Davies 2011c
Engineering Geology 121 (2011) 97—99
ELSt’1 ER Discussion
Contents lists available at Science Direct Engineering Geology
journal homepage: www.elsevier.com/locate/enggeo
ENGINEERING GEOLOGY
Richard Davies, Michael Manga. Mark Tingay, Richard Swarbrick
INDONESIA
Kami memuji Tanikawa et al. (2010) terhadap studi mereka tentang permeabilitas, porositas dan besarnya potensi overpressure (permeability, porosity and potential magnitude of overpressure generation) yang dibentuk dalam banyak batuan yang berada di wilayah Lusi, dimana banyak diantaranya yang kami setujui.
Mereka memberikan wawasan baru yang kritis (provide critical new insights) dalam hidrodinamika sistem Lusi (hydrodynamics of the Lusi system), khususnya menyorotikemungkinan pembentukan overpressure (overpressure generation) yang baik dari lempung Formasi Kalibeng Atas dan karbonat yang dari tempat dalam (Upper Kalibeng Formation clays and deep carbonates).
Pada permeabilitas, porositas dan tekanan (The permeabilities porosities and pressures) yang ditinjau dinyatakan bahwa sumber utama cairan yang disemburkan dari Lusi (the primary source of fluid erupted from Lusi ) adalah karbonat dalam (the deep carbonates), dengan lumpur dari Satuan Kalibeng Atas 2
menjadi tertahan dalam cairan ini menuju ke permukaan (becoming entrained within these fluids en route to the surface).
Karena itu hasil penelitian ini merupakan kunci dari kemajuan penting dalam pemahaman kita terhadap gunung lumpur Lusi (the results of this study represent a key advance in our understanding of the Lusi mud volcano).
Namun, mereka dari hasil kami tidak setuju dengan interpretasi penulis, khususnya terkait dengan pemicu semburan dan usulan terhadap dari lekuifaksi (triggering of the eruption and the suggestion of liquefaction).
Data yang dimiliki dari studi tersebut dan hasil mengungkapkan bahwa disipasi tekanan
dalam Unit Kalibeng Atas 2 lempung lambat (pressure dissipation within the Upper Kalibeng Unit 2 clays is slow), dengan demikian, selaras dengan analisis sebelumnya oleh Davies et al. (2008) yang menyatakan bahwa goyangan gempa tidak bisa memicu Lusi, dengan efeknya lebih lebih kecil dari dari pasang surut (that earthquake shaking could not have triggered Lusi as its effect was less than the tides).
Manga et al. (2009) menunjukkan bahwa likuifaksi dari lempung Kalibeng Atas (that liquefaction of the Upper Kalibeng clays ) tidak mungkin berasal dari gempa bumi Yogyakarta.
Selain itu, hasil yang disajikan di sini, ditambah dengan hasil dari penelitian lainnya,
menunjukkan bahwa Lusi kemungkinan besar dipicu oleh tekanan pori yang diinduksi reaktivasi dari patahan Watukosek (Lusi was most likely triggered by pore pressure-induced reactivation of the Watukosek fault).
Namun, peningkatan tekanan pori terkait dengan tendangan di sumur Banjar Panji 1
antara 117 dan 329 kali lebih besar daripada yang dihasilkan oleh Gempa Yogyakarta (the pore pressure increase associated with the kick in the Banjar Panji I well was between 117 and 329 times larger than that generated by the Yogyakarta earthquake).
Fakta-fakta ini dapat digabungkan dengan pengamatan baik yang baik sehingga diketahui bahwa gempa bumi sebelumnya tidak memicu Lusi, meskipun relatif lebih besar dan lebih dekat (Davies et al, 2008).
Jadi, meskipun memberikan hasil yang menarik dan kritis (despite providing intriguing and critical results), kesimpulan dari Tanikawa et al. (2010) gempa di Yogyakartamemicu gunung lumpur Lusi, sebgai tidak memiliki kredibilitas ilmiah (the conclusion by Tanikawa et at. (2010) that the Yogyakarta earthquake triggered the Lusi mud volcano lacks any scientific credibility).
Gunung lumpur Lusi (Lusi mud volcano) di Sidoarjo Jawa Timur mulai menyembur pada tanggal 29 Mei 2006 dan telah menyebabkan sekitar 13.000 keluarga telah diungsikan.
Kontroversi yang ada apakah semburan gunung lumpur tersebut disebabkan oleh pengeboran sumur eksplorasi gas Banjar Panji-1 (drilling of the Banjar Panji-1 gas exploration well) (Davies et al., 2007; Manga, 2007:
Davies et al., 2008; Tingay dkk., 2008) atau karena gempa di Yogyakarta (terjadi pada 05:54 pada 27 Mei 2006 (Mazzini et al., 2007: Sawolo et al., 2009)
Sebagai konstrain adalah: (a) tekanan pori dalam lapisan sedimen sebelum pengeboran (pore pressure in sedimentary strata prior to drilling): (b) perubahan tekanan pori (changes in pore pressure) yang terjadi akibat pengeboran sumur Banjar Panji-1 dan gempa bumi, dan (c) rute potensial dari fluida ke permukaan (potential routes for fluid to the surface) adalah sangat penting untuk menyelesaikan perdebatan ini (are critical for resolving this debate).
Hal tersebut ditangani pada makalah terbaru saat ini ditulis oleh Tanikawa et al. (2010), yang telah melakukan pemodelan pembentukan tekanan pori selama penguburan strata sedimen (who model pore pressure development during burial of sedimentary strata) pada lokasi semburan dan mengukur permeabilitas dan porositas dari contoh singkapan pada formasi yang mempunyai kesamaan umur dan litologi seperti halnya yang ditembus oleh sumur (measure the permeability and porosity of outcrop samples of formations of equivalent age and lithology as those penetrated by the well).
From this analysis, they conclude that overpressure developed within specific successions, in particular the Upper Kalibeng clays and a deep carbonate formation.
Dari analisis ini, mereka telah menyimpulkan bahwa overpressure yang berkembang dalam urutan tertentu (overpressure developed within specific successions), khususnya pada Formasi lempung Kalibeng Atas dan suatu formasi karbonat yang dalam (Upper Kalibeng clays and a deep carbonate formation).
Mereka berangkat dari alasan bahwa tekanan berlebih (overpressure) telah menyebabkan satuan yang kaya dengan lempung menjadi rentan terhadap lekuifaksi (the clay-rich unit susceptible to liquefaction) sebagai akibat deformasi berulang (cyclic deformation) saat gempa Yogyakarta, dan ini yang memicu gunung lumpur.
In this discussion we begin by summarizing the main conclusions made by Tanikawa et al. 2010) and then consider the validity of their most important conclusion that Lusi is a natural disaster.
Dalam diskusi ini kita akan mulai dengan meringkas kesimpulan utama (we begin by summarizing the main conclusions) yang dibuat oleh Tanikawa et al. (2010) kemudian mempertimbangkan validitas kesimpulan yang paling penting bahwa Lusi adalah sebuah bencana alam (then consider the validity of their most important conclusion that Lusi is a natural disaster).
Lastly. we reiterate the compelling evidence (Davies et al., 2008; Tingay et al, 2008) that Lusi is man-made and was caused by an underground blowout at the Banjar Panji 1 well.
Terakhir. kami mengulangi dari kompilasi bukti-bukti yang meyakinkan (Davies et al., 2008;. Tingay et al.,2008) bahwa Lusi adalah buatan manusia (Lusi is man-made)dan telah disebabkan oleh ledakan bawah tanah di sumur Banjar Panji-1 (that Lusi is man-made and was caused by an underground blowout at the Banjar Panji 1 well).
Kesimpulan utama dari makalah Tanikawa dkk. (2010) adalah bahwa overpressure dikembangkan dalam lempung abu-abu kebiruan (overpressure developed within the bluish grey clay) cenderung sebagai Satuan 2 dari Formasi Kalibeng Atas (Upper Kalibeng Formation Unit 2), dan oleh karenanya, lapisan-lapisan ini berada di bawah kompaksi (undercompacted) dan rentan terhadap remobilisasi (susceptible to remobilization).
Mereka menganggap bahwa karbonat pada posisi yang lebih dalam, istilah yang digunakan untuk Formasi Kujung Atas (termed the Upper Kujung Formation) sebagai sumber dari cairan yang paling mungkin (the most likely source of fluids).
(Catatan: pada makalah Lusi sebelumnya (misalnya Davies et al., 2007; Mazzini et al., 2007) menyebut karbonat ini sebagai Formasi Kujung. Namun, berdasarkan rasio isotop strontium (strontium isotope ratios) yang ada, menunjukkan umur 16-18 Juta Tahun (Kusumastuti dkk... , 2002), dan dengan demikian satuan tersebut bukan dari Formasi Kujung yang berumur Oligosen (Oligiocene Kujung Formation), melainkan harus disebut sebagai Formasi Prupuh atau Formasi Tuban (Tingay, 2010) dan dengan demikian pada diskusi ini selanjutnya disebut sebagai Formasi Prupuh).
Para penulis tersebut berhipotesis bahwa peningkatan tekanan berlangsung didalam Satuan 2 Formasi Kalibeng Atas, membuat sedimen menjadi lebih rentan terhadap tekanan amplitudo kecil statis atau dinamis yang disebabkan oleh gempa Yogyakarta (made the sediment more susceptible to the small amplitude static or dynamic stresses caused by the Yogyakarta earthquake). Sehingga adanya perubahan yang kecil saja dari tekanan sudah cukup untuk menginduksi likuifaksi (small changes in pressure were sufficient to induce liquefaction).
Masuknya dengan cepat gas dan cairan dari karbonat yang dalam (rapid influx of gas and liquid from the deep carbonates) yang mengalir melalui jalur keluar yang sebelumnya telah (flowed through pre-existing pathways) yang dibentuk oleh perekahan hidrolik alami (formed by natural hydraulic fractures). Hal ini yang menjelaskan mengapa pada Lusi terjadi semburan lumpur yang berlanjut (continuous mud eruption at Lusi).
Diskusi
Makalah ini memberikan informasi baru (The paper provides key new information) terhadap pada hidrodinamika Lusi (the hydrodynamics of Lusi), dan kami memang setuju dengan beberapa poin penting yang dibuat oleh penulis
Sebagai contoh data dari Banjar Panji 1 menguatkan pemodelan oleh Tanikawa et al. (2010) bahwa Unit Kalibeng Atas 2 adalah litologi berbutir halus yang mempunyai tekanan berlebih (overpressure) dan barada di bawah kompkaksi (undercompacted).
Kami juga setuju bahwa sumber yang paling mungkin dari cairan adalah karbonat dalam (most likely source of fluid is the deep carbonates). Lebih jauh lagi, kami setuju dengan hasil pemodelan (model results) yang menunjukkan bahwa overpressures yang signifikan telah ada di dalam dan dari di bawah dari karbonat Formasi Prupah (that significant overpressures exist in and below the Prupah Formation carbonates).
Sangat besarnya magnitut dari Overpressures (Very high magnitude overpressures) yang telah diamati pada penampang di sumur Porong 1, 6 km jaunya dari Lusi.
Meskipun kita sepakat pada wilayah ini, ada beberapa kesimpulan tentang bagaimana gunung lumpur itu dipicu, dimana bertentangan dengan penelitian kami yang sebelumnya telah dipublikasikan (Davies et al, 2007; Manga 2007; Davies dkk. 2008; Tingay dkk., 2008) dan diberikan dan skala bencana kemanusiaanini karena itu menjadi sangat penting hal tersebut menjadi tantangan (given the scale of this humanitarian disaster it is important that these are challenged).
Yang paling penting daripada hal tersebut, adalah alasan mereka bahwa gempa Yogyakarta memicu semburan daripada pengeboran sumur Banjar Panji-1.
Tanikawa et al. (2010) mengusulkan bahwa fluktuasi tekanan yang diinduksi oleh gempa Yogyakarta Mfi.3 (stress fluctuations induced by the Mfi.3 Yogyakarta earthquake) (2 hari sebelum semburan), menyebabkan lumpur di Unit 2 Formasi Kalibeng Atas kehilangan kekuatan dan mencairkan (caused the mud in the Upper Kalibeng Unit 2 to lose strength and liquefy) (abstrak mereka dan bagian 7.1).
Lainnya juga dipicu oleh memicu gempa (misalnya, Mazzini dkk .. 2007: Sawolo dkk,2009.). Kami berpendapat disini bahwa penalaran ini tidak benar (We argue herein that this reasoning is incorrect) dan, lebih jauh lagi, bahwa data yang mereka sajikan dapat tidakmembuktikan dari hipotesis mereka sendiri (that the data they present can be used to disprove their own hypothesis).
Karena jarak besar lokasi gempa dari semburan, tekanan yang bervariasi terhadap pada waktu menekankan diproduksi oleh penjalaran dari gelombang seismik (time-varying stresses produced by the passage of seismic waves) jauh lebih besar daripada perubahan tekanan statis disebagkan oleh slip pada ‘rutured fault’ (the static stress changes cause by slip on ruptured fault).
NASKAH ASLI DALAM BAHASA INGGRIS
DAVIES 2011B
Engineering Geology 121 (2011) 97—99
ELSt’1 ER Discussion
Contents lists available at ScienceDirect Engineering Geology
journal bomepage: www.elsevier.com/locate/enggeo
ENGINEERING GEOLOGY
Fluid transport properties and estimation of overpressure at the Lusi mud volcano, East Java Basin (Tanikawa et. al., 2010)
Sifat-sifat transportasi cairan dan perkiraan overpressure di gunung lumpur Lusi, Cekungan Jawa Timur (Tanikawa et al.., 2010)
Richard Davies, Michael Manga. Mark Tingay, Richard Swarbrick
The Lusi mud volcano in Sidoarjo. East Java. started to erupt on May 29th 2006 and has displaced 13,000 families.
Controversy surrounds whether the mud volcano was caused by drilling of the Banjar Panji-1 gas exploration well (Davies et al.. 2007; Manga, 2007: Davies et al.. 2008; Tingay et al.. 2008) or due to the Yogyakarta earthquake (has occurred at 05:54 am on the 27th May 2006 (Mazzini et al.. 2007: Sawolo et al.. 2009).
Constraining (a) pore pressure in sedimentary strata prior to drilling: (b) changes in pore pressure that occurred due to drilling the Banjar Panji I well and the earthquake, and; (c) potential routes for fluid to the surface are critical for resolving this debate.
These areas are tackled in the recent paper by Tanikawa et al. (2010), who model pore pressure development during burial of sedimentary strata at the site of the eruption and measure the permeability and porosity of outcrop samples of formations of equivalent age and lithology as those penetrated by the well.
From this analysis, they conclude that overpressure developed within specific successions, in particular the Upper Kalibeng clays and a deep carbonate formation.
They go on to argue that the overpressure made the clay-rich unit susceptible to liquefaction as a result of cyclic deformation during the Yogyakarta earthquake, and that this initiated the mud volcano.
In this discussion we begin by summarizing the main conclusions made by Tanikawa et al. 2010) and then consider the validity of their most important conclusion that Lusi is a natural disaster.
Lastly. we reiterate the compelling evidence (Davies et al., 2008; Tingay et al, 2008) that Lusi is man-made and was caused by an underground blowout at the Banjar Panji 1 well.
The main conclusions of the Tanikawa et al. (2010) paper were that overpressure developed within the bluish grey clay tended the Upper Kalibeng Formation Unit 2, and therefore, these strata were undercompacted and susceptible to remobilization.
They considered the deeper carbonates (termed the Upper Kujung Formation) as the most likely source of fluids.
(Note: earlier papers on Lusi (e.g. Davies et al., 2007; Mazzini et al., 2007) referred to the carbonates as the Kujung Formation however, strontium isotope ratios from them indicate they are 16—18 Ma (Kusumastuti et al., 2002), and thus are not the Oligiocene Kujung Formation, but rather should be termed the Prupuh or Tuban Formations (Tingay, 2010) and are thus termed the Prupuh Formation for the remainder of this discussion.).
The authors hypothesized that the elevated pressure within the Upper Kalibeng Formation Unit 2, made the sediment more susceptible to the small amplitude static or dynamic stresses caused by the Yogyakarta earthquake, so that small changes in pressure were sufficient to induce liquefaction.
The rapid influx of gas and liquid from the deep carbonates flowed through pre-existing pathways formed by natural hydraulic fractures, and this explains the continuous mud eruption at Lusi.
In summary, Tanikawa et al. (2010) hypothesize that pore fluid pressure changes caused by the Yogyakarla earthquake caused liquefaction of undercompacted shales and fluid flow occurred through natural fractures.
The paper provides key new information on the hydrodynamics of Lusi, and we indeed agree with several important points made by the authors.
For instance data from Banjar Panji 1 corroborates the modelling by Tanikawa et al. (2010) that the Upper Kalibeng Unit 2 is a fine grained lithology that is over pressured and under compacted.
We also agree that the most likely source of fluid is the deep carbonates. Furthermore, we agree with the model results suggesting that significant overpressures exist in and below the Prupah Formation carbonates.
Very high magnitude overpressures were observed in the section in the Porong 1 well, 6 km away front Lusi.
Although we are in agreement in these areas, there are several conclusions on how the mud volcano was triggered which are at odds with our published research (Davies et al., 2007; Manga. 2007; Davies et at.. 2008; Tingay et al., 2008) and given the scale of this humanitarian disaster it is important that these are challenged.
The most important of these, is their reasoning that the Yogyakarta earthquake triggered the eruption rather than drilling of the Banjar Panji-1 well.
Tanikawa et al. (2010) propose that stress fluctuations induced by the Mfi.3 Yogyakarta earthquake (2 days prior to the eruption) caused the mud in the Upper Kalibeng Unit 2 to lose strength and liquefy (their abstract and section 7.1).
Others have also invoked an earthquake trigger (e.g., Mazzini et at.. 2007: Sawolo et at., 2009). We argue herein that this reasoning is incorrect and, furthermore, that the data they present can be used to disprove their own hypothesis.
Owing to the large distance of the earthquake from the eruption site, the time-varying stresses produced by the passage of seismic waves are much larger than the static stress changes cause by slip on ruptured fault.
Davies et at. 2008) and Tingay et al. (2008) calcutate stress fluctuations of 2l +-kPa (with a static stress change of 30 Pa).
This is comparable to the amplitude of stress changes for ocean tides, sotid earth tides, hydrological loading and barometric pressure changes from large typhoons.
Yet these other phenomena did not trigger Lusi. Despite their similar amplitude, it may be reasonable to suggest that earthquakes might still initiate liquefaction because the period of the stress changes associated with tides or weather effects is much greater than that of earthquakes, and hence the pore-pressure changes produced by tong period stress changes may have time to diffuse and not initiate liquefaction.
In other words, it may be suggested that the response to long period stress changes may be drained, hut the response to earthquake may he undrained.
However, using the Tanikawa at al. (2010) measurements for the Upper Kalibeng Formation Unit 2 of 10 ‘8m2 for permeability, 10-18 Pa l for specific storage, and a pore pressure diffusion length scale of 100m, implies a drainage time scale of 10(-11 (3 kyr).
This drainage time scale is much longer than the period of tidal forcing and, thus, nullifies any suggestion that earthquakes may have been able to trigger liquefaction due to their shorter period.
Hence, the analysis by Tanikawa et al. (2010) again highlights the key issue that, if tides and weather did not initiate an eruption, it is not likely that similar pressures induced by the Yogyakarta earthquake could have.
Tanikawa et al. (2010) suggests that cyclic deformation may have weakened the mud, resulting in liquifaction.
Manga et al. (2009) measured how cyclic deformation affects the strength of mud erupted at the Lusi mud volcano. Manga et aI. (2009) found that strain amplitudes greater than were required to initiate a loss of strength for the range of seismic frequencies (between 0.1 and 10 Hz).
This strain amplitude is much greater than that experienced by the mud. In summary, the magnitude of time varying stresses produced by the Yogyakarta earthquake is fat’ too small to initiate an eruption.
Furthermore, these results suggest that Lusi was likely not triggered by liquefaction of the Upper Kalibeng Unit 2 clays, but rather that Lusi has resulted from over pressured fluid release from the deep carbonates.
Clay was simply being entrained by the fluids en-route to the surface, plausibly by processes similar to ‘piping’, where water erodes a conduit as is observed in some clay filled embankment dams (Fell CE al., 2003) or by wall sediment erosion of existing or new fractures.
There are a few other (minor) issues about which we disagree. First, Tanikawa et at. (2010) used a sonic log rather than the density log to estimate porosity in the Upper Kalibeng unit 1 (volcanic section).
Based upon the density log from Banjar Panji-1 we estimate porosities of 10—13% rather than in excess of 20% (Tanikawa et al.. 2010: Tingay, 2010).
Second, we know of no mechanisms to create such high magnitude overpressure in the volcanic sequences (Upper Kalibeng Unit 1) because they are relatively incompressible so, we question whether this should be included in overpressure modelling and their prediction of overpressure in this formation.
Tanikawa et al. (2010) also proposed that fluids utilized pre-existing hydraulic fractures. Indeed, the model of Lusi’s plumbing system being along hydraulic (tensile) fractures (albeit drilling initiated) is a component of the first published model for the development of Lusi proposed by Davies c’t al. (2007).
This may well be correct for fluid flow at depth. but at the surface eruptions were initially aligned in a NE—SW orientation parallel to the trend of the Watukosek fault (Mazzini et at.. 2007: Roberts et al,, in press) and later eruptions were aligned in an east—west direction parallel to the structural trend (Roberts et al.. in press).
Neither of these orientations is consistent with a mode I (tensile) fracture as the present-day stress tensor in the Lusi region, from data in the World Stress Map Project, is most likely a strike—slip stress regime with a maximum horizontal stress oriented NNE—SSW (Heidbach et al., 2010: Tingay et at.. 2010).
Under this stress regime, a tensile fracture would he expected to he oriented NNE—SSW (parallel to the maximum horizontal stress).
However, the hypothesized NE— SW oriented fault would be optimally orientated for reactivation under the observed stress regime.
This is a minor point, but does have implications and the conclusions made by Tanikawa et al. (2010) on the triggering of Lusi.
The Banjar Panji-1 well and the Lusi mud volcano are often considered lobe located above the structural culmination (shallowest occurrence) of the Prupah Formation and have thus been considered as the logical leak point for fluid from the deep carbonates (Tanikawa et al., 2010).
However, the Porong-1 welt, drilled in 1993. penetrated an adjacent structural culmination, with the same Prupah Formation (Kusumastuti et al., 2002).
The long-lived, high volume mud flow of water, mud and gas from Lusi indicate a large connected aquifer supplying high temperature, high pressure fluids encompassing both culminations (Davies et at., 2007).
The top of the Prupah was found at 2575 mat Porong-1. white the Banjar Panji-1 well may possibly have just penetrated it at 2833 in (Davies et at., 2007), or the carbonate could have been deeper than this (Mazzini et at.. 2007).
Assuming both wells were targeting the crest of the carbonate structure, then the Prupah Formation is at least 258 in shallower at the Porong-l well location and, therefore, this would be the natural hydraulic “leak point”.
So if one is looking for a natural leak point (rather than a leak point through a borehole), the Porong-1 location provides this, rather than the Banjar Panji carbonate culmination.
Indeed, it is probably no coincidence that Porong-1 location shows evidence for a circular shaped collapse feature overlying the carbonates, which is consistent with a palaeo-mud volcano (Kusumastuti et al.. 2002: Stewart and Davies. 2006) and is good evidence that this breach has occurred in the past.
The results presented by Tanikawa et al. (2010), coupled with the understanding of the subsurface geology and vent locations developed by many studies (notably Mazzini et al., 2007; Manga et al.. 2009: Tingay. 2010: Roberts et at.. in press) result in subtle, yet critical, modifications to the geological models of the Lusi eruption.
In particular, alt Lusi models should now consider that the fluid source is primarily the carbonates of’ the Prupah Formation; that initial fluid flow to the surface was probably via the Watukosek fault, and: that liquefaction is an unlikely mechanism for remobilization of the Upper Kalibeng clays.
Taking these key modifications into account indicates that the drilling and earthquake triggering models, often assumed to be completely different, are actually somewhat similar in terms of the probable route taken by fluid to the surface.
The earthquake-trigger hypothesis suggests that shaking by the Yogyakarta earthquake resulted in a transient pore pressure increase (effective stress decrease) that caused reactivation of the Watukosek fault that, in turn, opened up a fluid flow pathway from the deep carbonates to the surface.
The drilling-trigger hypothesis suggests that the kick in the Banjar Panji-1 well pumped high pressure fluid into the Watukosek fault, causing the fault to reactivate and open up fluid flow pathways to the surface (Davies et al., 2010; Tingay. 2010).
In other words, both models, in essence, suggest that Lusi results from reactivation of the Watukosek fault due to increased pore fluid pressures (decreased effective stress).
If this is the case, then the real question behind the triggering debate is actually whether it was the Yogyakarta earthquake or the kick in Banjar Panji-1 that caused the decrease in effective stress (higher pore pressures) that reactivated the Watukosek fault.
Indeed, Tingay et al. (2008) stated that “analysis of the Lusi eruption trigger must primarily examine mechanisms for the initiation and or reactivation of NE—SW-oriented faults and fractures beneath the eruption site”.
The pore pressure increase associated with each hypothesized triggering mechanism is reasonably well constrained. Seismic shaking from the Yogyakarta earthquake is calculated to be at most 21 MPa (Davies et at., 2008). In stark contrast, the absolute minimum value for the pore pressure increase associated with the Banjar Panji-1 kick is 2.42 MPa (with a potential maximum value of 6,9 MPa).
Hence, the pore pressure increase associated with the kick in Banjar Panji I is. between 117 and 329 times greater than that generated by the Yogyakarta earthquake.
We applaud Tanikawa ct at. (2010) for their study of the permeability, porosity and potential magnitude of overpressure generation within many rocks encountered in the Lusi region much of which we agree with.
They provide critical new insights into the hydrodynamics of the Lusi system, in particular highlighting the likelihood of overpressure generation in both the Upper Kalibeng Formation clays and deep carbonates.
The permeabilities porosities and pressures highlight that the primary source of fluid erupted from Lusi is the deep carbonates, with clays from the Upper Kalibeng Unit 2 becoming entrained within these fluids en route to the surface.
Thus, the results of this study represent a key advance in our understanding of the Lusi mud volcano.
However, we disagree with the authors’ interpretations of their results, particularly with respect to the triggering of the eruption and the suggestion of liquefaction.
The study’s own data and results reveal that pressure dissipation within the Upper Kalibeng Unit 2 clays is slow and, thus, confirms earlier analysis by Davies et at. (2008) that earthquake shaking could not have triggered Lusi as its effect was less than the tides.
Manga et al. (2009) demonstrated that liquefaction of the Upper Kalibeng clays was not possible from the Yogyakarta earthquake.
Furthermore. the results presented herein, coupled with results from other studies, indicate that Lusi was most likely triggered by pore pressure-induced reactivation of the Watukosek fault.
However, the pore pressure increase associated with the kick in the Banjar Panji I well was between 117 and 329 times larger than that generated by the Yogyakarta earthquake.
These facts can then be coupled with the well known observation that other earlier earthquakes did not trigger Lusi, despite being comparatively larger and closer (Davies et al.. 2008).
Thus, despite providing intriguing and critical results, the conclusion by Tanikawa et at. (2010) that the Yogyakarta earthquake triggered the Lusi mud volcano lacks any scientific credibility.
MAKALAH TANIKAWA 2010, YANG DIGUNAKAN OLEH DAVIES DKK., (2011) UNTUK MELAKUKAN TINJAUAN DAN KOMENTAR, SEBAGAI BASELAINES MAKALAH (HEAD TO HEAD DISSCUSSION)
https://sites.google.com/site/lusilibraryhardi2010/tanikawa
Fluid transport properties and estimation of overpressure at the Lusi mudvolcano, East JavaBasin
Sifat-sifat pengangkutan fluida dan estimasi dari overpressure pada mud volcano LUSI, Cekungan Jawa Timur
Tanikawa et al., (2010)
PREVIEW ALL PAPER: 2010_ENGEO3292_tanikawa_final.pdf View
Generation and maintenance of overpressure can prevent sediments from compaction and weaken sedimentary rocks in deep basins.
Excess fluid pressure is one of the key factors to explain the disastrous mud eruption that took place in Sidoarjo, East Java, on 29 May 2006, though the mechanism by which it developed is not well known.
We measured permeability and specific storage at a confining pressure of 100M Pain outcrop samples from the East Java Basin. Both permeability and specific storage in our samples showed large stratigraphic variations.
The mudstone of the Upper Kalibeng Formation that is thought to be the source of mud at Lusi had the lowest permeability of our samples at around 10−19–10−20m2, and the permeability of the Upper Kujung Formation limestone was 10−16m2, which is two orders of magnitude larger than that of the Lower Kujung Formation limestone.
In addition, the permeability and porosity of cemented sedimentary rocks showed low sensitivity to effective pressure.
From numerical basin analysis of the Lusi site together with laboratory data, we evaluated the evolution of pore pressure and porosity histories and their present distributions.
Our results show that high over pressure was generated below the mudstone of the Upper Kalibeng Formation and almost reached lithostatic levels.
The modeled fluid pressure variations consistent with the observed data. The long-lived overpressure at depth is mainly caused by the existence of thick low-permeability sediments and a high sedimentation rate.
overpressurization may have caused the mud to lose strength and cause liquefaction (and hydro fracturing) as a result of small stress fluctuations induced by the Yogyakarta earthquake, which may have ended up causing the mud eruption.
Sari (Indonesia)
Ditelaah dan dialihbahasakan ke Indonesia oleh Dr. Hardi Prasetyo untuk LUSI LIBRARY
Pembangkitan dan pemeliharaan kondisi tekanan berlebih ‘overpressure’ (generation and maintenance of overpressure) dapat mencegah sedimen mengalami kompaksi (pemadatan) dan terjadinya pelemahan batuan-batuan sedimen di cekungan-cekungan yang dalam (weaken sedimentary rocks in deep basins).
Tekanan fluida yang berlebih (excess fluid pressure) merupakan salah satu dari faktor-faktor kunci untuk menjelaskan semburan lumpur yang menimbulkan bencana (disastrous mud eruption) yang tampaknya telah mengambil tempat di Sidoarjo, Jawa Timur, pada 29 Mei 2006. Namun, melalui mekanisme yang bagaimana ia berkembang tidak diketahui dengan baik.
Kami mengukur permeabilitas dan penyimpanan yang khusus pada pembatasan tekanan (confining pressure) 100 MP pada contoh singkapan (outcrop samples) dari Cekungan Jawa Timur (East Java Basin). Kedua pemeabilitas dan penyimpanan khusus dari contoh-contoh memperlihatkan variasi stratigrafi yang luas (large stratigraphic variations).
Batulumpur dari Formasi Kalibeng Atas (The mudstone of the Upper Kalibeng Formation) yang ditentukan sebagai sumber dari lumpur Lusi (source of mud at Lusi) mempunyai permeabilitas yang rendah, dari contoh-contoh yang dianalisis menunjukkan sekitar 10 - 19 -10-20m2. Sedangkan permeabilitas dari batugampung Formasi Kujung Atas (the Upper Kujung Formation limestone) adalah 10-16m2. Nilai ini adalah dua kali lebih besar daripada batugamping Formasi Kujung Bawah (Lower Kujung Formation limestone).
Sebagai tambahan, permeabilitas dan porosity dari batuan-batuan sedimen yang tersemenkan (permeability and porosity of cemented sedimentary rocks) memperlihatkan sensitivitas yang rendah terhadap tekanan efektif (effective pressure).
Dilakukan analisis cekungan secara numerik dari lokasi Lusi bersamaan dengan data laboratorium, evolusi dari tekanan pori dan sejarah porositas (pore pressure and porosity histories) dan sebarannya saat ini dievaluasi.
Hasil kami memperlihatkan bahwa tekanan berlebih yang tinggi telah dibangkitkan di bawah batulumpur dari Formasi Kalibeng Atas dan hampir mencapai level litostatik (lithostatic levels).
Pemodelan variasi tekanan fluida konsisten dengan pengamatan data lapangan. Overpressures pada kedalaman berjangka panjang pada kedalaman (The long-lived overpressure at depth) terutama disebabkan oleh keberadaan sedimen dengan permeabilitas yang rendah yang tebal dan kecepatan sedimen yang tinggi (existence of thick low-permeability sediments and a high sedimentation rate).
Formasi Kalibeng Atas yang berada di bawah tekanan karena overpressurization mungkin telah menyebabkan lumpur kehilangan ketegasannya dan menyebabkan likuifaksi (may have caused the mud to lose strength and cause liquefaction) dan perekahan hidro (and hydro fracturing) sebagai suatu hasil tekanan yang kecil secara berfluktuasi diinduksi oleh gempabumi Yogyakarta. Kemungkinan sebagai penyebab akhir dari semburan lumpur (which may have ended up causing the mud eruption).
Kesimpulan
Data laboratorium memperlihatkan bahwa permeabilitas pada Formasi Kalibeng, dimana yang ditentukan sebagai sumber lumpur Lusi (mud source for Lusi), dengan nilai rendah yang berkisar antara 10/-19 sampai 10/-20 m2.
Formasi Kujung Atas (Upper Kujung Formation) bersifat poros dan permeabel (porous and permeable), tapi Formasi Kujung Bawah (Lower Kujung Formation) lebih kecil lagi.
Analisis cekungan (basin analysis) yang dilakukan di daerah Lusi memperlihatkan bahwa overpressures dibangkitkan dan dipelihara di dalam Formasi Kalibeng Atas, karena dampak kombinasi dari kecepatan sedimentasi yang tinggi dan permeabilitas yang rendah (rapid sedimentation rate and low permeability) dari Formasi Kalibeng.
Karakteristik dari sebaran tekanan sama dengan yang perkiraan dari pemboran.
Pengurangn dari strength dari batuan sebagai hasil dari tekanan tinggi yang berlangsung pada yang jangka panjang (long term high pressure) mungkin telah berhubungan dengan semburan lumpur.
Karena strength yang rendah lebih ideal untuk membangkitkan likuifaksi dan perekahan hidro (generation of liquefaction and hydrofacturing) pada level dari fluktuasi dinamika tekanan pori (dynamic pore pressure fluctuation). Hal ini sebagaimana halnya yang telah dipicu oleh gempabumi Yogyakarta.
Kami percaya bahwa masukan gas dan cairan dalam jumlah yang masif (rapid and massive influx of gas and liquid) berasal dari dari Formasi Kujung,yang mengalir melalui jalankeluar yang sebelumnya telah ada (flowing through pre-existing pathways) dibentuk oleh perekahan (formed by fracturing) selama evolusi cekungan (basin evolution). Sehingga dapat menjelaskan mengapa semburan dapat terus berlanjut (the continous eruption of Lusi).
Evolusi regional berjangka panjang dari tekanan fluida di dalam lapisan lumpur yang tebal (the regional long-term evolution of fluid pressures in thick mud layers) telah memainkan bagian yang besar dalam pembentukan mud volcano di pulau Jawa Indonesia (plays a great part in the formation of mud volcano on the island of Jawa in Indonesia).