http://www.sciencedirect.com/science/article/pii/S0012821X1100673X
Mazzini 2011
Earth and Planetary Science Letters
Volumes 317-318, 1 February 2012, Pages 305-318
A new hydrothermal scenario for the 2006 Lusi eruption, Indonesia. Insights from gas geochemistry
Adriano Mazzinia, , , Giuseppe Etiopeb, Henrik Svensena
a. Physics of Geological Processes, University of Oslo, Sem Sælandsvei 24, Box 1048, 0316 Oslo, Norway
b. Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma Italy and Faculty of Environmental Science and Engineering, Babes-Bolyai University Cluj-Napoca, Romania
Received 5 July 2011; revised 17 November 2011; Accepted 18 November 2011. Editor: R.W. Carlson. Available online 29 December 2011.
The 29th of May 2006 gas and mud eruptions suddenly appeared along the Watukosek fault in the north east of Java, Indonesia.
Within a few weeks several villages were submerged by boiling mud. The most prominent eruption site was named Lusi. To date (November 2011) Lusi is still active and a ~ 7 km2 area is covered by the burst mud breccia.
The mechanisms responsible for this devastating eruption remain elusive. While there is consensus about the origin of the erupted mud, the source of water is uncertain, the origin of the gas is unknown and the trigger of the eruption is still debated.
In order to shed light on these unknowns, we acquired a wide set of data of molecular and isotopic composition of gas sampled in several Lusi vents, in the surrounding mud volcanoes, in the closest natural gas field (Wunut), and in the hydrothermal vents at the neighbouring volcanic complex in the period 2006–2011.
The boiling fluids erupted in the crater zone are apparently CO2-dominated, while colder CH4-dominated and C2–C3 bearing fluids are identified at several sites around the crater zone.
(Gas genetic diagrams, maturity plots and gas generation modelling suggest that the hydrocarbons are thermogenic (δ13C1 up to − 35‰; δ13C2 up to − 20‰), deriving from marine kerogen with maturity of at least 1.5%Ro, for instance in the ~ 4400 m deep Ngimbang source rocks.
(CO2 released from the crater and surrounding seeps is also thermogenic (δ13C from − 15 to − 24‰) related to kerogen decarboxylation or thermal CH4 oxidation in deep rocks, although three vents just outside the crater showed an apparent inorganic signature (− 7.5‰ < δ13C = − 0.5‰) associated to mantle helium (R/Ra up to 6.5).
High CO2–CH4 equilibrium temperatures (200–400 °C) are typical of thermally altered hydrocarbons or organic matter.
The data suggest mainly thermally altered organic sources for the erupted gases, deeper sourced than the mud and water (Upper Kalibeng shales).
These results are consistent with a scenario of deep seated (> 4000 m) magmatic intrusions and hydrothermal fluids responsible for the enhanced heat that altered source rocks and/or gas reservoirs.
The neighbouring magmatic Arjuno complex and its fluid–pressure system combined with high seismic activity could have played a key role in the Lusi genesis and evolution. Within this new model framework, Lusi is better understood as a sediment-hosted hydrothermal system rather than a mud volcano.
Highlights
► Gas from Lusi eruption shows that CO2 and CH4 have a deep thermogenic origin.
► Thermally altered Ngimbang source rocks (> 4400 m depth) could generate the erupted gas.
► Lusi hydrocarbons derive from the Ngimbang–Kujung petroleum system.
► Mantle He from Lusi suggests deep magmatic intrusions from Arjuno–Welirang volcano.
► Lusi is not a mud volcano but rather a sediment-hosted hydrothermal system.
Keywords: Lusi eruption; sediment-hosted hydrothermal system; mud volcanoes; gas origin; CO2 and CH4; mantle
Munculnya semburan gas dan lumpur sepanjang Patahan Watukoksek: dampaknya
Pada 29 Mei 2006 secara tiba-tiba semburan gas dan lumpur (gas and mud eruptions) muncul sepanjang patahan Watukosek di utara timurlaut Pulau Jawa, Indonesia.
Dalam beberapa minggu kemudian beberapa desa telah digenangi oleh lumpur yang mendidih (boiling mud). Sampai waktu November 2011 Lusi terus aktif dan wilayah seluas sekitar 7 km2 telah ditutupi oleh luapan breksi lumpur (area is covered by the burst mud breccia).
Misteri asal mula dan pengendali mekanisme semburan Lusi:
Mekanisme yang bertanggung jawab untuk semburan yang merusak (devastating eruption) ini tetap sulit untuk dipahami (remain elusive). Sementara ada konsensus terkait asal-usul dari semburan lumpur (the origin of the erupted mud), sumber dari air tidak jelas (the source of water is uncertain), asal mula gas tidak diketahui (the origin of the gas is unknown) dan pemicu dari semburan tetap diperdebatkan (trigger of the eruption is still debated).
Pengambilan data dengan berbagai metoda dan lokasi:
Dalam rangka lebih memperjelas dari ketidak pahaman tersebut, penulis makalah telah mengambil himpunan data yang lebih luas dari komposisi molekul dan isotop contoh gas (molecular and isotopic composition of gas sampled) pada beberapa pusat semburan lusi, di sekitar mud volcano, di tempat terdekat dari lapangan gas (Wunut gas field), dan pada kawah panasbumi pada komplek berdekatan (hydrothermal vents at the neighbouring volcanic complex) pada periode 2006-2011.
Hasil dan analisis data:
Fluida mendidih yang disemburkan pada zona kawah didominasi oleh CO2, sedangkan dominasi CH4 dan fluida mengandung C2-C3 diidedentifikasikan pada beberapa lokasi di sekitar zona kawah (crater zone).
Pemodelan menunjukkan hidrokarbon termogenik berasal dari batuan sumber Ngimbang:
Diagram genetik gas, plot kematangan dan pemodelan pembentukan gas ditentukan (Gas genetic diagrams, maturity plots and gas generation modelling) bahwa hidrokarbon adalah termogenik (hydrocarbons are thermogenic (δ13C1 up to − 35‰; δ13C2 up to − 20‰)), berasal dari kerogen marin (marine kerogene) dengan kematangan sekurang-kurangnya 1,5%Ro. Tampak berasal dari batuan sumber Ngimbang pada kedalaman sekitar 4.400 m (deep Ngimbang source rocks).
Sumber CO2 termognik dan tiga lokasi menunjukkan ciri helium selubung:
CO2 yang dilepasakan dari kawah dan rembesan disekitarnya juga termogenik (δ13C from − 15 to − 24‰) berhubungan dengan decarboxylation atau oksidasi CH4 pada batuan dalam, walaupun tiga semburan diluar kawah menunjukkan tanda-tanda suatu kedapatan (apparent inorganic signature) (− 7.5‰ < δ13C = − 0.5‰) yang berasosiasi dengan helium selubung (mantle helium (R/Ra up to 6.5)).
Ciri hidrokarbon hasil alterasi panas atau unsur organik
Tingginya temperatur (200–400 °C) keseimbangan CO2-CH4 adalah ciri-ciri dari hidrokarbon alterasi panas atau unsur organik (typical of thermally altered hydrocarbons or organic matter).
Data menunjukkan bahwa erupsi gas terutama bersumber dari organik yang dirubah oleh panas (mainly thermally altered organic sources for the erupted gas). Sumber lebih dalam daripada lumpur dan air (Upper Kalibeng Shales).
Intrusi magmatik dalam dalam meningkatkan panas
Hasil ini konsisten dengan skenario kedudukan dalam (>4000m) intrusi magmatik (with a scenario of deep seated (> 4000 m) magmatic intrusions and hydrothermal fluids) dan fluida panasbumi bertanggung jawab untuk meningkatkan panas yaitu merubah batuan sumber dan atau reservoir (hydrothermal fluids responsible for the enhanced heat that altered source rocks and/or gas reservoirs).
Peran penting komplek magmatik, sistem tekanan fluida dan tingginya seismisitas
Komplek magmatik Arjuno yang berada di dekatnya dan sistem fluida-tekanannya dikombinasi dengan aktivitas seismisitas yang tinggi telah memainkan suatu peran kunci pada pembentukan dan evolusi Lusi (have played a key role in the Lusi genesis and evolution).
Lusi sebagai sedimen pembantu pada sistem panasbumi
Dalam kerangka model yang baru ini Lusi lebih baik dipahami sebagai suatu pelaku sedimen sistem panas bumi daripada suatu mud volcano (a sediment-hosted hydrothermal system rather than a mud volcano.)\
Ringkasan (Higlights)
· Gas dari semburan Lusi memperlihatkan bahwa CO2 dan CH4 mempunyai asal susul termogenik dalam (deep thermogenic origin).
· Alterasi panas dari batuan sumber Ngimbang (kedalaman >4400m) dapat membangkitkan semburan gas.
· Hidrokaarbon Lusi berasal dari sistem perminyakan dari Ngimbang-Kujung (the Ngimbang-Kujung petroleum System).
· Keberadaan He selubung (Mantle He) dari Lusi menunjukkan intrusi magmatik dalam (deep magmatic intrusions) dari gunung api Arjuno–Welirang volcano.
· Lusi bukan suatu mud volcano (Lusi is not a mud volcano) tapi lebih tepat sebagai sedimen-induk sistem panasbumi (a sediment-hosted hydrothermal system).
1. Introduction
3. Lusi phenomena: open questions
4. Methodology
4.1. Field work sampling and measurements
4.2. Laboratory analyses
5. Results
5.1. Molecular and isotopic composition of gas
5.2. Temperature data
6. Discussion
6.1. Methane and heavier alkanes
6.2. Carbon dioxide and helium
6.3. Towards a new model of Lusi seepage system
6.4. Seismicity and the volcanic complex
6.5. A new triggering scenario
6.6. Predictions of Lusi longevity
7. Conclusions
Fig. 1.
(A) Elevation map of eastern Java Island. Highlighted the position of some known mud volcanoes, and main volcanoes and volcanic vents. Note: the orientation of the main vents of the Arjuno–Welirang volcanic complex has the same NE–SW direction of the Watukosek fault that also hosts a large escarpment as well as other mud volcanoes;
(B) satellite image of Lusi the 14 of June 2011, courtesy of Crisp, NUS 2011. Note the brownish areas on the outskirts of the crates represent dry zones where it is possible to access. The central part closer to the crater remains muddy. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2.
(A) Helicopter view of Lusi (19 May 2011), on the background the Watukosek escarpment and the Penanggungan volcano. Note: the broken containment dam that was destroyed 5 h after the 26-04-2011 earthquake is aligned along the Watukosek fault system connecting Lusi and the volcanic complex;
(B) View of the close volcanic complex from Lusi site. From left to right: Arjuno, Welirang (erupting white smoke), and Penanggungan;
(C) View of erupting Lusi crater approached walking on dry erupted mud;
(D) One hour after the 26-04-2011 earthquake two distinct craters formed at Lusi site clearly indicating a correlation between the two events; (E) Helicopter view with detail of the damaged dam after the earthquake suggesting reactivation of the strike-slip fault.
A, B, D, E courtesy of S. Hadi.
Fig. 3.
Genetic zonation diagrams of methane (A: Schoell plot; Schoell, 1983) (B: Bernard plot; Bernard et al., 1978). To: thermogenic with oil; Tc: thermogenic with condensate; TD: dry thermogenic.
Fig. 4.
Natural gas plot, with the δ13C sequence of C1–C4 alkanes of Lusi and Wunut gas field data.
Fig. 5.
(A) Ethane vs. propane maturity plot (vitrinite reflectance, % Ro; model by Berner and Faber, 1996). Initial carbon isotope value of marine (Type I–II) kerogen is taken as (a) − 30‰; (b) − 28‰ (that is the mean value for East Java Basin source rocks) and (c) − 26‰. The terrestrial (Type III) kerogen precursor is taken as − 26‰ (mean for terrestrial East Java Basin kerogen). “Mud source” indicates the conditions of the Lusi mud (Upper Kalibeng shales; Mazzini et al., 2007).
(B) Thermogenic gas formation modelling for ethane vs propane, from default Type II kerogen (calculated using GeoIsochem Corp. GOR-Isotopes software 1.94; heating rate of 5 °C per million year; Tang et al., 2000). Bold and thin lines refer to cumulative and instantaneous gas generation, respectively.
(C) Same modelling for ethane vs methane from default Type I–II–III kerogen (calculated as in B). Maturity values estimated independently in (A) and (B) are similar, and the measured isotopic composition of ethane (δ13C2) would correspond to methane with δ13C1 around − 35‰, which is consistent with that measured in the Lusi crater (seeTables 1A, B).
Fig. 6.
Schematic cartoon summarising the known stratigraphy at Lusi site and depicting the Lusi plumbing system with the fluids contributions from different depths. Geochemistry shows the evidence of a deeper sited magmatic intrusions and/or hydrothermal fluids that feeds the Lusi system.
This link may also explain why Lusi activity still increases after earthquakes even when occurring at significant distances. We suggest that these earthquakes likely affect the plumbing system of the magmatic chamber at depth resulting in overpressure buildup that periodically enhances Lusi eruption. TOC values from the East Java Basin are extracted from Satyana and Purwaningsih (2003).
Fig. 7.
(A) He isotopic ratios vs. δ13CCO2. Dotted lines indicate minimum and maximum values of δ13CCO2 (analysed in 2006, 2007 and 2008; R/Ra not available). Carbon dioxide isotope zonation after Etiope et al. (2011c). AO, aerobic hydrocarbon oxidation; KD, kerogen decarboxylation; AC, alteration of marine carbonates; BSM, biodegradation and secondary methanogenesis. (B) δ13CCH4 versus difference in stable carbon isotope composition [δ13C(CO2–CH4)] between carbon dioxide and methane of Lusi, Wunut and other MV gases. Relation between fractionation of stable carbon isotope and temperature from Bottinga (1969).
Fig. 8.
Cartoon not to scale of the shallow plumbing system at Lusi site. The fluids rising through the main conduit have high temperature (at least 100 °C) and are CO2 dominated. During the fast rise from units with high T and P, the fluids cannot cool significantly and the sudden pressure drop allows the exsolution mechanism to be very efficient. This implies that the CO2 dissolved in the interstitial waters is liberated as gaseous phase once it reaches the surface. The satellite seeps, located along the Watukosek fault trend or on the rims of the caldera collapse, are CH4 dominated with low temperatures. The fluids seeping at these locations branch off from the main conduit and have sufficient time to cool down hence interrupting the exsolution mechanism.
Table 1A. Molecular composition of sampled gas in vol.%. N2 for samples JV06-07 and JV06-10 is respectively 1.703 and 1.736.
bdl: below detection limit, nd: not determined.View Within Article
Table 1B. Isotopic composition of sampled gas. Isotopic data: δ13C: ‰, VPDB; δD: ‰, VSMOW; atm. R/Ra = (3He/4He)sample/(3He/4He)atmosphere; Ra = 1.39 × 10−6; nd: not determined. ◊ helium gas extracted from water bottles collected at seepage sites. † indicates samples with He/Ne < 1.
RELATED ARTICLE
http://www.sciencedirect.com/science/article/pii/S0264817209000476
Strike-slip faulting as a trigger mechanism for overpressure release through piercement structures. Implications for the Lusi mud volcano, Indonesia
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.
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.
Keywords: Java, Indonesia; Lusi mud volcano; Faulting; Shearing; Analogue and mathematical modeling
1. Introduction
2. Methods
2.1. Fieldwork
2.3.1. Brittle rheology
2.3.2. Ductile rheology
3. Results
4. Discussion
5. Conclusions
Appendix 1.
Appendix 2.
Appendix 3.
Appendix 4.