When sediments are deposited and then buried, they start to compact and undergo chemical changes that turn them into sedimentary rock. Some of the water located between individual grains, along with the fluid and gas produced by the chemical reactions, escapes to the surface. In some cases, this movement of fluid and gas occurs through focused regions a few meters to a couple of kilometers across. During the ascent from depths of up to 5 km (3.1 mi), the fluid and gas can mix with sediment (sand or mud) that has not yet turned into rock. The result is an eruption of sediment, fluid, and gas at the surface. There are thousands of mud volcanoes on Earth, but they are poorly understood phenomena because (1) we cannot witness most of the processes directly as they are occurring underground, (2) little is known about the geological conditions prior to and during eruptions, and (3) unlike igneous systems, there are few fossil mud volcanoes that have been exposed on the surface of the Earth that can be examined in detail.
There are two basic forces that drive the water, gas, and mud mixture to the surface. One is the natural development of abnormally high pressure within sedimentary rock, termed overpressure. The second is the expansion of gas that comes out of solution as the mixture rises to the surface and its pressure decreases. The expansion of gas bubbles gives the mixture sufficient buoyancy to rise. We know little of the detailed structure of the feeder conduits that allow the flow to occur, but they probably consist of a complex system of fractures. In some mud volcano systems, the fluid source does not coexist with the mud source beds. Instead, the fluid comes from deeper strata and then passes through mud strata that are susceptible to being picked up by the flow underground (Fig. 1).
Fig. 1 Cross section through a mud volcano system, showing a source of fluid (aquifer) linked to a source of mud. Mud is eroded and brought to the surface through fractures in the sedimentary rock strata. The mud volcano has a lentoid (lens) shape. (1 m = 0.3 ft; 1 km = 1.61 mi.)
Location and dimensions
The Lusi mud volcano is located in the Porong subdistrict of Sidoarjo in eastern Java, and is the best-known mud volcano on Earth. It started to erupt at about 5 a.m. on May 29, 2006, with the main eruption vent located 150 m (500 ft) from a gas exploration well, Banjar Panji-1. The eruption consists of 79% water and 21% mud and other sedimentary rock fragments. By March 2008, the mud volcano had displaced about
30,000 people, covered an area of approximately 7.0 km2 (2.7 mi2), and was approximately 20 m (66 ft) thick at its center (Fig. 2).
Fig. 2 Crisp satellite image of the Lusi mud volcano in February 2008. (1 km = 1.61 mi.)
Trigger
Despite its unprecedented catastrophic impact on the local population and high media and scientific profile, one of the most fundamental questions about Lusi has yet to be resolved: Was the eruption triggered by the Banjar Panji-1 exploration well or by the Yogyakarta earthquake that occurred on May 27, 2006, two days before the eruption started? Determining the cause is important, as it may determine whether the drilling companies should be compensating those affected and be responsible for remediation work. The destruction caused by the mud volcano has been estimated at hundreds of millions of dollars.
The Yogyakarta earthquake of May 27, 2006 had an epicenter 250 km (155 mi) from the eruption and a moment magnitude of 6.3. It is well known that earthquakes can trigger mud volcano eruptions. Comparison of the magnitude and distance of this earthquake from the mud volcano with historical records of earthquakes that have caused eruptions shows that the Yogyakarta earthquake was either too far away or too small to have been the cause. Also, calculations of the impact that the earthquake would have had on the pressure of underground fluid show that pressure changes caused by the earthquake would have been negligible. Most powerful in this debate are the actual data from the exploration borehole. There are two key facts that point to the exploration borehole as the cause. Firstly, the uppermost 1091 m (3579 ft) of the hole was protected by steel casing and the lowermost 1743 m (5718 ft) was not. Secondly, on May 27, 2006, a decision was made to pull the drill bit out of the borehole. This is commonplace in drilling operations. While this was being done, there was an influx of water and gas into the hole (termed a “kick”). This is also not uncommon, but the kick was not noticed for several hours and potentially dangerous fluids and gas could eventually have reached the surface. Therefore, emergency valves known as blowout preventers were closed at the wellsite. The pressure in the wellbore was measured at the surface, while these valves were shut. These measurements showed that the pressure in the unprotected part of the borehole had passed critical levels and that the pressure in the borehole was slowly dropping. These data demonstrate that the pressure was sufficient to cause the rocks to fracture and crack and for the fluid in the wellbore to start to leak into them. This phenomenon has occurred in other boreholes and is termed a subsurface blowout. In some cases the fluid in the borehole moves toward the surface, working its way up through the cement between the protective casing and the surrounding rock. The wellbore probably provided the connection between water-bearing rock strata at the bottom of the hole (2834 m or 9298 ft) and layers of mud higher up. As the fluid moves from 2834 m depth upward, it passes through mud located at between 1219 to 1828 m (3600 to 5900 ft) depth and picks the mud up in the flow, bringing it the surface. The entrainment of huge volumes of mud is not a common aspect of subsurface blowouts.
Next developmental stages
A region that goes beyond the extent of the mud volcano is now undergoing subsidence. The rate of subsidence is a few centimeters per day. It probably occurs due to the removal of sediment and fluid below the surface and because of the weight of the erupted mud at the surface. New eruptions of fluid and gas are occurring within the area of the mud volcano and around its periphery. These eruptions may be the result of new geological faults forming as the region collapses or because dormant faults are reactivated.
Longevity
The length of this eruption could be estimated, but no calculation has been published to date. The eruption is probably driven by the pressure of the source of the water being higher than the downward pressure of a column of the mud and water that fills the fractures and links to the surface. The water-mud mixture also contains gas, and the expansion of the gas during the ascent of the mixture will be a contributory lift mechanism. Once the pressure of the aquifer has dropped, the main drive for the volcano will stop. But if there are still fractures in the rock providing a pathway to the surface, then the expansion of gas in the fluid will allow the volcano to continue to erupt for many years to come. One could also calculate the volume of the source of the water and also calculate how long it would take for the pressure to deplete to normal levels. If one uses other examples of subsurface blowouts as an indicator as to how long this will continue, then some sort of fluid and gas escape can be expected for decades.
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How to cite this article
Richard J. Davies, "Lusi mud volcano," in AccessScience, ©McGraw-Hill Companies, 2009, http://www.accessscience.com