* University of Trisakti ** Royal Holloway University of London *** University of Wollongong

IPA17-125-G

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Forty-First Annual Convention & Exhibition, May 2017

EXTENSION IN THE KUMAWA BLOCK, WEST PAPUA, INDONESIA

Ramadhan Adhitama*

Robert Hall** Lloyd T. White***

ABSTRACT The Kumawa and Aru basins are part of a narrow extensional system within the Aru trough in the northern Australian continental margin, located east of the U-shaped Banda Arc–Australia boundary. The study area is bounded by the Misool–Onin–Kumawa Ridge and Lengguru fold belt to the north, the Seram fold and thrust belt to the west, the Aru shelf to the east, and it connects to the Tanimbar trough to the southwest. Interpretation of 2D seismic and multibeam bathymetry data shows the Neogene history of extension. In the west of the study area the Kai Arch is a horst block bounded on its west side by the frontal thrust of the offshore Seram fold and thrust belt. The east side of the study area is bounded by a major normal fault downthrown to the east. The arch is capped by a carbonate platform now at depths of almost 1 km. East of the Kai Arch, there are more than 5 seconds TWT of Upper Miocene to Recent sediments. Structures are dominated by N-S to NNE-SSW-trending normal faults which can be traced SSW into the Tanimbar trough. The normal faults terminate in the north at a WNW-ESE-trending fault, which is partly buried beneath disturbed sediments transported into deeper water from the shelf south of the Lengguru fold belt. The Tarera-Aiduna fault zone is imaged on the seabed and dies out to the west, south of the Lengguru fold belt. It is a young structure with little displacement. Basin formation started during the Late Miocene and the basin underwent several periods of subsidence, marked by multiple unconformities within the syn-extension units. There is petroleum potential with three possible plays: (1) Mesozoic sandstone reservoirs in tilted fault blocks; (2) Cenozoic carbonates at the edge of the Kumawa and Aru basins; (3) Upper Miocene Klasafet Formation in the Seram fold and thrust belt. INTRODUCTION The Kumawa and Aru basins are located within the Aru trough (Figure 1) in the eastern part of Indonesia.

They are close to the collision zone of the Australian continent and the Banda Arc to the east of the Outer Banda Arc islands. The area is located south of the Bird’s Head of West Papua, between the Lengguru fold belt in the north, the Seram fold and thrust belt in the west, and the Aru Islands to the east (Figure 1). The Aru trough narrows to the south towards the Tanimbar trough. There has been little research specifically concerned with the geology of the offshore Kumawa block but there are a few studies of onshore geology in adjacent areas relevant to the Kumawa block (Tanimbar, Bird’s Head, Seram). Geology and palaeomagnetic data (Klootwijk et al., 1987; Thrupp et al., 1987; Pairault et al., 2003) suggest little relative motion during the Cenozoic between the Bird’s Head and the rest of New Guinea, which forms part of the Australian plate. The collision zone of northern Australia and the Banda Arc has a unique concave westward geometry which results from slab rollback processes into the Banda embayment (Spakman and Hall, 2010; Hall, 2012). This lies near the junction of the Pacific, Australian and Eurasian plates. Fold and thrust belts (FTB) around the Banda arc (Seram FTB and Timor-Tanimbar FTB) verge towards the outside of the “U” shape of the arc. The boundary between the Banda Arc and the Australian continent is a complex deformation zone. A series of folds, thrust faults and strike-slip faults dominate within the west part of the zone (e.g. Pairault et al., 2003; Teas et al., 2009), but to the east of the fold and thrust belt zone deep basins formed and there is evidence of major extension in the Aru trough region (e.g. Schlüter and Fritsch, 1985; Jongsma et al., 1989; Charlton et al., 1991; Adhitama et al., 2016). REGIONAL TECTONICS The Kumawa and Aru basins are within the northwest part of the Australian continent with a

well-documented geological history from the Paleozoic (e.g. Visser and Hermes, 1962; Pieters et al., 1983; Fraser et al., 1993). In the early Mesozoic (Triassic–Late Jurassic) various fragments separated from northern Gondwana accompanied by the closing of the Meso-Tethys ocean and opening of the Ceno-Tethys (e.g. Metcalfe, 2013). During this interval the region of the Aru trough experienced extension and rifting. In the Late Jurassic a number of blocks separated from the Australian margin to form the Banda embayment which left the Aru trough within the eastern passive margin of the oceanic embayment which was floored by Jurassic oceanic crust of the proto-Banda Sea (Hall et al., 2009; Hall, 2012). Australia–SE Asia collision began in the Early Miocene when the Sula Spur, a continental promontory north of the Banda embayment, made contact with the North Arm of Sulawesi (Hall, 2002, 2012). During continued northward movement of Australia, the Java Trench became aligned with the northern continent-ocean transition of the Banda embayment and the trench was able to propagate east. The subduction zone rolled back into the embayment along a lithospheric tear (Spakman and Hall, 2010; Hall, 2012). Seram recorded dramatic extension during the Banda rollback, exhuming the upper mantle, causing ultra-high temperature metamorphism, and melting continental crust of the embayment’s northern margin (Pownall et al., 2013, 2014; Pownall and Hall, 2014). The final stages of rollback formed the Weber Deep (Hinschberger et al., 2003; Pownall et al., 2016) to the west of the Aru trough and the Seram FTB. Collision processes and convergence are still active (Stevens et al., 2002; Bock et al., 2003). The multibeam and seismic data (Figures 2 and 3) offer the opportunity to assess the roles of extension and collision on the development of the Aru trough region, particularly during the Neogene when subduction rollback from the west began to impact the eastern Banda Arc. DATA AND METHODOLOGY A 2D time-migrated seismic dataset was provided by TGS together with a multibeam bathymetry. The seismic dataset included forty-nine lines in total: 33 dip-lines (W-E) with an overall spacing of 5 km, 7 strike-lines (N-S) with an average spacing of 25 km, and 9 oblique regional lines (Figure 3). Five horizons were interpreted. Kingdom 8.8 and Opendtect 6.0.5 was used to interpret the seismic sections, ArcGIS 10.2 and QGis 2.12.1 were used to interpret the seafloor structure. Acquisition information concerning the multibeam bathymetry provided by

TGS is given in Teas et al. (2009). After the project was completed additional multibeam bathymetry was provided by GeoData Ventures Pte. (Figure 2) which was used to trace the major faults identified during the project further to the south and east. SEISMIC STRATIGRAPHY The Kumawa and Aru basins are stratigraphically closely related, and similar, to the northern Australian continent, especially West Papua. Sedimentary rocks range in age from Paleozoic to Cenozoic. Numerous researchers have studied the sedimentary rocks of West Papua, including Visser and Hermes (1962), Dow and Ratman (1981), Pigram and Panggabean (1981), Pigram et al. (1982), Pieters et al. (1983), Fraser et al. (1993), Pairault et al. (2003), Riadini et al. (2010) and Sapiie et al. (2012). Here, the names of geological formations are adopted from the work of Pairault et al. (2003), which largely followed Fraser et al. (1993). Five seismic units were used (Figure 4). Unit 1 includes all lithologies older than Late Cretaceous and Unit 2 corresponds to the Jass Polysequence of Fraser et al. (1993). Unit 3 is the New Guinea Limestone, at the top of which is a major unconformity. This is overlain by Units 4 and 5 which are equivalent to the Klasaman and Klasafet Formations of Late Miocene and Plio-Pleistocene age. A simplified chronostratigraphic chart of the Kumawa and Aru basins is shown in Figure 4. The pre-kinematic units consist of metamorphic rocks, and sedimentary rocks such as the Jass Polysequence and New Guinea Limestone (Figure 3). Based on the seismic interpretation, formation of the Kumawa and Aru basins began approximately at the beginning of the Late Miocene after deposition of Unit 3 (equivalent to the New Guinea Limestone). During subsidence, syn-kinematic units (Unit 4 and Unit 5), equivalent to Klasafet (Late Miocene) and Klasaman Formation (Pliocene – Recent), were deposited. STRUCTURAL STYLES The multibeam images were used to define the structural elements of the sea bed of the Aru trough. Morphologically the study region can be separated into a number of parts (Figure 5). To the west of the approximately N-S trending Kai Arch is the Seram FTB. To the east of the Kai Arch is the Aru trough which has two deep sub-basins in it: a northern area of the Kumawa basin and to the east the Aru basin. The northeastern part of the area slopes towards these two sub-basins from a shallow shelf, termed here the Lengguru shelf. Young and probably ongoing

processes of deformation are recorded by structures visible on the seabed. Seismic sections were used to validate the structures seen on the seabed and investigate their significance at depth. In the Kumawa and Aru basins the dominant structures are normal faults, although minor contractional structures are also observed within some areas (Figure 5). Kai Arch The Kai Arch is bounded by normal faults on its east side (Figures 5 and 6) and a narrow N-S trough marking the thrust front of the Seram FTB on its west side (see below). At its northern end the arch is capped by an almost flat surface at approximately 1 km depth. We interpret this as a carbonate platform which is approximately 30 km long in a N-S direction and about 6 km wide (Figures 5 and 9). The platform top is deeper in the north at about 1100 m and shallower in the south at about 900 m; it also dips at a low angle to the west. The morphology of the top of the Kai Arch suggests the narrow platform formed above an earlier larger platform which is approximately 36 km long in a N-S direction and about 12-14 km wide. This formed above an erosional unconformity (Figure 7) and rests directly on Unit 3, the New Guinea Limestone. The ages of the carbonate platforms are unknown, except that they postdate the New Guinea Limestone, but we suggest they are probably of similar age to the carbonate platform tilted into the Seram trough further west (Patria and Hall, 2017) which is Late Pliocene to Pleistocene in age. The unconformity is interpreted as the intra- Pliocene unconformity identified by Pairault et al. (2003). The subsidence of the Kai Arch carbonate platforms is attributed to loading by east-directed thrusting of the Seram FTB. NNE-SSW normal faults On the east side of the Kai Arch is a series of linked normal faults dipping steeply east and trending NNE-SSW to NNW-SSE (Figure 5). These and numerous other normal faults with broadly NNE-SSW strike directions are the dominant structures in the block, and are distributed over almost all of the area (Figure 6). These faults cut through Paleozoic to Cenozoic rocks. The faults are hard linked at the basin edges on the west and east sides creating basin-bounding faults, defining the Aru trough which widens northwards. Some of the large faults are slightly curved (Figure 6). Many of them are segmented, dipping to the east and to the west, creating horst and graben structures (Figure 7).

NW-SE to WNW-ESE normal faults A NW-SE to WNW-ESE fault system forms the northernmost border fault of the Kumawa basin. It separates the Kumawa basin from Adi Island, and the southern margin of the Misool–Onin–Kumawa Ridge (Fraser et al. 1993; Pairault et al., 2003; Decker et al., 2009; Sapin et al., 2009) in the Kumawa Peninsula. This fault system bounds the NE side of the Kumawa basin and is seen clearly on the Top Unit 3 time-structure map (Figure 6).Adi Island consists of Cenozoic marine and terrestrial clastic sediments, mainly fine-grained shales as well as coal; while the Kumawa Peninsula consists of Neogene reef facies and bioclastic limestones. The bathymetry of the slope descending to the Kumawa and Aru basins shown by the multibeam images suggests this fault continues WNW-ESE beneath the disturbed surface of the slope towards the east side of the Aru trough (Figures 2 and 5), but the quality of the seismic below this slope is inadequate to identify structures. Gravity-driven fold structures Fold and thrust structures interpreted as gravity-driven are present in the eastern parts of the Kumawa basin. The axial traces of the folds trend NW-SE (Figure 5). The structures are confined to the late syn-extension unit (Unit 5) which has its maximum thickness in the Kumawa basin (Figures 7 and 8). In the southern part of the Kumawa basin the folds have an en echelon configuration. The distribution of the folds and the bathymetry of the Kumawa and Aru basins suggests that the basin had a deep triangular shape, narrowing southwards from the WNW-ESE northern faulted margin. Rapid subsidence is suggested to have led to failure of the former southern Lengguru shelf and its upper slope which moved downslope into the deeper basin towards the SSW. Folds formed with axes parallel to the strike of the slope. The present morphology of the upper surface of Unit 5 in the Kumawa basin (Figure 7) is seen most clearly on 3D images of the multibeam bathymetry (Figure 9). The irregular morphology is interpreted as the result of a mass transport complex of wet overpressured sediments moving downslope towards the SSW. Mud diapirism is also suggested by small volcanic shapes, pock marks and other features on the seabed that indicate fluid escape. Fold and thrust belt On the west side of the Kai Arch is a narrow trough oriented roughly N-S (Figure 5). This is the trace of the thrust front of the Seram FTB in the western part

of the area. In the study area there are sets of thrusts and folds with axial traces that strike N-S. These form the eastern part of the Seram FTB and extend to the west of the Kai Arch where the Seram FTB finishes abruptly. The N-S trough links to a WNW-ESE trending structure in the north just beyond the study area (Figure 2). In the study area the thrust front has previously been traced by different authors on both the west and east sides of the Kai Arch. The multibeam bathymetry and seismic lines show clearly that thrust front is traceable southwards on the west side of the Kai Arch, and out of the study area to the south between the islands of Kai Kecil and Kai Besar, as previously shown by Charlton et al. (1991). The steep feature on the east side of the Kai Arch is an east-dipping normal fault, not a thrust (Figures 5, 6 and 7). The seismic lines crossing the eastern part of the Seram FTB and thrust front show that deformation resembles a sequence of thin skinned imbricate fans (Figure 7). The multibeam images (Figures 2, 5 and 9) also display numerous structures that indicate widespread and young mud volcanism as reported from the Kai Islands (Charlton et al., 1991). Tarera-Aiduna strike-slip fault system The sinistral Tarera-Aiduna strike-slip fault system is well known in eastern Indonesia. The fault is often shown as a single fault which trends E-W south of the Bird’s Head from the western end of the Central Ranges. It is commonly projected to join the Seram trough, or even to cut it and continue westwards into the Banda Sea (e.g. Hamilton, 1979; Silver et al., 1985; Linthout et al., 1991, 1997; Charlton et al., 1991). The fault zone has only minor expression on land (Hamilton, 1979; Katili, 1986). Teas et al. (2009) mapped the southern strand of the fault zone and showed its sinistral character using multibeam data in the study area (Figure 9). Based on seafloor bathymetry and seismic observations at the northern side of the basin the southern strand of the Tarera-Aiduna fault zone cuts the Lengguru shelf and has a small normal offset downthrown in different places to both south and north. This, with features seen on multibeam bathymetry images, supports the interpreted sinistral strike-slip displacement and suggest the fault is locally transtensional and transpressional. This E-W strand of the Tarera-Aiduna fault zone does not reach the Seram trough or cut the deformation front; its orientation and position suggest it may change direction or terminate near Adi Island (Figure 5). The northern strand mapped on land must continue

westwards offshore south of the Bird’s Head coast in an area not covered by the multibeam data. This fault strand may terminate in the southern end of the Lengguru fold belt although one N-S seismic line shows a probable E-W fault NE of Adi Island which could be this strand. Age and effect of faulting Unit 3 (New Guinea Limestone) has a similar thickness across most of the study area except beneath the young carbonate platform capping the Kai Arch where it is thinner below an erosional unconformity (Figure 7). Most of the normal faults cut Unit 3 and many terminate at the unconformity. Some normal faults elsewhere in the basins appear to terminate at the base of Unit 4, whereas others cut Unit 4, and several important faults cut Unit 5 and reach the seabed. Unit 5 thickens significantly to up to 5 seconds TWT in several graben (Figure 8) within the Kumawa and Aru basins. The downslope movement that produced the irregular surface of the mass transport complex and gravity-driven folds indicates rapid young subsidence. The Aru trough is the site of young seismicity and CMT solutions indicate present-day E-W extension. These observations indicate the phase of broadly E-W extension that produced the Aru trough began after Unit 3, was active during deposition of Units 4 and 5, and continues at present. It is possible that extensional faults have reactivated older structures in Australian margin as implied by maps in Charlton et al. (1991) but it is not possible to resolve this from the seismic lines. The extensional features terminate south of the Kumawa Peninsula and Lengguru shelf. To the north are the Misool–Onin–Kumawa Ridge and Lengguru fold belt. We suggest that the NW-SE to WNW-ESE fault system identified above is the main northernmost border fault of the Aru trough and can be mapped westwards towards Seram. The E-W Tarera-Aiduna fault zone is further north, relatively young and less important. It may be taking over from the WNW-ESE fault system. A simple sequential 2D palinspastic restoration of two E-W lines was carried out to assess extension in the study area using Midland Valley Move software. Compaction was ignored in the bedding restoration. Figure 10 summarises the results for the extended region east of a pin on the Kai Arch. A Late Miocene–Pliocene phase caused 4.1 km (3%) extension on the northern line and 2.4 km (4.4%) on the southern line. A Pliocene–Recent phase caused an additional 15.1 km (14.2%) extension on the

northern line and 7.0 km (17%) on the southern line. The total extension estimated is 19.2 km and 9.41 km on the northern and southern lines respectively. PETROLEUM POTENTIAL The Kumawa and Aru basins contain a maximum of 7 seconds of Upper Neogene sedimentary rocks. The thickness and extreme depth of the basin increases the chance of young sedimentary rocks entering the oil and gas window. Significant subsidence in the late stages of basin formation mean traps formed long before oil and gas expulsion. Potential source rocks are Mesozoic to Early Pliocene in age. The similarity of the geology to the north (Salawati and Bintuni basins) and to the south (Tanimbar area) suggests three possible petroleum plays within the Aru trough (Figure 11): (1) Mesozoic sandstone in tilted fault blocks, (2) Tertiary carbonates at the periphery of Kumawa and Aru basins, (3) Upper Miocene Klasafet Formation in the Seram fold and thrust belt. Play 1: Mesozoic sandstone in tilted fault blocks The Mesozoic play in the Kumawa block is basically the petroleum play proposed by Pertamina BPPKA (1996) and Roberts et al. (2011) for the Tanimbar area. This suggests an oil and gas kitchen under the Seram FTB, with subsidence and flexure caused by thrust loading. Hydrocarbons may have migrated from the kitchen towards the east, into Mesozoic tilted fault blocks near the Kai Arch horst. Hydrocarbons could also have been generated in the Kumawa and Aru basins. Active subsidence on the major faults bounding the basin will increase the chance of the faults acting as conduit to the Kai Arch. Potential reservoir rocks are sandstones of the Sebyar polysequence, Roabiba polysequence, and Inanwatan polysequence (Fraser et al., 1993). The potential seals for the sandstone reservoirs are the shale-dominated Cretaceous–Paleocene Jass polysequence and possible intra–Mesozoic seals within other polysequences. Play 2: Tertiary carbonates at the periphery of Kumawa–Aru basin Potential source rocks range in age from Mesozoic to Late Cenozoic. Mesozoic sources could be important near the southern boundary of the Kumawa basin, where subsidence is less than in the northern area. Mesozoic sources in the northern and central Kumawa basin are likely to be over-mature, but sources could include Upper Miocene to Pliocene

sedimentary rocks of Units 4 and 5 (Klasafet and Klasaman Formations). The main reservoir unit for this play is the New Guinea Limestone Group. Possible traps include three-way dip closure anticlines cut by normal faults. The folds were produced by normal faulting during the early stage of subsidence of the Kumawa basin (Late Miocene). The potential seal is the shale-dominated Lower Klasafet Formation. Play: 3 Upper Miocene Klasafet Formation in the Seram FTB For this play an oil and gas kitchen is located under the Seram fold and thrust belt, like play 1. There is no influence from the Kumawa basin. Potential source rocks are Mesozoic shales, reservoirs are sandstones, and potential seals are intra-Klasafet Formation shales. Possible traps are anticlines related to the formation of the Seram fold and thrust belt. CONCLUSIONS The Kumawa and Aru basins contain up to 7 seconds of Upper Neogene sedimentary rocks. The basins are dominated by NNE-SSW trending normal faults. Significant extension began in the Late Miocene and major subsidence occurred between the Pliocene and present day. A NW-SE to WNW-ESE fault system forms the northern border of the Kumawa basin. The E-W Tarera-Aiduna fault zone is relatively young and less important. Mass transport complexes and folding are gravity-driven structures produced by rapid Pleistocene subsidence of the basin which exceeded sediment supply. The thrust front of the Seram fold and thrust belt is on the west side of the Kai Arch not on the west side of the Aru trough. There is petroleum potential with three possible plays: (1) Mesozoic sandstone reservoirs in tilted fault blocks; (2) Cenozoic carbonates at the periphery of Kumawa and Aru basins; (3) Upper Miocene Klasafet Formation in the Seram fold and thrust belt. ACKNOWLEDGEMENTS We thank TGS for providing the multibeam and 2D seismic data for this study. We also thank GeoData Ventures Pte. Ltd. for additional multibeam data. The consortium of oil companies who support the Southeast Asia Research Group funded the MSc of Ramadhan Adhitama at Royal Holloway University of London. We are grateful to colleagues in the SEARG for discussion.

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Figure 1 - Tectonic elements of eastern Indonesia (modified from Hall, 2012). The study area is marked by

the red box. Faults are marked by solid black lines; subduction zones are indicated by toothed black lines; dashed black lines mark the troughs around the Banda Arc. LFB: Lengguru fold belt; SFTB: Seram fold and thrust belt.

Figure 2 - Multibeam bathymetry coverage of the Aru trough with principal features named. The study area is outlined in black.

Figure 3 - Overview of the seismic dataset used in this study.

Figure 4 - Summary of seismic stratigraphic units used in this study for the Seram FTB, Kai Arch and the Kumawa and Aru basins.

Figure 5 - Major faults and folds in the study area shown on the coloured multibeam bathymetry map with

simplified structural summary.

Figure 6 - Time-structure map of Top Unit 3 (New Guinea Limestone) showing major faults mapped from 2D seismic lines.

Figure 7 - Interpreted N-S and E-W seismic sections crossing the study area with details of the Kai Arch.

Figure 8 - Time-thickness map for Unit 5.

Figure 9 - Multibeam bathymetry map of the study area in centre, with 3D images of bathymetry above and below showing Kumawa and Aru basins, mass transport complex interpreted to have moved SSW, and details of the Kai Arch carbonate platform and the Tarera-Aiduna fault zone.

Figure 10 - Summary of 2D palinspastic restoration of two E-W lines using Midland Valley Move software to assess timing and amount of extension in the study area.

Figure 11 - Hydrocarbon potential of the Kumawa and Aru basins.