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 overpressured 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 overpressured 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 m 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 overpressured 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. INTRODUCTION 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 (overpressure). 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; Milkov, 2000). 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 km, (b) covers an area of ~3.6 km and is up to ~10 m thick, (c) has buried 4 villages and 25 factories, and (d) displaced 11,000 people. 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: Figure 1. Map of Java, showing the location of the eruption in the Porong subdistrict and Purwodadi and Sangiran Dome, where other mud volcanoes have been documented.
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Jul 29, 2011
Bruce Gill + 4
Open Access