick clay seam, sands, shales, volcanic debris and finally into carbonate rocks. The well was cased roughly the top 40% of its total depth of 2,834 m.
A M6.4 earthquake took place on May 27 around 250 km SW of the drill site. Sources of what happened next document, a small loss of drilling mud took place at the well around seven minutes after the quake, or about the same time the seismic waves arrived. There was a second mud loss that seemed to be connected to a pair of aftershocks and a kick during drill string removal, all three events suggest a change in the subsurface condition of the region. Note that problems in drilling arise from time to time and are generally not catastrophic.
A number of mud volcanoes in the vicinity of the neighboring Watukosek fault 40 km NE of LUSI became active around the time of the earthquake. Three of these were in line with the direction of the fault. On May 29, a small amount of water, steam and erupted around 200 m from the well. Two additional eruptions took place on June 2 and 3 around 800 – 1,000 m NW of the well. These stopped a couple days later. H2S, methane gas and hot mud (around 60 C) were released from these small eruptions. May 29 is generally tabbed as the initial eruption date.
The mud eruption took place around 200 m from the well. The initial eruption ejected mud, hot water and clouds of steam. Discharge rate was less than 5,000 m3/day. By June, the crater had grown, and discharge rate increased to 50,000 m3/day. Water temperatures were as high as 97 C. Low viscosity mud continued to erupt in August. At the time, perhaps 70% of the erupted material was water, making the mud extremely low viscosity, spreading the mud widely rather than building a cone. The highest discharge rate was 156,000 m3/day in Oct. By May 2007, retaining walls were built to contain the flow, averaging 100,000 m3/day. A ring dike built around the main vent to contain the flow failed and subsided by July. Discharge rate in July was down to 40,000 m3/day. Diameter of the main crater by Jan 2010 was 120 m, flowing continuously at 30 – 50,000 m3/day. From time to time, material is ejected at 2-3 locations along the Watukosek fault. By Jan 2011, flow rate and steam clouds were significantly reduced to less than 10,000 m3/day. The eruption is transitioning to a mature and quiescent phase. It also building a dome around the main vent.
It is the contention of recent researchers that the mud volcano was triggered by natural processes connected to over pressurized shale and seismically activated faults as conduits. Initial speculation that it was an underground blowout of the well (drilling company’s fault) was shown to be premature and based on incomplete data. Rather, it was due to a seismic reactivation of the neighboring Watukosek fault (nature’s fault). A 2014 paper by Istandi, et al discussed the event in great detail and concluded that reactivation of existing faults in the area caused the eruption.
The eruptions took place along a 1,200 m NNE – SSW lineament starting around 200 m away from the exploratory well. The lineament is contiguous with the underlying Watukosek fault zone. High flow rates coincide with a series of earthquakes, leading to the theory that the fault was reactivated.
In less than a year, the mud volcano displaced some 24,000 people, inundating local roads, villages, roads, and destroying at least one natural gas pipeline. The eruption caused subsidence over 5 cm/day in the area next to the main eruption. The subsidence is expected to impact an area within 3 km of the main eruption and disrupt the subsurface structure. It is expected to be as deep as 60 m before the eruption sequence ends.
Geology of Mud Volcanoes
Geometry of mud volcanoes is variable. They can be up to a few kilometers in diameter and several hundred meters tall. The main elements of a mud volcano are the crater, periphery mud flows, irregularly shaped terrains, and mud lakes. The “classic” mud volcano is conical with a main crater, mud flow stratification, periodic eruptions, stiff mud neck protrusion (high viscosity mud), steep hills, swampy area as the lower viscosity mud spreads out, a collapsed depression, and a muddy crater lake. These elements vary based on the viscosity of the mud and the stage of development.
Typical activity has differing cycles of activity. These include catastrophic initial events like LUSI, and periods of relative quiet with moderate activity as the pressurized system is unpressurized. It appears that each mud volcano has its own life cycle that includes a period of catastrophic activity. This activity is controlled by local pressure within the subsurface sedimentary source, and the eruptive mechanism and evolution of the eruption is dependent on gas content within the fine-grained sediments sourcing the eruption.
While there are about 1,100 known mud volcanoes, some estimates that well over 10,000 more may exist on continental slopes and abyssal plains in the ocean. The largest known structures are 10 km in diameter and up to 700 m above the neighboring terrain. Mud volcanoes are often related to active petroleum systems, particularly if the natural gas is generated by deep decomposition of organic matter (thermogenic). The release of natural gas powers these systems. Mud volcanoes that release large amounts of CO2 are driven by magmatic activity which may not indicate the presence of significant hydrocarbon reservoirs. Many large onshore hydrocarbon fields were discovered after drilling around mud volcanoes in Europe, the Caspian Sea and the Caribbean.
Depending on size and level of activity, mud volcanoes may pose significant ecological hazards and create localized environmental destruction. They typically eject breccias, mud flows and/or flames in association with earthquakes. Active mud volcanoes have emitted large quantities of CO2 and methane, the latter which explodes and burns from time to time. When the viscosity of the mud is low, the eruption may flood large areas, inundating villages, homes, roads, farms, factories and residents.
In the end, the LUSI mud volcano was the most recent of at least 16 mud volcanoes in east Java and the seventh known along the Watukosek fault system. It was the first to be observed from its beginning and the seismic logs before and during drilling will greatly assist in future hazard mitigation.
The close proximity of two major volcanic systems – Penanggungan and Arjuno-Welirang – may contribute to geothermal properties of LUSI, though those connections have not yet been fully explored. A 2018 article in EOS describes a recent 3D model of the region’s subsurface that identified a hot plume of hydrothermal fluids that extend from the LUSI crater at a depth of at least 6 km. This plume is connected to Arjuno-Welirang by an elongated subsurface feature along the Watukosek fault system from the volcanoes to the back-arc basin below LUSI. The mud volcano system is fed from two different depths, 1.5 km and greater than 4 km (the well only made it to 3 km).
While there is obvious geothermal evidence of the connection between the volcanic and LUSI systems, there was no clear image connecting the two until two years ago. That changed in 2018 when analysis from the 3D modeling identified a 6 km deep hydrothermal plume reaching the surface at LUSI. The plume is connected at depth beneath Penaanggungan via a well-defined 3 km wide low velocity corridor extending through the region at 4 km. This corridor is interpreted as a magmatic intrusion and hydrothermal fluid migration. In this case, the hot volcanic fluids reached the source rock, baked the organic-rich sediments and overpressurized the system. Note that rocks at depth in Indonesian oil and natural gas are generally hot without the assistance of volcanic systems, with 135 C at 2,500 m listed in one source.
Mount Penanggungan Volcano
The closest volcanic system to LUSI is the Mount Penanggungan volcano. It is a small stratovolcano located about 40 km S of Surabaya, one of Indonesia’s most sacred mountains, and the youngest of the Arjuno-Welirang complex. The volcano hosts numerous ruins of sanctuaries, monuments and sacred bathing places dating at least 977 AD. These are mostly on the N and W flanks, which may or may not indicate an active hydrothermal system, as I have not yet come across anything indicating hot springs or pools.
Lava flows from flank vents descend on all sides. There are pyroclastic deposits that form an apron around it. It is similar in age to Arjuno-Welirang and Semeru volcanoes. The most recent eruption is thought to be 1,800 years ago. It tops out at 1,631 m, and has just over 416,000 within 10 km, 4.6 million within 30 km and nearly 26 million within 100 km. Like its neighbors, Pengnggungan is an andesitic / basaltic / trachyandesitic subduction volcano that is part of the volcanic arc created by the Java Trench. It is currently considered to be an extinct volcano.
Due to its location, gentle slopes and numerous shrines, Penanggungan is a favorite for day hikers. The older structures are Hindu / Buddhist sanctuaries, named ‘candi’, and at least 80 of them are found all over the mountain. It is a very wet mountain with spring water in most of the candis. Many of these have baths where worshipers, clergy and local royalty bathed. Some of these have been restored. Many have not so their purpose is unknown. Hikers recommend hiring a guide so as not to get lost on the 4-hour hike. Some of the hiking articles on Penanggungan are quite detailed.
One of the ancient Indonesian legends has the holy Mt Mahameru transported from India to Java to hold the islands in place. It was not an entirely successful journey as the mountain began to break apart during the journey with its dropped pieces forming a chain of volcanic peaks. The base became Mt Semeru, Java’s highest mountain. Its summit became Mr Penanggungan. Ancient Javanese viewed Mt Penanggungan as a reflection of sacred Hindu mythology.
Arjuno-Welirang is a complex of at multiple coalesced eruptive centers in East Java, 50 km SE from Surabaya. There are 6 km between the peaks of Arjuno to the S and Welirang to the N, and four peaks over 3,000 m in the system. It is primarily an andesitic system that has also produced basalts and basaltic andesites. Arjuno is named after Arjuna, a hero in the Mahabharata epic. Welirang is the Javanese word for sulphur.
Nomenclature is a bit odd, as the volcanic centers are referred to as crowns. Eruptive centers include Welirang, Arjuno, Kembar I – III, Bakal and a side vent eruption from Bakal. The massif also has evidence of a massive eruption, ring fracture and collapse zone. This preceded construction of the current Arjuno-Welirang, which built Kembar I and II, Bakal (SE trending activity). The last magmatic eruption came from Kembar II located between Arjuno and Welirang.
There are multiple faults in the region. These appear to control the trend of activity over time but location of current hot springs and hydrothermal system. There are at least two ring faults, Ringgit and the neighboring Anjasmoro Caldera to the immediate west. The Ringgit fault may identify the location of a flank collapse on the original Arjuno-Welirang massif. The ancestral volcanoes are Mount Ringgit to the E and Mount Linting to the S.
Welirang has an estimated diameter of 10.4 km and an estimated volume of 102 km3. Oldest volcanic rocks date around 200,000 years old.
Welirang has been investigated for geothermal energy and has a large, active hydrothermal system. The most recent eruption from Welirang took place from the central vent. There are active fumaroles and solfataras in the central vent, and a pair of hot springs on the flanks. There appears to be strong gas/water interaction between meteoric waters and deep volatiles from the magma system below.
The most recent explosive eruptions took place in 1950 (VEI 2) and 1952. They were both on the NW flank. There was a possible emission from Welirang in 1991. Steam plumes were photographed from space in Sept 1991 and Nov 1994. Thermal alerts from MODIS triggered in Aug – Oct 2002 in the summit area. Ostein Lund Andersen Photography posted multiple photos of steam plumes from Welirang in 2013, 2014 and 2017.
In Feb 2018, seismologists measured what is described as A-type volcanic earthquakes, interpreted as magma moving to the upper part of the conduit. This was accompanied by volcanic gasses / steam from the crater. There were two sources, what was interpreted as magmatic in the crater and hydrothermal on the flanks. The flank activity was accompanied with a crack and some release of steam. Activity was very shallow, at several hundred meters depth.
Like neighboring Penanggungan, Arjuno-Welirang is also an active climbing / hiking destination. The most popular hikes start from neighboring Tretes and follows a trail to a sulphur carriers camp at a water source in the saddle between the two peaks. Hikes range from just a few hours and kilometers to 2-3 days and up to 35 km. Tretes is a resort town with an array of hotels and guest houses. The lower slopes of the volcanoes are forested. The plateau is covered with grasses and alpine pine trees and has a population of deer and wild pigs. The peaks are bare. There are a number of temples, graves and other historical sites on the lower flanks and surrounding countryside still visited by local residents.
Tectonics of Indonesia are complex, with four plates involved: The Australian Plate subducting under the Eurasian Plate generally from the south, and the Philippine Sea and Pacific Plates being involved in the N and E of the collision zone.
Java was built by accretion of what is called the East Java Microplate to the western part of the island. The current subduction line is the Sunda Trough to the S of the island. Volcanic activity in Java is considered back arc volcanism and is typically andesitic in nature.
Northward movement of the Australian Plate slowed significantly 35 – 20 Ma. This increased the angle dip of subduction under Java and moved the volcanic line. During this, the South China Sea became an active seafloor spreading center.
The Indian – Australian Plate boundary shifted south 20 – 5 Ma, and continuous volcanic activity took place along almost the entire length of the island of Java. Back arc basins developed in the northern part of the island along with significant faulting in the island. These basins quickly filled with sediments and became one of the most prolific hydrocarbon potential areas in Indonesia.
The LUSI mud volcano appears to be an example of interaction between an active volcanic system and a neighboring hydrocarbon reservoir. It was not the first system in the general area. It will not be the last one. I am astounded that something so destructive took place in close proximity to nearly 26 million residents and took so few lives.