PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Fortieth Annual Convention & Exhibition, May 2016

IPA16-546-G PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Fortieth Annual Convention & Exhibition, May 2016 BASIN AND PETROLEUM SYSTEM MODELING OF OFFSHORE TANIMBAR REGION: IMPLICATIONS OF STRUCTURAL DEVELOPMENT HISTORY Afif Saputra* Michio Ohara* ABSTRACT Basin and petroleum system modeling is one of the most powerful tools to understand the implication of basin evolution to the hydrocarbon generation, migration and accumulation. The modelling can reduce the risk in oil and gas exploration especially in the frontier area such as in the offshore Tanimbar region. To build a geologically plausible model of petroleum system in this region, tectonic development history integrated with source rock evaluation is the key factor to understand the hydrocarbon distribution. 1D basin modelling in several key locations was performed to understand the maturity history throughout the study area. The study area is located in the offshore Tanimbar region and Northern Bonaparte Basin. The basin has a complex development history since the Mesozoic rift system followed by the Late Neogene collision between Banda Arc and Australian margin. The oil and gas occurrences in the Southern Banda Outer Arc on the Australian passive margin indicates the presence of another petroleum province, although the Northern Bonaparte Basin is considered as a mature gas prone petroleum province such as Abadi, Sunrise-Troubadour and Lynedoch gas fields. This paper illustrates the contribution of Masela Deep and Malita Graben kitchens for Abadi gas field, in contrast, offshore Tanimbar region mostly charged by the potential northern kitchen. The differences of hydrocarbon province between both areas are considered to be explained according to differences in thermal history and thrust fault activity through the Late Mesozoic to the Quaternary as well as variation of source rock type. Although uncertainties are remaining to be considered since the area is frontier and its complicated tectonic development history, the study implies the not well known play in Banda Outer Arc. Keywords: Banda Arc, Basin Modeling, Tectonic, Tanimbar INTRODUCTION The history of basin development is very important to understand the hydrocarbon generation and migration in a certain area. A particular tectonic setting has its own unique of petroleum system concept. Eastern Indonesia is one of the most complex tectonic histories which resulted a complex source rock maturity history as well. Several authors reported and discussed the proven and potential play concept in the Eastern Indonesia (Satyana, 2014; Sumantri and Syahbuddin, 1994; Kartaadiputra and Samuel, 1988; Barber et al., 2003; Roberts et al., 2011). Satyana (2014) identified that the play concepts are very dependent on and controlled by the geology evolution of the region. Basin modeling is one of a useful tool to get the understanding of the proven petroleum system together with the underexplore play concept. This paper presented one of possible modeling which could explain the petroleum generation, migration, and accumulation in the offshore Tanimbar region as well as simulated the differences of maturity history throughout the Northern Bonaparte graben system and the Banda Outer Arc fold-thrust belt (FTB) area. Geological Background The offshore Tanimbar region is located in the south eastern part of Indonesia which bounded by Australia territorial to the south. It is consist of Babar Selaru Block at the northern part and Abadi field at the southern part. Geologically, offshore Tanimbar region is located in the Banda Outer Arc, Timor-Tanimbar Trough and the Northern Bonaparte Basin (Figure 1). * INPEX Corporation

Ohara et al. (2015) presented the detail tectonic setting of the offshore Tanimbar region. It is consist of continental passive margin sequences at the Northern Bonaparte Basin and FTB at the Southern Banda Outer Arc. Ohara et al. (2015) described the tectonostratigraphic of this region particularly from Mesozoic to Recent based on the regional seismic interpretation (Figure 2). The Figure 3a & b shows the regional cross section through the study area in dip and strike direction. It illustrate the structural configuration of the study area. The Mesozoic Rift is represented by normal fault cut through the Mesozoic (Figure 3a) which later on some of this fault were reactivated during the Plate bending at the Plio Pleistocene due to collision effect. The sediment depocenter is shifted from the southern/south eastern of Abadi Field at the Cretaceous to north/northeast of Abadi Field at the Paleogene. Figure 4 illustrates the 3D diagram of key event of basin evolution from the Late Cretaceous to recent at the study area. Mesozoic The Mesozoic is very important time for this region because most of the major source rock and the reservoir is deposited during this time. Regionally, rifting occurred along the northwest Australia (Longley, 2002) at the Jurassic, it caused the activation of Masela Deep and Calder Graben in the offshore Tanimbar area (Ohara et al., 2015). The sediment was concentrated on the sag area within the graben and Babar Selaru block area was located on the structural high (Ohara et al., 2015). Ohara et al. (2015) suggested the culmination of Calder and Malita Graben at the Late Cretaceous based on regional seismic interpretation. The Malita Graben were become major depocenter at this time (Ohara et al., 2015; Cadman and Temple, 2004). Paleogene There is no fault activity has been reported on the previous author at the Paleogene. However, depocenter was shifted to Masela Deep (Ohara et al., 2015). This region is mostly located on the passive margin at this time (Longley, 2002). The sediment deposited were comprises of prograding carbonate of Johnson, Hibernia, and Cartier Formations. Neogene Present-day Tectonic reconstruction from Hall (2012) illustrates how complex the plate movement at the Neogene in the Eastern Indonesia. It is relating three major plates boundary interaction between Eurasian, Pacific, and Australian Plate. In the Miocene period, in offshore Tanimbar area is relatively stable which marked by the development of carbonate and no significant thickness change (Ohara et al., 2015). Perdana et al. (2016) introduces the development of carbonate within the area. A collision between Banda Arc and Australian Continent resulted FTB in the Banda Arc, Timor- Tanimbar Trough and reactivation of Mesozoic normal fault in the south on the Australian continental margin (Ohara et al., 2015). Petroleum System Modeling The petroleum system modeling combines the basin history based on tectonic and stratigraphy, source rock properties, and boundary conditions. The simulation is in 1D modeling located in the Masela Deep and surrounding of Abadi field area (Figure 1) to figure out the maturity history, hydrocarbon generation and expulsion within the offshore Tanimbar region. Total 21 of 1D basin modeling location were simulated. The modeling result is taken into one from many possible scenarios due to some uncertainty of parameters such as heat flow estimation, rock properties, and source rock kinetics. However, it can be utilized as consideration to explain the proven petroleum system established in this area and future potential of exploration activity in this region. Stratigraphy The stratigraphy data were utilized from geology interpretation based on regional 2D vintage seismic and INPEX Babar Selaru broadband 3D seismic data which is tied to Abadi wells (Figure 5). The lithology assignment is guided by the interpretation of paleo-depositional environment. The rock properties input is based on general known as default including thermal conductivity, radiogenic heat, heat capacity, mechanical-chemical compaction. The interpretation quality in the southern part of study area is relatively good but there is a difficulty in the northern part (thrust area) of 3D seismic area due to some unclear reflectors. It increases the seismic interpretation and stratigraphic unit uncertainties. Source Rock and Geochemical Characterization

Regionally, there are several possible source rock levels within the area. Marine shales of Plover Formation equivalent is considered as the main source rock of Abadi Field (Nagura et al., 2003). Calder-Malita Graben and Masela Deep were suggested to be the main kitchen (Nagura et al., 2003). Based on the internal evaluation, Type III kerogen is predominantly plotted on the Van Krevelen diagram. The marine shale source rock is influenced by the terrestrial input. The second potential source rock that has been considered in this model is the Triassic Aitutu Formation equivalent which is reported by Charlton (2002). Oil occurrences in the Banda Arc area prove that oil prone petroleum system is established in this region (Charlton, 2002). Charlton (2002) cited an old paper from Netherland ('t Hoen and van Es, 1928) which describing source rock properties of the Aitutu Formation from the West Timor (Timor Island). The TOC of Aitutu Formation on the sample location is 23.3%. He also cited Robertson (1998) source rock evaluation which resulted up to 8% TOC and HI 396 mghc/g TOC from the same area. Oil family analysis was conducted by JOGMEC-INPEX (2011) indicating that the oil stains from Tanimbar Archipelago are sourced by Pre-Jurassic source rock. Boundary Condition Model boundary conditions include paleo-water depth, sediment-water interface temperature, and heat flow. The paleo-water depth is based on the paleo environment analysis guided by Abadi well and seismic interpretation. Paleo Water Depth and Sediment-water Interface Temperature The paleo water depth is defined by geological interpretation based on Abadi well geological report and seismic interpretation. Generally, the bathymetry is started around outer neritic at the Triassic (Pertamina-Core Laboratories, 1995) and followed by shallow marine at the Jurassic (Aswan et al., 2012). The deep water facies were developed as thermal subsidence at the Cretaceous. At the Paleogene, the water depth is started to fall and getting shallower up to the Miocene. The thrust uplifted the northern area, however, the southern area (Timor-Tanimbar Trough) was rapidly deepening due to plate bending (Ohara et al., 2015). Present-day sediment-water interface temperature (SWIT) data is available in Abadi wells. The seabed temperatures are from wireline log and DST data. Temperatures at the sediment-water interface through time were calculated within PetroMod by an algorithm (Wygrala, 1990) that relates mean paleosurface temperature and geologic age as a function of plate tectonic reconstructions at presentday latitudes and depth (Hantschel and Kauerauf, 2009). Heat Flow Heat flow is critical for the thermal modeling. It is determined the evolution of the thermal history of the basin and the organic maturity. The present-day heat flow was calculated from the regional well in surrounding area and the paleo-heat flow was assumed and estimated by tectonic history which is based on Allen and Allen (2005). The present-day heat flow in the Abadi area is around 60 65 mw/m2 while to trough area, the heat flow is relatively low due to the crustal thickening of thrust formation as shown on Figure 1. The Figure 6 shows the paleo heat flow estimation from Allen and Allen (2005) based on the tectonic history. There are some paleo heat flow scenarios that have been tested which are calibrated by the well data from Abadi. The paleo heat flow estimation which presented in this paper is one of most likely case based on the calibration in Abadi Well data. The heat flow is relatively high during the Mesozoic rifting at around 80 s mw/m2 and decreasing gradually as thermal subsidence occurred at the Cretaceous to Paleogene (50-60 mw/m2). The heat flow is interpreted to increase again at the Late Miocene-Pliocene since the reactivation of Mesozoic normal fault due to Australian Plate bending. TGS-NOPEC (2008) reported the presentday heat flow at the offshore Tanimbar areas ranging from 30.2 63.1 mw/m 2. This quite wide range is taken as the uncertainty of the model. Modeled Maturity Calibration and Analysis The maturity modeled result calibrated against the measured maturity data from Abadi Wells (Figure 7). In the Cretaceous section, the Ro data is shifted become low values which are suspected due to less vitrinite maceral component since the Wangarlu Formation is deposited in fully marine environment. Therefore, the calibration is focusing on the Jurassic Plover Formation which has type III kerogen which is more valid Ro value.

The modeled result indicate differences in maturity history (Figure 8) which might control by burial history differences spatially and heat flow variation throughout the area. The sediment thickness is varying throughout the Masela Deep, Abadi field, and Babar Selaru Region. At the thrust area is more complicated than Abadi area due to stratigraphy duplication by thrust. The thin skinned type of thrust deformation caused thicker overburden due to stratigraphy duplication. On the other hand, the Jurassic and Triassic sequences are uplifted. It means that there are possibilities of high-grade maturity for below the decollement source rock but retain maturity for uplifted source rock. Therefore, the restored section becomes a key to understand the present-day maturity of source rock in the thrust area (Figure 9a). The paleo 1D model using restored section resulted from 0.7 0.85%Ro for Jurassic source rock and 0.85 1%Ro for Triassic source rock (Figure 9b). Before the thrust occurred, the Jurassic source rock in Babar Selaru region is relatively less mature than Abadi and Masela Deep area. The temperature and Ro history shows the overlapping at the Late Cretaceous. The significant thickness differences caused the rate of temperature and Ro in Abadi increase much than Babar Selaru region (Figure 9c& d). The Babar Selaru region is relatively in topographically high at the Cretaceous with less sediment thickness compare to Abadi field area (Figure 10a). While at the Paleogene and Neogene the sediment was deposited northward (Figure 10b). Figure 11 illustrates the evolution of the Lower Jurassic source rock from the Late Cretaceous to the present-day. The maturity map is made by extrapolation of 1D basin modeling which is guided by overburden map through time. The overburden map is created from restored depth map based on geological interpretation assumption. The first active kitchen area was located in Masela Deep/Calder Graben at the Late Cretaceous while Abadi Field and Babar Selaru Block are still immature. The Jurassic source rock at the deepest part of the Masela Deep reached the wet gas window. At the Oligocene, the sediment depocenter was shifted northward and start to increase the maturity level of Jurassic source rock in the Abadi field. It was continued up to Late Miocene, where Jurassic source rock was at the oil window all over the Abadi Field area. Just before the thrust occurred (~2Ma), Jurassic source rock at the Masela Deep reached the dry gas window and some part of southern Abadi Field already in the wet gas window. Meanwhile, the Abadi Field and Babar Selaru area are entering the Middle to late oil window stage. The Jurassic source rock in Masela Deep was fully transformed into dry gas window in all over the graben and the wet gas window cover all area of southern Abadi field at the present-day. Generation, Migration, and Accumulation The Masela Deep/Calder Graben is the earliest kitchen pod that entering gas window at around the Cretaceous. The gas generation in Masela Deep/Calder Graben was not that significant during the time. This condition is consistence until at around the Miocene. At the Late Miocene, most of Masela Deep and southern of Abadi field is already in the gas window. At this time, the Abadi structure is not formed yet. Based on the restored paleo depth map of Plover Formation equivalent, the Abadi structure is formed at the very young around Pliocene - Pleistocene as part of plate bending due to Banda Arc Australian Continent Collision. The Pleistocene to Recent is the crucial timing of migration accumulation in the Abadi Field. The big fault of Masela Deep is seemed likely to be sealed since the offset is significant (Figure 3a). Therefore, the most likely migration pathway for Abadi is from the south or south east where the fault offset is decreasing (Figure 12). In the Babar Selaru area, the generation and expulsion are poorly known due to the complexity of thrust fault. However, before the thrust occurred in the Jurassic and Triassic is still in the oil window. Therefore, there is a big chance to have oil generation in the uplifted sourced rock. The thrust fault could be a good way for vertical migration during the activity. Ohara et al. (2016) reported the indication of thermogenic hydrocarbon occurrences from seabed as a prove of hydrocarbon migration through thrust fault. Meanwhile lateral migration is also can be expected within any porous reservoir potential in the Babar Selaru region as Ohara et al. (2016) proposed. CONCLUSIONS The source rock evaluation resulted possible Type II source rock in the northern area of Babar Selaru block as outcropped in Banda Arc area. It is as old as Late Triassic which deposited in the marine environment. It is also supported by oil seepages in Timor Island and oil stain in Tanimbar Island which is coming from expected source rock. Based on 1D basin modeling, Triassic and Jurassic source rocks are still remain in the oil window in

the Babar Selaru region before the thrust occurred and there is a possibility of oil generation up to date at the uplifted sequence of Triassic and Jurassic source rock. Based on the maturity history and migration simulation, The Abadi field is most likely charged from the Masela Deep and/or Calder Graben during Late Neogene and possibly to date. Based on the 1D simulation for entire area in offshore Tanimbar Region, the structural development and depositional history is the main factor to investigate the maturity history regionally. The migration scenario in thrust complex is recommended to be modeled in 3D sense because the deformation history is complex. ACKNOWLEDGEMENTS We would like to thank MIGAS, SKKMIGAS, our partner Pertamina Hulu Energi and INPEX Corporation for their support and permission to publish this paper. The authors thank JOGMEC for their support in conducting the geological and geophysical surveys onshore and offshore Tanimbar Archipelago. The authors thank TGS-NOPEC Geophysical Company for permission to publish proprietary information. We also would like to thank Mr. Matsui, Mr. Inaba and Exploration geoscientists in INPEX Jakarta for their support and useful discussions. REFERENCES Allen, P. A., and Allen, J. R., 2005, Basin Analysis, Principles and Applications, 2 nd Edition, Blackwell Science Ltd. Aswan, Zaim, Y., Kihara, K., Hadianto, K., 2012, Depositional Facies of Plover Formation in the Abadi Field, Eastern Indonesia Based on Core Sedimentology, AAPG International Convention and Exhibition, Extended abstract, Singapore, 16 19 September 2012 Barber, P. M., Carter, P. A., Fraser, T. H., Baillie, P. W., and Myers, K., 2003, Paleozoic and Mesozoic Petroleum System in The Timor and Arafura Seas, Eastern Indonesia, IPA03-G-169, Proceeding Indonesian Petroleum Association, 29 th Annual Convention and Exhibition Cadman, S. J., and Temple, P. R., 2004, Bonaparte Basin, NT, WA, AC & JPDA, Australian Petroleum Accumulations Report 5, 2nd Edition, Geoscience Australia, Canberra. Charlton, T. R., 2002, The Petroleum Potential of East Timor, The Australian Petroleum Production and Exploration Association Journal, 42, 1, 351-369. Corelab DB: Pertamina-Core Laboratories, 1995, The Petroleum Geology and Hydrocarbon Potential of the Foreland Basins of Eastern Indonesia Hall, R., 2012, Late Jurassic-Cenozoic Reconstructions of the Indonesia region and The Indian Ocean, Tectonophysics, 570-571, 1-41. Hantschel, T., and Kauerauf, A. I., 2009, Fundamental of Basin and Petroleum System Modeling, Springer Dordrecht Heidelberg London New York. Kartaadiputra, L. W., Samuel, L., 1988, Oil Exploration in Eastern Indonesia, Facts and Perspective, IPA88-11.20, Proceedings Indonesian Petroleum Association, 17th Annual Convention and Exhibition. Longley, I. M., Buessenschuett, C., Clydsdale, L., Cubitt, C. J., Davis, R. C., Johnson M. K., Marshall, N. M., Murray, A. P., Somerville, R., Spry, T.B., and Thomson N. B., 2002, The North West Shelf of Australia - A Woodside Perspective, in KEEP, M. & MOSS, S. J. (Eds), The Sedimentary Basins of Western Australia 3: Proceedings of the Petroleum Exploration Society of Australia Symposium, Perth, WA, 2002, 27 88. Nagura, H., Suzuki, I., Teramoto, T., Hayashi, Y., Yoshida, T., Bandjarnahor, H. M., Kihara, K., Swiecicki, T., and Bird, R., 2003, The Abadi gas field, IPA03-G-141, Proceedings Indonesian Petroleum Association, 29th Annual Convention and Exhibition. Ohara, M., Nakamura, K., Sasaki, Y., 2015, The Structural Evolution of Babar Selaru Region In The Southern Banda Outer Arc, Eastern Indonesia, IPA15-G-180, Proceedings Indonesian Petroleum Association, 39th Annual Convention and Exhibition. Perdana, L. A., Fatwa, A., Ohara, M., Saputra, A., and Fujimoto, M, 2016, 3D Seismic Geomorphology Interpretation of Cenozoic Carbonate Succession in Offshore Tanimbar Region, Eastern Indonesia, IPA16-G-547, Proceedings Indonesian Petroleum Association, 40th Annual Convention and Exhibition.

Roberts, G., Ramsden, C., Christoffersen, T., Wagimin, N., Muzaffar, Y., 2011, East Indonesia: Plays and Prospectivity of the Wast Aru, Kai Besar and Tanimbar Area Identified from New Long Offset Seismic Data, AAPG Annual Convention and Exhibition, Houston, Texas, USA, Expanded Abstract, 10 13 April 2011 Satyana, A. H., 2014, Successful and Prospective Exploration Play Concept of Indonesia: Lessons From History and Recent Progress Anticipating Future Challenges, IPA14-G-276, Proceedings Indonesian Petroleum Association, 38th Annual Convention and Exhibition. Sumantri, Y. R. and Sjahbuddin, E., 1994, Exploration Success in the Eastern Part of Indonesia and Its Challenges in the Future, IPA94-1.0-051, Proceedings Indonesian Petroleum Association, 23 rd Annual Convention and Exhibition. TGS-NOPEC, 2008, Jamdena Regional Geochemical Survey SGE Program Interpretive Report, Technical Report #08-2032. Wygrala, B. P., Yalchin, M. N., Dohmen, L., 1990, Thermal histories and overthrusting Application of numerical simulation technique, Org. Geochem. Vol. 16, Nos 1-3, pp. 267 285.

Figure 1 - Geography of surrounding Babar Selaru PSC from eastern Timor to Tanimbar Island. The background bathymetry map based on Shuttle Radar Topography Mission (SRTM) data (NASA, 2000) with 500m increment of depth contour line (after Ohara et al., 2015).

Figure 2 - General Stratigraphy and basin evolution surrounding Abadi Field, Masela Block in the Northern Bonaparte Basin compiled by Ohara et al. (2015) based on (Fujimoto et al., 2014) for Babar Selaru region tectonic evolution and the tectonic event from Longley et al. (2002).

Figure 3 - Schematic regional geological cross sections through Babar Selaru and Masela blocks. (a) A-A : W-E cross section from Kelp Deep-1 ST1 to Barakan-1 through Troubadour and Abadi gas fields, (b) B-B : N-S cross section from thrust zone in Babar Selaru block to Lynedoch gas field through Abadi gas field. Significant sag area of Calder Graben/Masela Deep expected to be the kitchen for Abadi Gas Field. The sediment depocenter is shifted from southern of Abadi at the Late Cretaceous to north/north east of Abadi at the the Paleogene.

Figure 4-3D diagram of basin evolution at the Abadi and Babar Selaru region (BBS) and surrounding area based on structural evolution of Babar Selaru Region from Ohara et al (2015). Thick deposit was located at the southern and western of Abadi Field at the Cretaceous. At the Paleogene, the depocenter is shifted north/northeastward. Through the Miocene, Carbonate is well developed at the stable continent setting. The collision deformed the Southern Banda Arc created southward verging FTB at the northern part of Babar Selaru region.

Figure 5 - Formations, ages, lithology and thickness data input is determined from seismic interpretation guided by Abadi well information. Geological interpretation allowed the lithology distribution prediction on the pseudo wells. The Jurassic source rock is compiled from Abadi Field and Triassic source rock is referenced to Charlton (2002). R = Reservoir rock; C= Cap rock; S=Source rock.

Figure 6 The paleo heat flow estimation based on the tectonic history from Allen and Allen (2005) which is guided by tectonic reconstruction form Hall (2012). (1) Jurassic rifting, (2) Cretaceous Early Neogene Passive margin, (3) Late Neogene collision between Australia and Banda Arc. The paleo heat flow has a wide range uncertainty which caused some possible scenario. The dark blue is represent the maximum case and light blue is represent the minimum case based on the Allen and Allen chart (2005).

Figure 7 - The modeled maturity calibrated against measured vitrinite reflectance data on the Abadi well and porosity data from Abadi-1 core at the Jurassic Plover Formation. Both calibration methods have good agreement with the model result.

Figure 8- Burial history curve with vitrinite reflectance maturity overlays of Sweeney and Burnham (1990) in the (a) Northern (Thrust margin); (b) Central of Masela Deep and (C) representative of Abadi Field.

Figure 9 - (a) Restored section of NW-SE across the thrust zone in Babar Selaru Region to Abadi Field from Ohara et al. (2016). (b) The paleo maturity modeled at the thrust zone of Babar Selaru Region before thrust occurred (2Ma). (c) The temperature and vitrinite reflectance model of thrust zone area at the Babar SelaruRegion and Abadi Field area. (d) Locations of temperature and maturity model of figure (c); red circle (1, Trough); light blue circle (2, Abadi Field).

Figure 10 - Schematic 3D diagram with 2D cross section along the Offshore Tanimbar Region (Babar Selaru Block Abadi Field) area at the Late Cretaceous (a) and Late Miocene (b). Basin dynamics resulted in variation of maturity history across this area. Figure 11 - The Jurassic source rock maturity map based on the extraction 1D basin modeling result guided by overburden map through time (Late Cretaceous to Recent).

Figure 12 - The migration scenario for Jurassic reservoir at the Pleistocene and Recent. Blue arrows show the general hydrocarbon pathway. The background base map (a) is restored depth map of Jurassic Plover equivalent on Pleistocene and (b) is Recent.