Structural and stratigraphic development of Offshore NW Sulawesi, Indonesia

13 The area of the Offshore NW Sulawesi lies between eastern Sundaland (Borneo) and the North Arm of 14 Sulawesi. Possible influences on the basins include Paleogene rifting in the Celebes Sea and Makassar 15 Strait, Neogene subsidence and uplift in Borneo, late Neogene subduction at the present-day North 16 Sulawesi Trench, and displacements related to the Palu-Koro Fault. This study presents the results of 17 structural and stratigraphic framework of the Offshore NW Sulawesi based on interpretation of 2D 18 seismic surveys offshore and multibeam bathymetry. In this study, the Offshore NW Sulawesi region is 19 divided into three parts: the Deepwater Tarakan Basin in the NW, the Muara Sub-basin in the west, and 20 separated from these by the Palu-Koro Fault, the North Sulawesi Fold-Thrust Belt in the east. There is no 21 continuation of the left-lateral strike-slip Palu-Koro Fault to the adjacent area of Borneo via the Muara 22 Sub-basin and Deepwater Tarakan Basin. Both basins developed after extension began in the Middle 23 Eocene associated with oceanic spreading in the Celebes Sea. Since then, sediment was fed to the basins 24 from the east and south, with several episodes of subsidence, particularly during Early Miocene in the 25 Muara Sub-basin. Rapid prograding shelves from eastern Borneo are linked to regional inversion and 26 uplift on land since the Middle Miocene and led to gravity-driven movement in the Deepwater Tarakan 27 Basin which formed toe-thrust faults in the latest Miocene. Deformation in the North Sulawesi Fold28 Thrust Belt is interpreted to have occurred in the latest Miocene or Pliocene to present-day with 29 subduction of Celebes Sea at the North Sulawesi Trench and movement on the Palu-Koro Fault. 30 31


INTRODUCTION 32
The region of Offshore NW Sulawesi between the North Arm of Sulawesi and Eastern Sundaland (Borneo) 33 has been enigmatic in terms of the nature of the basin and structural development within the area. This 34 area is located in a region where there is a transition between Eastern Sundaland which mainly consists 35 is equivalent to the Middle Eocene Sembakung Formation and which include deformed volcaniclastics 163 and Danau Formation basement rocks (Achmad and Samuel, 1984;Sunaryo et al., 1988). Possible 164 siliciclastic facies of mixed shale and sandstone are mentioned by Achmad and Samuel (1984) and Wilson 165 et al. (2007). 166 167

Unit A 168
Unit A is characterized by variable amplitudes, discontinuous and chaotic reflections, and a low frequency 169 seismic package ( Figure 6). In several seismic lines, this unit is characterized by continuous reflection 170 with growth strata package terminating at a normal fault which indicates a syn-extensional unit ( Figure  171 9). This unit is deformed into a series of faulted blocks (Figure 7-Figure 9). The top of this unit is 172 interpreted as Top Late Eocene based on stratigraphic control from Makassar-A1 well (Camp et al., 2009). 173 According to Achmad and Samuel (1984) and Sunaryo et al. (1988), Unit A is an Upper Eocene marine 174 siliciclastic facies with localized carbonate build ups. 175 176

Unit B 177
Unit B is characterized by high amplitudes, discontinuous and mounded reflections, mound geometry, 178 and a low frequency ( Figure 6). On several seismic profiles, this unit is observed as onlapping and 179 downlapping on Unit A (Figure 7-Figure 9). The top of this unit is marked by high amplitude reflections 180 which can be recognized easily within the study area as a regional carbonate unconformity equivalent to 181 the Top Early Oligocene (Achmad and Samuel, 1984;Sunaryo et al., 1988). This unit is less deformed than 182 Unit A and several internal reflection terminate at faults. Build ups and mounded features are the most 183 common seismic reflection pattern (Figure 7-Figure 9). The unit is interpreted as a relatively shallow 184 marine carbonates. 185 unit is interpreted as a local unconformity which is equivalent to the Top Late Oligocene based on 192 stratigraphic well control of Karang Besar-1 described by Sunaryo et al. (1988). This unit is equivalent to 193 an Upper Oligocene unit which consists of siliciclastic facies or pelagic and hemipelagic mud. According to 194 Achmad and Samuel (1984) and Sunaryo et al. (1988) Miocene based on the Karang Besar-1 well described by Sunaryo et al. (1988). The depositional 204 environment and lithology based on seismic stratigraphy is a shallow to a deep marine environment 205 dominated by pelagic and hemipelagic mud. This unit is equivalent to the Lower Miocene Birang 206 Formation of Achmad and Samuel (1984) and Sunaryo et al. (1988). 207 control from Karang Besar-1 described by Sunaryo et al. (1988). This unit is equivalent to the Upper 223 Miocene Menumbar Formation described by Achmad and Samuel (1984) and is a marine siliciclastic 224 facies from a prograding delta shelf. 225 226

Unit G 227
Unit G is characterized by high amplitudes and continuous, and sigmoidal reflections ( Figure 6). Internal 228 reflections show as progradation geometry with high frequency downlapping towards Unit F (Figure 7-229 Figure 9). The top of this unit is the seabed. This unit is equivalent to the Pliocene-Recent according to 230 stratigraphic well control in the Karang Besar-1 well illustrated by Sunaryo et al. (1988). It is mixed 231 siliciclastic and carbonate facies from a prograding delta. 232 233

Seismic stratigraphy of the Deepwater Tarakan Basin 234
The Deepwater Tarakan Basin seismic sequence has been divided into several seismic packages. 235 Stratigraphic control from one well (Bougainville-1) taken from published paper (Putra et al., 2018) and 236 unpublished post-drill report (BP, 2000) was used to build the seismic stratigraphic framework in the 237 basin ( Figure 11). However, the Bougainville-1 well only reach the Upper Miocene stratigraphic unit as 238 the total depth of the well. Thus, the age of seismic stratigraphy units older than Late Miocene was 239 predicted in this paper (see section 4). Six seismic units have been identified within the Deepwater 240 Tarakan Basin area which are, Unit X1, Unit C1, Unit D1, Unit E1, Unit F1, and Unit G1 ( Figure 12). 241 242

Unit X1 243
Unit X1 has variable amplitudes, discontinuous and chaotic reflections, and low frequency ( Figure 13-244 Figure 15). The top of this unit shows mounded geometry possibly related to an underlying volcanic 245 edifice at an average depth of 9 secs TWT which can be easily traced within this area as a basal 246 unconformity. The internal seismic pattern has localized high amplitudes with parallel reflections. This 247 might be basaltic sills. The lithology of this unit is interpreted as oceanic crust with several local parallel 248 basaltic sills. 249 250

Unit C1 251
Unit C1 is characterized by variable amplitudes with continuous and subparallel reflections. The 252 reflection pattern of this unit becomes chaotic and contorted in the deformed zone of the Deepwater 253 Tarakan Toe-Thrust ( Figure 13-Figure 15). The low frequency seismic package downlaps towards Unit 254 X1. The unit is interpreted as marine siliciclastic facies of pelagic mud and sandstone. 255 256

Unit D1 257
Unit D1 has variable amplitudes with parallel and subparallel reflections. The reflection pattern is partly 258 continuous but becomes contorted in the Deepwater Tarakan Toe-Thrust ( Figure 13-Figure 15). The 259 seismic frequency is relatively low and reflection downlaps towards Unit C1. This unit is interpreted as 260 marine turbidites with mixed siliciclastic facies. 261 Tabul and Santul Formations described by Achmad and Samuel (1984) which could be equivalent to the 276 Upper Menumbar Formation and is a marine siliciclastic prodelta facies from a prograding delta shelf. 277

Unit G1 279
Unit G1 is characterized by high amplitudes, parallel continuous reflections, and high frequency ( Figure  280 13- Figure 15). Internal features within this unit include, for example, local incisions infilled with 281 sediment. This unit also shows growth strata packages on top of the Deepwater Tarakan Toe-Thrust with 282 onlap onto Unit F1. The bottom part of Unit G1 downlaps on Unit F1, especially in the basinward section. 283 According to the stratigraphic well control from Bougainville-1 (BP, 2000), the top unit is equivalent to 284 the Top Pliocene-Recent which also pointed to the Tarakan and Bunyu Formations described by Achmad 285 and Samuel (1984). The unit is marine shale facies with thin limestone beds of a prodelta environment 286 (Achmad and Samuel, 1984). 287 288

Seismic stratigraphy of the North Sulawesi Fold-Thrust Belt 289
The North Sulawesi Fold-Thrust Belt seismic sequence has been divided into two main seismic units 290 which are named the Lower Unit and Upper Unit. There is no stratigraphic well control within this area. 291 The seismic stratigraphy is based on seismic reflection characteristics. 292 293

Lower Unit 294
The Lower Unit is characterized by variable amplitudes with parallel and chaotic reflections, and low 295 frequency ( Figure 17-Figure 18). The most obvious feature of this unit is a highly deformed character that 296 is related to a fold-thrust belt. Several erosional truncations are observed within this unit. Possible 297 lithologies for this unit include mixed basement and cover rocks with volcaniclastic-siliciclastic facies. 298 299

Upper Unit 300
The Upper Unit is characterized by variable amplitudes with parallel reflections and high frequency 301 seismic reflections (Figure 17-Figure 18). Growth strata are obvious from reflection patterns in this unit. 302 The lower part shows stratal termination on the Lower Unit (e.g. downlap and onlap). The unit is 303 interpreted as a marine pelagic and hemipelagic mud dominated facies. 304

Deepwater Tarakan Basin 331
The Deepwater Tarakan Basin is dominated by a fold-thrust belt which is characterized by an imbricated 332 fault-propagation fold system ( Figure 13-Figure 15). In the contractional deformation province, the 333 sequence of thrusting tends to younger basinward. This is interpreted as a gravity-driven structure as 334 observed in several seismic lines ( Figure 15). Unit X1 is a relatively undeformed unit with morphology 335 features that are interpreted as a volcanic edifice forming part of the Eocene oceanic basement ( Figure  The oldest part of the Celebes Sea is underlain by basaltic basement according to Silver and Rangin 376 (1991) and, based on the studies of the geochemistry from the two holes drilled in the Celebes Sea during 377 ODP Leg 124, has a Mid-Oceanic Ridge Basalt (MORB) affinity. The age of the Celebes Sea crust, based on 378 magnetic anomalies immediately east of the Deepwater Tarakan Basin, is Eocene (Weissel, 1980). This is 379 supported by overlying pelagic sediments which contain radiolarians of Middle Eocene age (Nichols and 380 Hall, 1999;Rangin et al., 1990) and by seismic refraction measurements (Murauchi et al., 1973). Middle-Upper Miocene quartz rich turbidite sandstones are discussed by Nichols and Hall (1999). The 403 provenance of the quartz rich sandstones indicates these turbidites were derived from erosion of 404 continental crust (Nichols and Hall, 1999). Plate tectonic reconstructions of the Celebes Sea region which 405 is bordered by Borneo in the west, suggest no relative motion between the Deepwater Tarakan Basin and 406 present-day adjacent areas (Hall, 1996(Hall, , 2002(Hall, , 2011(Hall, , 2012. This implies that sediment influx during the 407 Middle-Late Miocene, as suggested by Nichols and Hall (1999), came from Borneo. Hamilton (1979) also 408 supports the idea that sediments were likely derived from Borneo to the Celebes Sea. 409  In eastern Borneo and adjacent areas, extension began in the Middle Eocene (Hall, 2002(Hall, , 2012 which led 566 to the formation of oceanic crust in the Celebes Sea (Weissel, 1980). The extension phase extended 567 towards the Makassar Strait which initiated the Eocene rifting separating West Sulawesi from Borneo 568 (Hall, 2002(Hall, , 2012. Extension is also observed in the Muara Sub-basin where rifting initiated in this area 569 at the same time. Tarakan Basin seems to have accommodated Unit C1 which was transported from the west to the basin. 583 The depositional environment is interpreted as deep marine with mixed bathyal shales and fine 584 interbedded siliciclastics. Late Eocene radiolarian pelagic and hemipelagic mud which indicates deep 585 marine environment were also found in the Celebes Sea (Nichols and Hall, 1999) which is closer to the 586 Deepwater Tarakan Basin. In the Muara Sub-basin, rifting in the Middle Eocene accommodated Unit A 587 which is interpreted as mixed bathyal shales and fine interbedded siliciclastics transported to the basin 588 from southwest which in the landward direction is dominated by marginal marine coals and siliciclastics 589 (Wilson and Evans, 2002). 590 591

Early Oligocene -Late Oligocene stage 592
In the Oligocene, there was a deep marine area to the east of Borneo (Hall, 2002). During this period 593 there was no significant tectonic activity in the Muara Sub-basin and Deepwater Tarakan Basin. The 594 Muara Sub-basin and the adjacent area was mostly dominated by widespread carbonate build ups 595 (Achmad and Samuel, 1984;Sunaryo et al., 1988;Wilson and Evans, 2002). Unit B, which is interpreted as 596 carbonate, was unconformably deposited on Unit A. The contact of Unit A and Unit B is interpreted as the 597 breakup unconformity. Deposition continued until the Late Oligocene, and in the Muara Sub-basin Unit C 598 fills the basinal area which was surrounded by carbonate builds up of Early Oligocene age. 599 600 The Deepwater Tarakan Basin was a deep marine environment. Little sediment was shed to the east of 601 Borneo during the Oligocene (Hall, 2002). There was no significant change in the sediment source in the 602 Deepwater Tarakan Basin. During this period, Unit C1 is interpreted as transported into the Deepwater 603 Tarakan Basin from the west. Unit C1 is interpreted as a basinal area east of a prograding shelf where 604 bathyal shales and fine interbedded siliciclastics were deposited. 605

Early Miocene stage 607
The Early Miocene was marked by a change of sedimentation character in Borneo (Hall, 2002). A large 608 amount of clastic sediments eroded from Central Borneo fed eastern Borneo particularly the Kutai Basin 609 initiating a prograding delta to the east (Hall, 2002) report of the Bougainville-1 well summary in the Deepwater Tarakan Basin, there was poor hydrocarbon 708 shows with minor gas peaks only. The section below detachment possibly mature source rock but no 709 migration path to charge to the anticline (BP, 2000). Thus, the Bougainville-1 well is considered as dry 710 well and it is interpreted that the well was drilled far down dip of the crest of the anticline structure and 711 below the hydrocarbon water contacts (Atzeni and Guritno, 2003). 712 713 However, the Aster-1 well in the Deepwater Tarakan Basin shows promising result. According to Atzeni 714 and Guritno (2003), the Aster-1 well, which was situated relatively near the crestal position of the 715 anticline structure, consists of 6 oil and gas levels with the main discovery level was the Upper Miocene 716 AST200 that has 10m of oil. The biomarkers of the oil sample show a terrestrial origin of the source rock 717 generated the oil (Atzeni and Guritno, 2003). The oil and gas discovery from Aster-1 well indicates the 718 effectiveness of the petroleum system in the Deepwater Tarakan Basin (Atzeni and Guritno, 2003). 719