Strike-slip faulting as a trigger mechanism for overpressure release through piercement structures. Implications for the Lusi mud volcano, Indonesia

A. Mazzini ^a^,^,,
A. Nermoen ^a,
M. Krotkiewski ^a,
Y. Podladchikov ^a,
S. Planke ^a^,^b,
H. Svensen^a
- - ^a Physics of Geological Processes, University of Oslo, Sem Sælandsvei 24, Box 1048, 0316 Oslo, Norway
  - ^b Volcanic Basin Petroleum Research, Oslo Research Park, 0316 Oslo, Norway
  Received 18 June 2008, Revised 26 February 2009, Accepted 12 March 2009, Available online 19 March 2009
  
  Abstract
  
  On May 29th 2006 a mud volcano, later to be named ‘Lusi’, started to form in East Java.
  
  It is still active and has displaced > 30,000 people. The trigger mechanism for this, the world's largest and best known active mud volcano, is still the subject of debate.
  
  Trigger mechanisms considered here are (a) the May 27th 2006 Yogyakarta earthquake, (b) the drilling of the nearby Banjar Panji-1 gas exploration well (150 m away), and (c) a combination of the earthquake and drilling operations.
  
  We compare the distance and magnitude of the earthquake with the relationship between the distance and magnitude of historical earthquakes that have caused sediment liquefaction, or triggered the eruption of mud volcanoes or caused other hydrological responses.
  
  Based on this comparison, an earthquake trigger is not expected. The static stress changes caused by the rupture of the fault that created the Yogyakarta earthquake are a few tens of Pascals, much smaller than changes in stress caused by tides or variations in barometric pressure.
  
  At least 22 earthquakes (and possibly hundreds) likely caused stronger ground shaking at the site of Lusi in the past 30 years without causing an eruption.
  
  The period immediately preceding the eruption was seismically quieter than average and thus there is no evidence that Lusi was “primed” by previous earthquakes.
  
  We thus rule out an earthquake-only trigger. The day before the eruption started (May 28th 2006), as a result of pulling the drill bit and drill pipe out of the hole, there was a significant influx of formation fluid and gas.
  
  The monitored pressure after the influx, in the drill pipe and annulus showed variations typical of the leakage of drilling fluid into the surrounding sedimentary rock strata.
  
  Furthermore we calculate that the pressure at a depth of 1091 m (the shallowest depth without any protective steel casing) exceeded a critical level after the influx occurred.
  
  Fractures formed due to the excess pressure, allowing a fluid-gas-mud mix to flow to the surface.
  
  With detailed data from the exploration well, we can now identify the specific drilling induced phenomena that caused this man-made disaster.
  
  Mazzini 2009,
  
  Abstract
  
  Piercement structures such as hydrothermal vent complexes, pockmarks, and mud volcanoes, are found in various geological settings but are often associated with faults or other fluid-focussing features.
  
  This article aims to investigate and understand the mechanisms responsible for the formation of piercement structures in sedimentary basins and the role of strike-slip faulting as a triggering mechanism for fluidization.
  
  For this purpose four different approaches were combined: fieldwork, analogue experiments, and mathematical modeling for brittle and ductile rheologies.
  
  The results of this study may be applied to several geological settings, including the newly formed Lusi mud volcano in Indonesia (Mazzini et al., 2007).
  
  Lusi became active the 29th of May 2006 on the Java Island. Debates on the trigger of the eruption rose immediately. Was Lusi triggered by the reactivation of a fault after a strong earthquake that occurred two days earlier?
  
  Or did a neighbouring exploration borehole induce a massive blow-out? Field observations reveal that the Watukosek fault crossing the Lusi mud volcano was reactivated after the 27th of May 2006 earthquake.
  
  Ongoing monitoring shows that the frequent seismicity periodically reactivates this fault with synchronous peaks of flow rates from the crater.
  
  Our integrated study demonstrates that the critical fluid pressure required to induce sediment deformation and fluidization is dramatically reduced when strike-slip faulting is active.
  
  The proposed shear-induced fluidization mechanism explains why piercement structures such as mud volcanoes are often located along fault zones.
  
  Our results support a scenario where the strike-slip movement of the Watukosek fault triggered the Lusi eruption and synchronous seep activity witnessed at other mud volcanoes along the same fault.
  
  The possibility that the drilling contributed to trigger the eruption cannot be excluded.
  
  However, so far, no univocal data support the drilling hypothesis, and a blow-out scenario can neither explain the dramatic changes that affected the plumbing system of numerous seep systems on Java after the 27-05-2006 earthquake. To date (i.e. April 2008) Lusi is still active.
  Article outline
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  Abstract
  
  Keywords
  
  1. Introduction
  
  2. Methods
  
  3. Results
  
  4. Discussion
  
  5. Conclusions
  
  Acknowledgements
  
  Appendix 1.
  
  Appendix 2.
  
  Appendix 3.
  
  Appendix 4.
  
  References
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