Nam Con Son

Nam Con Son Basin is the southern most sedimentary basin offshore Vietnam. The majority of the basin is located in a shallow water (less than 200 meters) but to the north the bathymetry can goes deeper than 2000 meters.

Key seismic sections were published by Swiecicki and Maynard (2009):

Seismic expression of rifting events (Swiecicki & Maynard, 2009) shows 2 major rifting episodes:
1) Oligocene- Early Miocene rift system (Oligocene North-South extension)
2) Middle Miocene - Pleistocene rift system (Middle Miocene NW-SE extension)







Regional cross section
Lan Tay and Hoa Dao structure complex on a NW-SE regional seismic line by Restrepo-Pace (2007).

 1st Rift system
Tien Cau syn-rift and Cau post rift as indicated on seismic.









2nd Post rift deposits
Seismic details on Dua Formation, Early Miocene in age

Mang Cau Syn-Rift 2
Significant thickening towards Hoa Dao high is observed in this seismic section.

Nam Con Son Post Rift 2.












Bien Dong - Post rift sag phase
Progradational sequence on top of bathyal shales (transparant unit)

Bien Dong - Post rift sag phase
Progradational sequence on top of bathyal shales (transparant unit)

Bien Dong channels
Time slice showing the channel complex.








Eearly Pliocene (LBD) Channel System
Time slide and seismic section in both strike and dip directions.

Upper Bien Dong progradational system
Petronas Tower as a scale for the canyon system on the left.
Comparing seismic sections and time slice.







Upper Bien Dong progradational system












Reference: 
Swiecicki, T. and Maynard, K., 2009, Geology and Sequence Stratigraphy of Block 06/94, Nam Con Son Basin, Offshore Vietnam, SEAPEX

Song Hong / Yinggehai Basin

Fig. 2 Detail map of the Beibu Gulf with
Song Hong / Yinggehai Basin,
showing wells and seismic locations

Fig.1 Sedimentary basins offshore Indochina.

The southeast extension of Red River Fault generated a rift basin offshore west of Hainan Island (Fig. 1). The Vietnamese called it Song Hong Basin and the Chinese called it Yinggehai Basin. The basin has NW-SE orientation, parallel to the Red River Fault system.

The maximum water depth in Beibu Gulf is less than 200 m and it is getting deeper towards the south when it is open up to the South China Sea. A number of wells have been drilled in this area with oil and gas discoveries (Fig. 2). Several seismic lines were published:

A & B are perpendicular to Red River Fault system.
C is parallel to the Red River Fault system.

Fig. 3. Section A, located to the north of the basin. A graben 

system was opened up as part of the Red River Fault system

 ( Nielsen et al, 1999)

Fig. 4. Section B, Line 89-I-36A shows a series of horst 

and graben system in the lower section, below B horizon  

( Nielsen et al, 1999)

Fig 5. Section C. 90-I-C65 line, SW-NE orientation, 

perpendicular to Red River Fault system ( Nielsen et al, 1999)

Fig. 6. Section D. Line 89-I-62 with NW-SE orientation, 

parallel to Red River Fault system ( Nielsen et al, 1999)

Reference: 

Nielsen, L.H., Mathiesen, A., Bidstrup, T., Vejbñk, O.V., Dien, P.T., Tiem, P.V., 1999. Modelling of hydrocarbon generation in the Cenozoic Song Hong Basin, Vietnam; a highly prospective basin. Journal of Asian Earth Sciences 17, 269±294.

The Lesser Sunda Islands

Seismic Expression of Tectonic Features in the Lesser Sunda Islands, Indonesia

Herman Darman

Introduction

The Sunda Arc is a chain of islands in the southern part of Indonesia, cored by active volcanoes (Fig. 1). The western part of the Sunda arc is dominated by the large islands of Sumatra and Java, and is commonly called ‘the Greater Sunda Islands’. The tectonics in this part is dominated by the oceanic subduction below the Asian continental plate. Towards the east the islands are much smaller and are called ‘the Lesser Sunda Islands’. The transition from oceanic subduction to continent-island arc collision developed in this area.

The Sunda Arc has long been considered as a classical accretionary margin system where the Indo-Australian oceanic plate is underthrust beneath the Asian continent, active since the Late Oligocene (Hamilton, 1979). At the eastern end of the Sunda Arc the convergent system changes from oceanic subduction to continent-island arc collision of the Scott Plateau, part of the Australian continent, colliding with the Banda island arc and Sumba Island in between.

Figure 1. Map of Southeast Asia showing the different crustal type in the region and the location map of the Lesser Sunda Islands (after Doust & Lijmbach 1997). 

The Lesser Sunda Islands are also called the inner-arc islands. The formation of these islands are related to the subduction along the Java Trench Bali Island is located in the west of the Lesser Sunda Islands and Alor Island at the east end (Fig. 1). To the south of the inner-arc islands, an accretionary wedge formed the outer-arc ridge. The ridge is subaerially exposed in the east as Savu and Timor Island. The northwest of the Lesser Sunda Islands are underlain by a Late Cretaceous Accretionary Crust, which changes to an oceanic crust in the northeast (Doust & Lijmbach, 1997). The Sumba Island has a unique orientation and the origin of the island is still debated (Rutherford et al., 2001).

The aim of this article is to provide a broad overview about the structures of the tectonic units based on some selected seismic lines. These lines also give a better geological understanding, including recent processes that developed in the area.

Seismic data

A number of surveys have been deployed to acquire seismic data in this area. Selected seismic data used for this article were acquired in the following expeditions:

- R. V. Vema cruise 28 and R. V. Robert Conrad cruise 11 (in Hamilton, 1979)

- Rama 12 expedition (Prasetyo, 1992; Scripps Institute of Oceanography, http://www.ig.utexas.edu/sdc/)

- R. V. Baruna Jaya late 90’s (Krabbenhoeft, A., 2010) for bathymetric data acquisition.

- R. V. Sonne, cruise SO190 (Lüschen et al, 2011)

- CGG Veritas Spec. Survey (Rigg & Hall, 2012)

- ION-GXT JavaSPAN 2008 (Granath et al, 2011)

Apart from the surveys mentioned above, there are other surveys which contributed significant pieces of information to help geoscientists in understanding the geology of the region. During the Snellius-2 Expedition, for example, Van Weering et al., (1989) have also acquired a number of seismic sections in the area, but later surveys have gathered improved seismic images. Prasetyo (1992) reported seismic reflection and gravity data from this area as well.

The earlier surveys, such as R. V. Vema and R. V. Robert Conrad in Hamilton (1979) provided limited data mainly confined to information on bathymetry and shallow depth of image. The later images, acquired by CGG Veritas are considered as a modern industry standard for seismic, providing seismic images down to 8 seconds Two-Way-Time. Recent long cable with improved technology by ION helped to acquire seismic more than 10 km deep. These ION deeper sections help geoscientists to acquire a better understanding about the basement.

Tectonic features

The Lesser Sunda Islands area consists of several tectonic units (Fig. 2). Several regional seismic sections were shot across these features. Some lines give a better geological understanding about the composition and the tectonic processes.

Figure 2.  Structural map of the Lesser Sunda Islands, showing the main tectonic units, main faults, bathymetry and location of seismic sections discussed in this paper.

1. Outer-arc Ridge

The outer-arc ridge or also called the fore-arc ridge is an accretionary wedge formed by the subduction of the Indian plate. In the west of the Lesser Sunda Island region, the Outer Arc Ridge formed about 3000 m below sea level, parallel to the Inner Arc. To the east, the outer-arc ridge exposed sub aerially as the outer-arc islands of Roti and Timor. (Fig. 2).These islands are mainly composed of raised shallow and deep marine sediments. Mud diapers and mud volcanoes are common in the outer-arc islands (Hamilton, 1979; Zaim, 2012). The outer arc is bounded by the Java Trench which marked the subduction point in the south. The northern margin of the Outer Arc Ridge is partly covered by the fore-arc basin sediment fill.

Figure. 3. Six 15 km deep seismic sections acquired by BGR from west to east traversing oceanic crust, deep sea trench, accretionary prism, outer arc high and fore-arc basin, derived from Kirchoff prestack depth migration (PreSDM) with a frequency range of 4-60 Hz. Profile BGR06-313 shows exemplarily a velocity-depth model according to refraction/wide-angle seismic tomography on coincident profile P31 (modified after Lüschen et al, 2011).

Figure 3 shows regional seismic sections acquired by the Sonne cruise in the region. Section - A, B, C and D in this figure (from Lüschen et al, 2011) show similar patterns of the outer-arc ridge. The subduction zone after the trench and the accretionary complex are well imaged. Lüschen (2011), also provide detail seismic images of Section B in Figure 4, showing the structures of the outer-arc ridge. The outer-arc ridge is a structurally complex unit with a series of thrust faults (Fig. 4A and further detail in Fig 4B).  Some of these faults generated topographical relief on top of the outer-arc ridge and formed ‘piggy-back basins’, which are filled with recent sediments from surrounding structural highs. On seismic these sediments appear as brighter and relatively flat reflectors all the way up to the surface (Fig. 4C).

Section E and F in the east of the area show different patterns compared to the western  4 sections.   The outer-arc ridge in Figure 3E has a gentle relief and the thrust faults are not as clear as the sections in the west. Figure 3F also shows a gentle relief but much wider (reformulate). The difference between the four seismic profiles in the west and the two in the east, reflect the transition from oceanic subduction to continent in the west to continent-island arc collision in the east (Kopp, 2011).

2. Fore-arc Basin.

Depressions in the seabed between the inner volcanic arc and the outer-arc are known as fore-arc basins. The fore-arc basin in the west is called the Lombok Basin and the water depth of this basin is about 4000 to 5000 m deep (Fig. 2). Savu Basin is the fore-arc basin located in the east of the Lesser Sunda Islands, separated from the Lombok basin by Sumba Island. In parts the water depth of Savu Basin is deeper than 2000 m.

Figure. 4. Detail sections of BGR06-303. A) Outer-arc ridge with thrust faults which formed the accretionary complex. B) Detail section of A) showing the trench sediment fill and the thrust faults in the north of the section. C) Detail section of A) showing the sediment fills of the Piggy-Back Basin, with relatively undisturbed flat surface on the north. The active fault has disturbed the continuation of the sediments in the south of the section.

A series of north dipping thrust faults are clearly seen in the close up of these seismic sections as shown in Figure 4. A closer detail display of fault system in the trench is shown in Figure 4. A small sediment accommodation space developed in the trench. Recent sediments have filled this small depocentre, indicated by flat sea bottom on seismic sections.

Lüschen et al, 2011, also indicated a Piggy-Back Basin developed in the centre of the Outer Arc Ridge by the thrust fault system. These basins are generally small and filled with recent sediments. Similar to the trench deposit, these basins are characterized by semi parallel reflectors with flat surfaces (Fig.4).

The Savu fore-arc basin developed in the east of the Lesser Sunda islands, where there is now a change from oceanic subduction to arc-continent collision (Rigg and Hall, 2012). The Savu Basin is bounded to the west by the island of Sumba and by a submarine ridge (the Sumba Ridge) that crosses the fore-arc obliquely in an NW-SE direction. The basin is narrowing to the east. To volcanic island arc bounded the north part of the basin (Fig. 1).

Figure 5. North-south seismic sections across Savu Basin. A) Rama expedition seismic, shows the relationship of the outer-arc ridge, Sumba Island high, Savu basin and Flores Island in the north. B) Another Rama expedition seismic in the centre of Savu Basin. C) A CGG Veritas seismic lines parallel to section 6B with higher resolution image with the seismo-stratigraphic unit interpretation in D).

Figure5A and 5B shows 2 regional seismic sections across the Savu Basin, acquired during Rama expedition in early 1980’s. The section on the west (Fig. 5A) shows the narrow part of the basin, with the southern flank of the volcanic arc (Flores Island) in the north and the east continuation of the Sumba Island high in the south. A detail section of the southern margin of the basin is shown in Fig. 5C with seismo-stratigraphic interpretation (Fig 5 D) by Rigg and Hall (2012). At the south end of this section Unit 1 is uplifted and thrust northwards towards the basin and Units 2, 3 and 4 are largely missing and interpreted to have been redeposited in the basin as Unit 4. Figure 5D shows a significant southward thinning of Unit 3 and 4. Steep dipping of the base of Unit2 are probably controlled by faults. Unit 3 contain a brighter reflective package which wedges out to the north. A rather transparent seismic package developed in the north part of the unit. The top of Unit 4 is relatively undisturbed in the distal part.

3. Inner Arc – Volcanic

The Inner volcanic arc islands are some of the simplest geological structures within this complex region, and are certainly simpler than the outer-arc islands. The islands arc is basically a chain of young oceanic volcanic islands, often ringed by reef limestones or by other sedimentary material that has eroded from the main body of the island and built up between the tongues of lava and other extrusions. In general, the origins or basal materials of these islands become progressively younger from west to east, following the evolution of the Banda Arc eastward from the Sumba Fracture (Monk et al, 1997).

Figure 6. Seismic-reflection profile across Bali-Lombok volcanic ridge, acquired by R. V. Robbert Conrad cruise 11 (Hamilton, 1979). The crest and north flank of the outer-arc ridge are mantled by pelagic sediments, whereas the south flank is not; this may record increasingly intensity of deformation within the mélange wedge southward toward the Java Trench. Strata within the outer-arc basin display basinal downfolding which decreases upward. The volcanic ridge is made irregular by volcanoes, fault blocks, and folds which affect the sedimentary cover.

Figure 6 shows a seismic section acquired between Bali and Lombok island by Robert Conrad cruise 11 (Hamilton, 1979). The volcanic ridge is made irregular by volcanoes, fault blocks, and folds which affect the sedimentary cover (Fig. 6). The southern flank of the volcanic ridge is rich of volcanic deposits. A smaller sea bottom high in the north is probably formed by volcanic intrusion (Fig. 7).

Figure 7. Block diagram of the southern part of Lombok Island. The surface is a gradients map of bathymetric data. Gradients are draped on perspective view of bathymetric relief. Trench, outer wedge, slope break and inner wedge are indicated. The sections is modeled based on sea bottom profile (after Krabbenhoeft et al, 2010).

4. Continental shelf edge

The Australian Continental Shelf is located in the southeast of the Lesser-Sunda Islands. The edge of this continent is interpreted to be in the north side of Sumba and Timor Island (Fig. 1, after Harris et al, 2009). Unfortunately the seismic images acquire in these area are either to shallow or too poor to see the edge of the edge of the Australian Continent Shelf.

The Sunda Shelf is located in the northwest of the studied area. A deep seismic section acquired by ION (Fig. 8) helped to understand the margin of the Sunda Shelf. Granath et al (2011) have interpreted the top basement based on this seismic image. The shallow basement in the WNW beneath NSA-1F well (Fig. 8A) and Kangean West-2 (Fig. 8B) is interpreted as the Sunda Shelf.  The deeper basement in the ESE has been interpreted as Late Cretaceous accretionary crust (Doust & Lymbach, 1997). Hamilton, 1979, identifies this area as Tertiary oceanic and arc crust.

Figure 8. Two WNW-ESE seismic lines in the north of the Lesser Sunda Islands showing the potential margin of the Sunda Shelf or Eurasian Continental crust margin. These seismic sections were acquired by ION (Granath et al, 2011). A) Seismic line between NSA-1F and SG P-1 well with significant drop of basement (Horizon A) about 35 km ESE of NSA-1F. An isolated basement high raised about 30 km WNW of SG P-1 well. B) Seismic line between Kangean West-2 and ST Alpha-1. A significant horse-graben system developed in the east of Kangean West-2 which brought the basement (Horizon A) deeper towards the ESE.

Figure 9. Detail sections of the profiles shown in Figure 8. A) located near to NSA-1F and B) located near to Kangean West-2 well. 

5. Flores Basin

A west-east trend normal fault, which is dipping to the south, developed in the north of the Lesser Sunda Islands and formed Flores Basin. The map in figure 1 shows that the water depth in this basin reaches about more than 4000 meters. A seismic section acquired by R. V. Robert Conrad (Fig. 10, Hamilton, 1979) shows a deep trench developed by the fault. Recent sediment accumulation is well imaged in this section at about 6.5 seconds. The Flores Basin is poorly understood as it is deep and covered only by sparse data.

Figure 10. A N-S seismic section from Lamong Doherty Geological Observatory, acquired by R. V. Robert Conrad cruise 11 (Hamilton, 1979). This section shows little sediment on the narrow floor of the Flores Sea or Flores Basin, in contrast to the thick strata on the platform between that sea and the South Makassar Basin which probably consist of carbonate units.

Prasetyo (1992) published a number of seismic lines which cover Flores Basin and discussed the Flores Thrust Zone in great detail. The thrust zone is a prominent E-W oriented structural feature extending from east to the west of the Flores Basin. The fault zone separated south dipping sedimentary sequences, including Paleocene rift and related sediments, from the complex deformed material to the south (Prasetyo, 1992)

6. Sumba Island

The position of the Sumba Island is unique. It is not part of the Sunda arc, which formed a lineation of volcanic islands in the north of Sumba. From the position it may be more related to Timor but it has different orientation (Fig. 1 and 2). The origin of the island is still a debate amongst worker on this area.

The Sumba Island is generally recognized as an exposed fore-arc basement which is located between the Inner and Outer Arc. Several workers have considered Sumba Island as a micro continent within a region of arc-continent collision (Audley-Charles, 1975; Hamilton, 1979), and more recently as accreted terrane (Nur and Ben-Avram, 1982; Howell et al., 1983). De Werff et al (1994) and Harris et al (2009) conclude that the Sumba Island is a continuation of Timor which is an arc-continent collision zone.

Tectonic activities

The Sunda Arc is known as an active convergence zone producing earthquakes, tsunamis and volcanic hazards. The Indo-Australian plate currently moves at 6.7 cm/a in a direction N11^oE off western Java and thus almost normal to the trench (Tregoning et al. 1994). Convergence speed slightly increases from western Java towards the east at a very subtle rate. The movement is reaching 7 cm/a of Bali (Simons et al, 2007) and has been active since Eocene (Hall & Smyth, 2008). The overriding plate is continental including Sumatra and western Java (Kopp et al, 2001) and the basement below the forearc basin offshore Bali and Lombok is probably a rifted crust of a continental character in transition to oceanic character at Sumbawa and further east (Banda Sea, Van der Weff, 1996).

The locations of the earthquake epicenters in the centre part of the Lesser Sunda Island reflect the subduction of the Australian Lithosphere under the Asian continenet (Fig. 11) . The Australian lithosphere, which is interpreted as Precambrian continental crust (Hamilton, 1979) moves northward. This subduction angle is also getting steeper northwards.

Figure 11. This plot shows the earthquake localizations on a South-North cross section for the lat -14°/-4° long 114°/124° quadrant corresponding to the Lesser Sunda Islands region. The localizations are extracted from the USGS database and corresponds to magnitude greater than 4.5 in the 1973-2004 time period (shallow earthquakes with undetermined depth have been omitted.

data source: USGS-NEIC; displayed in http://bigideasroots.wordpress.com/6-1/

Discussion

Two major tectonic discontinuities separate the Banda Arc from the Sunda Arc in this area. The Pantar Fracture extends approximately north-south between the island of Pantar and Alor, and the Sumba Fracture separates Sumba and Flores islands from Sumbawa (Nishimura and Suparka, 1986). Unfortunately the discontinuity of the arc, or the transition from Sunda to Banda arc is not clearly seen on seismic section. Nishimura and Suparka (1986) use ‘fracture’ to describe the separation, which indicates a  small offset and therefore may not be imaged well on seismic sections, especially by older sections

All seismic sections included in this are article were acquired sparsely offshore. Additional data around Sumba Island may improve the understanding of the origin of the island. This may bring the debate of the origin of the island closer to conclusion.

Closing Remarks

The Lesser Sunda Islands are a very active tectonic region, formed by the subduction of the Indian Oceanic plate in the west and a continent-island arc collision in the east.

This area is located between the Eurasian Continental Crust or also known as Sunda Shelf and the Australian Continental Crust or Scott Plateau. The geology in the north of the Lesser Sunda Islands is poorly understood as it is poorly covered by seismic and lack of well penetration.

The west part of the Lesser Sunda Island is generally less complex compared to the east. The transition from Sunda arc to Banda arc, with Sumba and Timor Island in the east make the geology more complicated.

References

Doust, H., & Lijmbach, G., 1997, Charge constraints on the hydrocarbon habitat and development of hydrocarbons systems in Southeast Asia Tertiary Basins, in Proceedings of the Petroleum Systems of SE Asia and Australasia Conference, Indonesian Petroleum Association.

Granath, J. W., Christ, J. M., Emmet, P. A., & Dinkelman, M. G., 2011, Pre-Cenozoic sedimentary section and structure as reflected in the JavaSPANTM crustal-scale PSDM seismic survey, and its implications regarding the basement terranes in the East Java Sea in: Hall, R., Cottam, M. A. &Wilson, M. E. J. (eds) The SE Asian Gateway: History and Tectonics of the Australia–Asia Collision. Geological Society, London, Special Publications, 355, 53–74.

Hamilton, W., 1979, Tectonics of the Indonesian region US Geo. Survey Prof. Pap. 1078, 1-345.

Hutchison, C., 1989, Geological Evolution of South-East Asia, Oxford University Press.

Krabbenhoeft, A., Weinrebe, R. W., Kopp, H., Flueh, E. R., Ladage, S., Papenberg, C., Planert, L., and Djajadihardja, Y., 2010, Bathymetry of the Indonesian Sunda margin-relating morphological features of the upper plate slopes to the location and extent of the seimogenic zone, Nat. Hazards Earth Syste. Sci., 10, p. 1899-1911.

Lüschen, E., Müller, C., Kopp, H., Engels, M., Lutz, R., Planert, L., Shulgin, A., Djajadihardja, Y. S., 2011, Structure, evolution and tectonic activity of the eastern Sunda forearc,Indonesia from marine seismic investigations, Tectonophysics, 508, p. 6-21

Monk, A., de Fretes, Y., Lilley, G. R., 1997, The ecology of Nusa Tenggara & Maluku, Periplus Edition.

Nishimura, S. and Suparka, S., 1986, Tectonic development of east Indonesia, Journal of Southeast Asian Earth Sciences 1, 45-47.

Prasetyo, H., 1992, The Bali-Flores Basin: Geological transition from extensional to subsequent compressional deformation, Proceedings of Indonesian Petroleum Association, 21th Annual Convention

Rigg, J. W. D. & Hall, R., 2012, Neogene development of the Savu Forearc Basin, Indonesia, Marine and Petroleum Geology 32, p. 76-94

Simons, W. J. F., Socquet, A., et al., 2007, A decade of GPS in Southeast Asia: resolving Sundaland motion and boundaries, Journal of Geophysical Research, 112.

Tregoning, P., Brunner, F. K. Et al., 1994, First geodetic measurement of convergence across the Java Trench, Geophysical Research Letters, 21, p. 2135-2138.

Van Weering, T. C. E., Kusnida, D., Tjokrosapoetro, S., Lubis, S., Kridoharto, P. and Munadi, S. (1989) The seismic structure of the Lombok and Savu forearc basins, Indonesia Neth. J. Sea, Res. 24, 251-262

Seram Sea and Bintuni Basin

After Herman Darman & Paul Reemst (Shell International E & P, The Hague), 2012, Berita Sedimentologi #23:

Seismic Expression of Geological Features in Seram Sea: Seram Trough, Missol-Onin Ridge and Sedimentary Basins

Introduction
The Seram Sea (Fig. 1) is located between Seram Island and the Bird’s Head of Papua, Eastern Indonesia. This sea extends to the east towards Bintuni Bay. Some part of the sea, between Seram and Misool are deeper than 2000 m. The Seram Island is mountainous with altitudes reaching 3000 m above sea level at the center of the island.


Figure 1. Regional Geological map of Seram Sea and vicinity, showing the structural elements in this area, outcrops and major faults. Seismic sections discussed in this article is shown in red. Main wells are also displayed as reference.  SR = Sekak Ridge; MOR = Misool-Onin Ridge; KBF = Kepala Burung Foreland Basin.

Several seismic surveys have been conducted to understand the geology of this region. The first seismic sections were published by Hamilton in 1979. These seismic sections were acquired by Western Geophysical for Phillips Petroleum. In 2000, Schlumberger published some seismic lines acquired in 1997 with an improved resolution improvement (Blunden, 2000). More higher quality seismic lines were acquired as part of non-exclusive and multi-client projects in the late 1990’s which provide a better geological understanding of the region and lead to several petroleum exploration opportunities.

This article discusses the seismic expression of several geological features in the Seram Sea vicinity based on published seismic sections. The offshore seismic sections cover part of the imbricated complex in the north of Seram Island, the Seram Trough, the Misool-Onin High and the sedimentary basins the east of the Misool-Onin Ridge, such as the Tamaloi-Malagot Basin, the Semai-Berau Basin and the Bintuni Basin (Fig. 1.)

Misool-Onin Ridge
The Missol-Onin Ridge is a structural high located in the Seram Sea between the island of Seram and the Bird’s Head of Papua. This feature is exposed above the sea as the Misool Island in the nortwest and the Onin Peninsula in the southeast. This high is truncated by the large Sula-Sorong Fault System in the north and the Terera Fault in the south. Both faults are interpreted as sinistral lateral faults (Fig. 1). At present the southern flank of this high forms a steep sea bottom relief towards the Seram Trough but the northern flank has been covered by younger sediments and does not show a significant bathymetric expression.

Several wells have been drilled on the Misool-Onin Ridge. Daram Selatan-1 penetrated the northern part of the Misool-Onin High and TBJ-1X was drilled in the south. Daram Selatan-1 tested a section of more than 1000 m of Triassic age dominated by limestones section (Wongsosantiko & Mertosono, 1996) and TBJ-1X encountered an interval of almost 200 m of Permian clastic and carbonate interval (Fraser et al, 1993).

Seismic section 1 (Fig. 2) located in the western part of the Seram Sea extends from the SW to the NE, and shows the Seram Imbricated Complex, the Seram Trough, the Kepala Burung Foredeep, the Misool Onin Ridge and the Berau Basin. The western part of the Misool-Onin Ridge has been penetrated by the Daram Selatan-1 well. Figure 3 shows a seismic section acquired for Amoseas and processed by Texaco in 1991 (Wongsosantiko and Mertosono, 1996). The structure and stratigraphy is very complex in this area and the seismic is very difficult to interpret as a result of relatively poor data quality. The two sections in figure 3 show the seismic interpretation prior to drilling of well Daram-Selatan-1 and the geological interpretation based on well data such as lithologies and stratigraphic data. The Top Triassic marker was interpreted significantly shallower and the structures are more complex than those in previous interpretations.

Figure 2. Section 1 across Seram Sea showing the seismic expression of the imbricated comples, Seram Trough, Kepala Burung Foredeep Basin, Misool-Onin Ridge and Bintuni Basin. CS-1X and Agung-1 well control occur in the north of the section. This section is modified after Pairault et al, 2003.

Figure 3. A detail seismic section across Daram Selatan-1 well acquired by Amoseas. Two interpretations are displayed: A) prior to the drilling result  and B) after the drilling result. Note the changes of stratigraphic and structures interpretation (after Wongsosantiko & Mertosono, 1996).

The northern part of the Misool-Onin Ridge has been uplifted as indicated by the missing Paleocene-Miocene stratigraphic section . Pliocene-Pleistocene interval covered the whole area. The eastern uplift is also indicated in Section 3 (Figure 4) at the center part of the ridge. Section 1 and 4 (Fig. 5) clearly show an unconformity that cuts through the Paleocene – Miocene interval. Section 3 (Fig. 4; Blunden, 2000), however, does not show any indication of erosion.

Based on stratigraphic reconstruction of Section 1 and 4 we identify at least two major uplift events in the Misool-Onin High area. The first one is a post Triassic age and is followed by a second event of post Cretaceous to Pleistocene time. Both uplift events mainly took place in the northern part of the ridge (Fig. 2 and 4).


Figure 4. Seismic section at the center of Misool-Onin Ridge acquired by Schlumberger Geco-Prakla in cooperation with the government of Indonesia in 1997 (after Blunden, 2000) 

Figure 5. Section 4, modified after Paurault et al (2003). The stratigraphic reconstruction shows a Permian paleo high and a tectonic uplift in the Misool-Onin High which caused the erosion of Tertiary section shown as unconformity in the south of the section. 



Figure 6. Section 5 across Berau Basin, Sekak Ridge and Bintuni Basin. TBE-1X is located on Inanwatan Ridge and Kalitami-1X on Puragi Ridge. These ridges are part of larger Sekak Ridge. 

I
mbricate Complex
The Imbricated Complex north of Seram Island and south of Misool-Onin Ridge is characterized by a highly complex fault system that generates poorly image seismic section. Some of the trust faults have been interpreted by Paurault et al. (2003) in Section 1, 3, 6 and 7 (Fig. 2, 4 and 7). Steeply dipping thrust faults can be seen on Section 6 and 7. The fault system generated a rough sea bottom and as a result mini basins developed between fault blocks that accommodate Pleistoce-Pliocene sediments (Fig. 2)

Blunden (2000) published a detail seismic section of the imbricated complex (Fig. 4). Unfortunately the quality of the seismic is poor. Figure 8 shows a higher quality seismic published by Searcher. The reflectors which cross the structures and almost parallel to the sea bottom indicate potential hydrate layers in this area.

Figure 7. Two seismic sections acquired by Fugro in the south of Misool-Onin Ridge, covering Kepala Burung Foldbelt system, Seram Trough and the imbricated complex in sourth. Both sections are SW-NE trends.  A) SM05-221 section shows 3 large anticlines in the complex. B) SM05-222 covers a larger area of imbricated complex.

Seram Trough
The northwest part of the Seram Trough is relatively narrow compared to the southeast. The deepest part of this trough can reach >2000 m water depth. Recent sediment supply is mainly accommodated the southeastern part of the trough, indicated by flat sea bottom as shown in Fig. 7 and 8. Bright amplitude in the northern part of section 6 (Fig. 8) is interpreted as a slope failure deposit from the Kepala Burung Foreland in the north.

Differences in thickenss of recent sediments deposited in the Seram Trough show that the locus of depocentres changed through time (Fig. 8). In some areas in the south sediments are thicker than the north. This suggests active tectonism and rapid deposition in the area.

Figure 8. A detailed section of Figure 7B showing the potential hydrate layer on the left of the section and the two major anticlines on the right. Potentially some limestone developed in the north of the area as shown on this figure.

Kepala Burung (Birds Head) Foreland
A foreland basin developed between the Seram Trough and the Misool-Onin Ridge. Generally this tectonic unit covers an area with water depths of about 200 to 2000 meters. Section 1 (Fig. 2) shows a structural high in west part of the foreland (Fig. 2). Further east, Section 3 shows a simple dipping foreland. Section 6 (Fig 7.A) shows 3 anticlinal features which developed locally. Just south of this section the anticlines disappear and the largest anticline is faulted (Fig. 7.B).

The structural map in Figure 1 also shows that Section 3 is located in the narrowest foreland area. The foreland developed well in the south of Onin Peninsula.

Berau Basin
The Berau Basin is located north of the Misool-Onin Ridge. Section 1, 3, 4 and 5 dissect this basin. All these sections shows a significant unconformity as a result of a major uplift in the south towards the Misool-Onin Ridge and the north. Another major unconformity is shown in Section 1, below the Oligocene-Paleocene unit. This unconformity mainly occurs in the south, close to the Misool-Onin Ridge.

Wells Agung-1 and CS-1X along Section 1 penetrated Tertiary to Permian sedimentary formations deposited in the northern part of the Berau Basin. In the south of the basin, wells North Onin-1 and Gunung-1 encountered a mainly Cenozoic unit but Gunung-1 also went through Permian clastics and carbonates section at the bottom of the hole (Fraser et al, 1993).

Sekak Ridge
The Sekak Ridge is a large anticlinal feature which separates the Berau Basin from the Bintuni Basin. Section 5 (Fig. 6) shows a seismic profile of this ridge. The ridge has several minor highs, that are penetrated by well TBE-1X and Kalitami-1X. Fraser et al. (1993) called the minor highs Inanwatan and Puragi ridge. Both wells penetrated a Jurassic interval at their deepest levels.TBE-1X found some sandstones with coal fragments and redish color shale indicating low terrestrial influence (Fraser et al, 1993). Kalitami-1 encountered more sands of a similar depositional setting in a Jurassic interval. The Cretaceous interval of both wells are very shally, deposited in an open marine environment.

The northern onshore extension of the Sekak Ridge was penetrated by wells Puragi-1, Tarof-2 and Ayot-1&2. Tarof-2 and Ayot-2 wells also encountered Permian clastics. The Mesozoic interval of Tarof-2 well is dominated by shale, but Ayot-2 well found some limestone. All wells reported the presence of a thick Miocene Kais limestone formation at shallower level.

Bintuni Basin
The sedimentary basin east of the Sekak Ridge is called the Bintuni Basin. This basin contains significant petroleum accumulation as discovered in the Vorwata, Wiriagar, Roabiba and Ofaweri fields. Towards the east, the Bintuni Basin is bounded by the north-south trending Arguni Fault.

Section 5 (Fig. 6) shows a seismic section across the western part of the Bintuni Basin. Several structures developed during the Mesozoic but do not continue into the Cenozoic part of the section.

Kepada Burung Foredeep Basin
A foredeep basin developed between the Misool-Onin High and the Seram Trough. This structural unit is shown in Section 1, 3, 6 and 7. Section 1 (Fig. 2) in the north of this unit indicats a Miocene-Oligocene remnant but it is not calibrated by any well. Section 3 (Fig. 4) shows a relatively steeply dipping Mesozoic interval towards the Seram Trough. Several structures developed in the southern part of this foredeep basin as shown in Section 6 and 7 (Fig. 7). Limestone build ups are potentially developed in the Upper Jurassic interval and generated discontinuous strong seismic reflectors. Tertiary deposits in this area have thin and continuous reflectors typical for distal marine deposits that are usually dominated by fine grained clastics (Fig. 8). Hydrate layers can also be recognized from the detailed seismic section and indicated low (< 0oC) temperature, which is typical for deep water deposits.

Well South Onin-1 drilled in this basin and reported Upper Cretaceous limestones at bottom hole with minor shale interbeds (Fraser et al, 1993). Slightly shallower the well encountered >500 m thick Paleogene limestone. Although there are some gas shows, the well is unfortunately considered as a dry well.

Seram Trough
A deep flat sea bottom characterizes the southern part of the Seram Trough (Fig. 7) indicating a recent sediment fill. In the north, Section 1 (Fig. 2) and Section 3 (Fig. 4) show a narrow trough with limited recent sediment fill. A strong amplitude anomaly in the north of Section 6 (Fig. 8) is an indication of slope failure debris flow deposits came from the north slope of the Kepala Burung Foredeep basin. This section also shows a shift of depocentres as some sediment packages are thicker in the south and some are in the north.

Imbricate Complex
The Imbricate Complex in the south of the study area is generally seismically poorly imaged due to intensive faulting as shown in Fig 2, 4, 7 and 8. The thrust faults are dipping to the south and some of them are seen on the seismic sections, especially at the front end of this structural unit. Figure 4 and 8 show the detail of the faults.

Small depositional centers developed between fault blocks in the southern part, capturing recent sediments supplied from Seram Island (Figure 2). At the sea bottom the active faults generate a rough bathymetry as seen in Figure 4a and Figure 7. Tighter anticlinal features occur as drag folds as a result of the faulting. This implies that the faults are currently still active.

A potential hydrate layer occurs in this area (Fig. 8) as can be interpreted from a reflector parallel to the sea bottom imaged in the south of the section. Several petroleum discoveries were made onshore Seram Island, with the Manusela Jurassic oolithic limestone unit as primary target (see K. Hill article in this volume).

Conclusion
The seismic sections reveal a complex tectono-stratigraphic history of the Seram Sea. Based on the interpretation of several key seismic lines, we propose the following sequence of events:
• Rift related faulting took place over an extensive area during the Permian, followed by partial uplift in the Triassic.
• During the Cenozoic, Paleogene and Miocene limestones developed extensively during this period of time.
• An inversion phase in Late Miocene – Early Pliocene indicated by transpression and folding, and reactivation of older extensional faults. Erosional process developed at the Misool-Onin Ridge during this stage.
• Emplacement of the Imbricate Wedge during Pliocene to Quaternary times.

Figure 9. Structural and depositional reconstruction of Misool-Onin Ridge. Faults are red for normal faults, green for inverted faults and blue for reverse faults. See Figure 5 for location of this section.

Reference
Amiruddin, 2009, A Review on Permian to Triassic Active or Convergent Margin in Southeastern most Gondwanaland: Possibility of Exploration Target for Tin and Hydrocarbon Deposits in the Eastern Indonesia, Jurnal Geologi Indonesia, Vol 4 No. 1, Maret 2009, p. 31-41

Blunden, T. (eds.), 2000, Indonesia 2000, Reservoir Optimization Conference, Schlumberger

Fraser, T. H., Bon, J., Samuel, L., 1993, A New Dynamic Mesozoic Stratigraphy for the West Irian Micro-coninent Indonesia and Its Implications, Proceedings Indonesian Petroleum Association, 22nd Annual Convention.

Hamilton, W., 1979, Tectonics of the Indonesian Region, Geological Survey Professional Paper 1078, US Government Printing Office, Washington.

Wongsosantiko, A. & Mertosono, S., 1996, Peran Teknologi Eksplorasi Mutakhir Sebagai Sarana Penunjang Strategi Bisnis Minyak dan Gas Bumi, Suatu Pengamatan dan Pengalaman di PT Caltex Pacific Indonesia, in Kumpulan Makalah Peran Sumberdaya Geologi Dalam Pembangunan Jangka Panjang II, Dalam Rangka Memperingati HUT Pendidikan Tinggi Teknik ke 50 Yogyakarta.

North Sulawesi Basin

Sulawesi Sea and its surrounding area is an active tectonic region. Many studies were done to understand the tectonics in this area. For this reason, several seismic vintages were acquired in the Sulawesi Sea (Celebes Sea) area. The seismic lines were published in: 1977 by Hinz: single multichannel reflection seismic (MRS) profile 1979 by Hamilton: single trace reflection seismic lines 1987 by Hinz and 1989 by Fechner: seismic data acquired by SONNE-cruise 49 in Mindanao area, at the eastern end of the Celebes Sea Basin. 1994 by Zen and Hinz: seismic data acquired by SONNE-cruise 94, sponsored by German institutions. 1997, by Beiersdorf et al.: seismic data acquired by SONNE-cruise 98 which also did geological and geochemical investigations.

This article shows the seismic expressions in the southern margin of the Sulawesi Sea, which is the subduction zone in the north of Sulawesi North arm and known as North Sulawesi Trench (Fig. 1)

Regional Tectonics

Regional cross section in Figure 2 shows the subduction of Sulawesi Sea oceanic late to the south and goes underneath the northern arm of Sulawesi which is dominated by calc-alkalic potassic (CAK) volcanic material. This section is constructed by Walpesdorf et al. (1998) based on seismic epicenters.

The seismicity of this area is very high and occurs in wide range of depth. Figure 3 shows the distribution of epicenters recorded by USGS. Close to the northern coast of Sulawesi, the seismicity is relatively shallow and it is getting deeper southwards. This evidence supports the regional cross section in Figure 2 is recorded by USGS in the seismicity map in Figure 3.

Fig. 3. USGS seismicity map of the study area

Seismic Sections

RV SONNE vessel cruises 98 acquired regional seismic sections across Sulawesi Sea Basin. One of the north-south sections displayed in Figure 4, shows general deepening of Sulawesi Sea basin to the south. The Miocene horizons clearly go beneath the thrusted zone or the accretionary complex in the north of Sulawesi.

From 2 November 1994 to 14 December 1994, RV SONNE vessel acquired seismic data in the north Sulawesi trench. The survey was chiefed by M. T. Zen (BPPT-Indonesia) and K. Hinz (Germany) who aimed to : Study the structure, the age and the geological evolution of Sulawesi Sea. Study the tectonic framework of the North Sulawesi and Mindanau continental margin which is still active. Understand the mechanism of the formation of the accretionary prism at the back zone of North Sulawesi and the west of Sangihe Island.

Two seismic sections were published as shown in Figure 5. These condensed sections shows relatively low angle of subduction beneath a 60 km accretionary complex. Overall the system has a steep dip subduction

zone. The structures formed part of the sea bottom, which indicates


Present day or recent tectonics activities. These evidences confirm USGS seismicity maps. Sediment supply from the onshore Sulawesi is also active. Line 28 shows an indication of recent sediment accumulation in the south of the section. Line 30 shows a steeper slope in the south of the section which may also caused by sediment supply from onshore Sulawesi.

A detail section of line SO98-28 displayed in Figure 6 shows the oceanic crust which is overlain by Middle Miocene turbidites and Latest-Miocene to Pliocene clastic interval go beneath the accretionary complex in the south. This complex composed of intense south dipping thrust faults. The faults are getting deeper from north to south and at the same time generating more complex features.

Conclusions

The studies in this area conclude that the structures are clearly shown in the front end of the accretionary zone because they are relatively young. Further south the structures are less pronounced as they have gone through more tectonic phases, the reflectors become too complex to be imaged by seismic. With limited and low data resolution, significant structure growth on seismic section which indicates fault timing is not well observed.

Some of the structures also offset the sea bottom, which indicates a relatively new fault or still on-going fault reactivation. This evidence support USGS seismicity map which show recent and active tectonic activities.