Tertiary Tectonic Evolution and Related Hydrocarbon Potential in the Natuna Area

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© IPA, 2006 - 13th Annual Convention Proceedings, 1984

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PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Thirteenth A n d Convention, May 1984 Search

TERTIARY TECTONIC EVOLUTION AND RELATED HYDROCARBON POTENTIAL IN THE NATUNA AREA Abiratno Wonpsantjko* Gatot K.Whjudo**

ABSTRACT Three major tectonic elements are recognized during Early Oligocene in the Natuna area, i.e. the West Natuna Basin, the East Natuna Basin and the Natuna Arch. The arch represents a N-Strending basement high which effectively separates the two basins. Basin development in West Natuna started during Early Oligocene where rifting and/or pull-apart produced predominantly SW-NE halfgrabens filled with nonmarine sediments. The features probably formed due to the extrusion of the Malay Peninsula from Asia during the early stages of the India-Asia conision. T& 'tensional forces of West Natuna however seemed to have little effect on the East Natuna Basin where Oligocene sediments are uniformly represented and thicken only regionally eastward toward the basin Center. Compressive forces began to dominate in the West Natuna basin during Early Miocene, resulting in reverse movement of once normal faults and corresponding uplift of areas which previously were half grabens. During this same time East Natuna was structured by predominantly tensio d forces which prclduced NW-SE right lateral movements coupled with SW-NE normal faulting. The differences in tectonic regimes between West and East Natuna during Early Miocene can again be explained by a changing pattern in the extrusion of Indochina from Asia during later stages of India's collision with Asia. The hydrocarbon potential of the West Natuna Basin is heightened by the presence ?f local thick sequences for both Oligocene and Miocene sediments which were tectonically controlled. Both sections offer good potential for source and reservoir rocks. In the West part of the East Natuna Basin the presence of thick, mature sediments is limited t o areas of Early Miocene grabens. INTRODUCTION The Natuna area discussed in this paper is restricted to the offshore area of Natuna Island within the Indonesian territory north of 3'N. It encompasses an area of about 200,000 square km (fig. 1). The area consists of three major tectonic elements, i.e. the West Natuna Basin, the East Na-

* Amoseas Indonesia Inc. ** Pertamina

tuna Basin, and the Natuna Arch which represents a N-S trending basement high effectively separating the two basins. Industry activities in the area during the last decade have provided abundant good quality well and seismic data allowing detailed structural analysis. The most comprehensive study dealing with the Tertiary structural development of the area is written by White and Wing (1978) which is primarily based on detailed structural analysis in the south half of the study area. Recently Tapponier, et. al. (1982); postulated "extrusion" tectonics, resulting from the collisio n of India against Asia, which would have influenced or even determined the tectonic evolution of the South China Sea region. The Natuna area includes parts of two major basins with different tectonic styles. It is also at the junction of two different structural systems, making it ideal for studying the Tertiary structural developments for interpreting the tectonic evolution of the South China Sea The authors hope that the data presented here will contribute to a better understanding of South China Sea tectonics and will advance the petroleum exploration in the general area.

FAULTING AND FOLDING A. West Natuna Basin

In the West Natuna Basin two prominent fault trends are present, i.e. the SW-NE, and NW-SE trends (fig. 2). The majority of these faults are typified by the changes in their sense of movement from reverse at shallow horizon to normal in the deeper section. Similarly their fold styles also change from anticlines at the top to half grabens at depth. These faults generally represent bounding faults for sedimentary wedges in the deeper section with marked thickening toward the faults themselves. Eubank and Makki (1981) point out that this typical structural style also exists in the Central and South Sumatra basins, and named it the Sunda Fold. The top Oligocene to top Basement Isochron Map (fig. 3) shows the distribution of sedimentary wedges with their bounding faults. The primary direction of these older faults is SW-NE with only minor faults trending @W-SE. The isochron map suggests that faulting in t6e West Natuna

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Basin occured at the latest during early Oligocene, as a result of extensional forces or puIl-apart. The resulting grabens and half grabens were subsequently fded wiih essentially continental clastics. In places these sedimentary wedges are up to 2.5 sec. (TWT) or approximately 15,000 f t thick. In contrast, the Mioceqe Isochron Map (fig. 4) shows marked thins which coincide with areas indicated as thicks in the Top Oligocene to Top Basement Isochron map. These t h i n s are brounded by reverse faults, which were previously normal faults. Many of these were reactivated faults which reversed their direction of movement. T h i s indicates that compressional forces were active during the Miocene which produced syndepositional folding and thrusting. The Miocene faults have two major trends. The dominant trend is SW-NE and the other is to the NW-SE (figs. 2,5). Both fault directions are associated with folds. However, folds associated with the SW-NE faults are parallel to the fault direction (figs 6,7), while those associated with &W-SE faults are oblique to the fault-direction (figs. 8,9). Some of these form en echelon thrusts and folds (fig. 9). Based on Moody’s concept (1973) the Miocene fault-fold system in the West Natuna Basin appear to be caused by a NW direction of primary stress. This stress would produce SW-NE priented primary folds and thrusts and NW-SE oriented fsrst order wrench faults associated with second order drag folds (fig. 10).

A late Miocene to Early Pliocene unconformity is present locally in West Natuna, particuIarly around anticlinal crests indicating temporary emergence and subsequent erosion. The overlying section consists of relatively undisturbed Pliocene to Recent marine sediments (fig.lOa). It is apparent that folding and thrusting in West Natuna ceased at the end of Miocene. In turn sedimentation was more or less controlled by differential compaction, as suggested by the existence of local thins overlying topographic highs or eroded zones. Pliocene faulting is essentially absent (fig. 1I). B. East Natuna Basin The structural style in the East Natuna Basin diffets considerably from that in the West Natuna Basin. The area seemed to be less affected by Oligocene rifting or pullapart. Oligocene half grabens exist only in the northernmost part of the area (fig. 3). These extensional faults also trend SW-NE, similar to the ones found in ihe West Natuna Basin. The rarer, and thinner graben fills in the area however, indicate that the rift magnitudes were less than those in the West Natuna Basin. With the exception of the above mentioned area, large parts of the East Natuna Basin have a relatively thin Oligocene section (less than 3000 ft thick) which thickens only sIightIy to the east: The Miocene isochron map (fig. 4) on the other hand shows some local thick’s which represent graben fiu. Many of the bounding faults are syndepositional or growth

faults which started to develop in Early Miocene time. A. typical example of these faults is illustrated by the seismic section (fig. lob). The Miscene sedimentary wedge thickens towards the west-bounding fault, and the underlying Oligocene section onlaps basement in the same direction, suggesting that the west-bounding fault started to develop during Early Miocene. The Miocene.faults in the &st Natuna Basin are grouped into two major trends, i.e the SW-NE and NW-SE trends (figs 2,12). Unlike those in the West Natuna Basin, all faults are extensional. The preponderance of tensional SW-NE faults in the northeast suggests that some of the NW-SE faults are right lateral. An example (fig. 13) confirms the interpretation. This right lateral fault system also occurs in Sarawak, southeast of the study area (ASCOPE/ CCOP, 1981), Only minor folds exist and they are generally associated with NW-SE right-lateral strike-slip faults, particularly around areas where the strike direction changes (fig. 14). The preponderance of normal faulting and the low intensify of folding indicates a prectominantly tensional structural regime in ~e East Natuna Basin during the Miocene. The Miocene section also thickens regionally eastward reaching thicknesses in excess of 10,000 ft (2.5 sec. TWT) (fig. 4). In the eastern and northeastern parts of this basin thick carbonates were deposited associated with reef buildups in response to basin subsidence in the east. Like in the West Natuna Basin,.the eastern and northeastern parts of the East Natuna Basin were tectonically inactive during the Plio-Pleistocene. However, in the southern part Pliocene faults are common, probably related to basin subsidence and rapid sedimentary loading in the east, as suggested from the marked thickening of PlioPleistocene sediments eastward. It appears that this rafiid sedimentary loading also resulted in diapirism in the southeast part of the basin (White and Wing, 1978).

TERTIARY TECTONIC HISTORY Several writers have speculated about the cause of basin formation and tectonic developments in the Natuna and adjacent areas. White and Wing (1978) proposed two possibilities on the cause of basin formation in the Gulf of Thailand and West Natuna: collapse of the Sunda Landmass by being stretched and rifted, or as a result of a mild compensatoIy downwelling as the subduction zone along the Lupar Line (east of the Natuna area) stopped at the end of Eocene time. Tapponier et al (1982) postulate a propagating ’extrusion’ tectonics model in Southeast and Eastern Asia caused by the collision between India and Asia. They suggest that the Tertiary basins in the Gulf of Thailand and southwestern China Sea were caused by pullapart and rift during the earliest phase of extrusion when the Malay Peninsula and Sunda Shelf were ’extruded’ southeastward (fig. 15).

163 The riftlpull-apart and 'extrusion' tecJonics theory seems t o fit fairly well with the observed Tertiav structural developments in the Natuna area. Figure 16 illustrates our postulated tectonic system in the Natuna and adjacent areas during Early Oligocene. The relative southeastward movements of the Malay Peninsula and Sunda Shelf relative t o the Asia mainland resulted in rifts and puI1-aparts in the Gulf of Thailand and West Natuna. NW-SE oriented grabens were developed in the Thai and Malay basins. These were formed by left-lateral movements associated with rifting, while SW-NE grabens dominate the West Natuna Basin as a result of pull-aparts or being stretched in response to an arrested separation of Sunda Shelf from Indochina. The pull-apart intensity decreased northeastward, with only mild effects evident in the northern part of East Natuna Basin. The Early-Middle Miocene tectonic system (fig. 17) depicts the southeastward movement of the western part of the Indochina block toward the Sunda Shelf. This action produced dominantly NW-SE right-lateral movements in the Thai and Malay basins associated with wrench-related folding (Hamilton, 1979). The same force resulted in more intensive folding and reverse faulting or wrenching in the West Natuna Basin. Applying Tapponier et.al.3 'extrusion' concept, the southeastward movement of the Indochina block is probably related to the next phase of India impinging against Asia. This time the 'extrusion' of the Malay Peninsula and Sunda Shelf stopped, but in exchange the 'extrusion' of the Indochina Block southeastward was activated. Structural analysis in the East Natuna Basin suggests, however, that the tectonic regimes in the area during the Miocene were mainly tensional which could mean that southeastward movement of the eastern part of Indochina block relative to the Sunda shelf was minimal. On the other hand sea floor spreading, which was active during Mid Oligocene to Mid Miocene time in the South China Sea (Taylor and Hayes, 1980; Halloway, 1981), could have had a dominant effect on the East Natuna Basin and may have caused the pull-aparts and rifts associated with right lateral strike-slipfaults. The southeastward movement of the western part of the Indochina block stopped in Late Miocene time causing the structuring in the Natuna and adjacent areas to cease. Meanwhile rapid sedimentation and subsidence continued to take place in the eastern part df East Natuna associated with some gravity faulting and diapirism. Tapponier et al suggest that the ending of Indochina block movement was followed by the next 'extrusion' stage, during which India penetrated deeper into Asia. By this time the extrusion of Indochina had stopped and the eastward extrusion of southern China started.

HYDROCARBON POTENTIAL

A. source-rocks The Oligocene grabens in the West Natuna Basin are

filled with thick continental clastics, reaching u p to 15,000 f t (2.5 sec TW). Geochemical data from existing wells suggest that this section contains good source-rocks for gas and fair to good source-rocks for oil, at least locally. Rifts and pull-aparts have also created high heat flow as reflected by the high geothermal gradient in the area (fig. 18). With an average gradient of 2S°F/100 ft, simiir to that of the present day, the lower and middle parts of the Oligocene section were probably mature for gas and oil generation during the deposition of Miocene sediments and subsequent folding. The lowest part of the section could have been mature before early structuring began. In East Natuna the Oligocene section is generally thinner, i.e. less than 3000 ft, except in Some local grabens in the northeast and, possibly, in areas to the east towards the central part of the basin. Source-rock maturation in the relatively thin Oligocene sediments would require much thicker overburden. Mature Oligocene sediments are therefore believed to be present only in areas of Miocene grabens and in the east and southeast parts of the basin where overlying sediments are sufficiently thick. Maturation probably occurred during Pliocene time at the earliest. The Miocene section, both in the West and East Natuna basins consists of sandstone and shale interbedded with some coals, possibly deposited in deltaic or paralic environment. Geochemical analysis mdicate that the sections contain fair to good source rock potential for both oil and gas. Although primarily gas prone, the 'coaly' beds are . reported to' contain up to 25% waxy sapropels which are known as oil source kerogen. The Miocew section in the West Natuna basin, however, is mostly immature. Mature Miocene sourcebeds are postulated o d y in the very deepest. synclinal areas in the westernmost part of the basin where the overlying PIio-Pleistocene sediments are thick In the East Natuna Basin, mature Miocene exists in the eastern and southeastern parts of the study area. Only the lowest part of the section may have reached thermal maturity in the Miocene grabens. The Bio-Pleistocene sediments in the West Natuna basin are generally thin, less than 4OOO ft thick. The section is therefore immature. A similar case also occurs in the East Natuna Basin. Although subsidenceand rapid sedimentation occurred in the eastern parts of the basin, resulting in up to 10,000 ft of Plio-Pleiqtocene sedimentary fa,the sequence is probably still immature. B. Reservoir Rocks

The Oligocene and Miocene sections in both the West and East Natuna Basins contain numerous sandstones with fair to good reservoir qualities. In the West Natuna however, the' Miocene sandstones generally have better reservoir qualities than those of the Oligocene because they are the products of recycled older sediments uplift%d during the Early Miocene (White and Wing, 1978). A s for the East Natuna Basin, the sandshale n t i ~ in both the Oligocene

164 and Miocene sections generally decreases eastward. They possibly shale out just east of the study area. Both Oggocene and Miocene sandstones constitute a main re.woir objective for the Natuna area. Beside the clastic reservoirs, the Mid Miocene to Lower Pliocene carbonate build-ups also provide excellent potential reservoirs for the East Na-

tuna Basin.

P

C. Traps

Traps m the West N a W Basin generally exist as antidines and as closures formed on the upthrown side of reverse faults related to Miocene compression and wrench-. mg. The East Natuna traps are generally related to normal faults and growth faults, old basement highs, or diapiric structures. Carbonate buildups also contniute important traps for the East Natuna Basin. -

The Oligocene grabens in West Natuna offer good. potential source rocks and early maturation for both oil and gas generation, while in East Natuna maturation was more or less controned by the presence of Miocene grabens and thick Pliocene-Pleistocene overburden. Miocene source rocks in West Natuna are generally immature, except in the deep synclines in the westernmost part. In .East Natuna, only in the easternmost parts of the area and in the deepest parts of local grabens is the Miocene section mature. Traps are present in West Natuna in anticlines or upthrown reverse fault closures caused by Miocene compression and wrenching, while in East Natuna they are associated with normal fault closures, growth faults, old basement highs, diapiric structures, or carbonate buildups.

SUMMARY AND CONCLUSIONS From the foregoing discussion, the tectonic evolution during the Tertiary in the Natuna and adjacent areas may be summarized as follows: 1. Basin developments in West Natuna and the Gulf of Thailand started during Early Oligocene time as a result of rifting and pull-apart of the Malay Peninsula and Sun& SheIf from Asia. The'CoIWon of hdia into Asia and the related 'extrusion' tectonic model of Tapponier et.al. might be the responsiile mechamm * The riftslpullaparts produced numerous grabens and half-grabens filled with thick continental sediments in the West Natuna Basins. The grabens are fewer in number and are fined by thinner sediments m the northeastern part of East Natuna Basin,reflecting the dimhidm . g e€fects of rifting northeastwards. Meanwhile, thick continental ctasticJ were also deposited in the gulf of Thailand (Thai and Malay basins) in grabens formed by left lateral faults.

.

2.

The earlylate Miocene folds and thrusts are associated with right-lateral strike-slip movements reflecting compressional and wrenching style tectonics in West Natuna and parts of the Gulf of Thailand. Many. of the normal faults were reactivated with reversed sense of motion. This was possiily caused by the southeastward movement of the Western Indochina block relative to Malay Pensinsula and Sunda Shelf; a movement which might be related to the next 'block extrusion' during the foIJoWiag sfage of India's impingement against Asia. In the meantime the active sea floor spreading in the South China Sea resulted in the formation of grabens and halfgrabens with some right-lateral strike-slip fauhings in the East Natuna Basin.

3.

The PliocenePTeistocenewas a period of transgression without any major tectonic events in the Natuna area. It suggests tbat the southeastward movement of Western Indochina relative to the Sunda Shelf had ceased.

ACKNOWLEDMENTS The authors wish to thank their colleagues at PERTAMINA and AMOSEASINDONESIA for the stimulatingdiscussions and suggestions made toward improving this paper. We would also like to thank the management of PERTAMINA, AMOSEAS INDONESIA, CHEVRON, AND TEXACO, for their permission to present this paper.

REFERENCES ASCOPE/CCOP, 1981, Tertiary Sedimentary Basins of the GuE of Thailand and South China Sea: Stratigraphy, Structure, and Hydrocarbon Occurrences (Compilation). The ASCOPE Secr., 72p.

Armitage, J.H. and C. Viotti, 1977, Stratigraphic Nomenclature Southern End Malay Basin- Indonesian Petrol. Assoc, Proc. 6th Annual conv, p69 - 94. Eubank, RT. and AX. Makki, 1981, Structural Geology of the Central Sumatra Back-Arc Basin. Indonesian Petrol. Assoc., Proc. loth Annual Conv., P153 - 196. Ilanoway ,N.H., 1981,The North Palaw an Block,Phillipines: Its Relation to the Asian Maidand and Its Role in the Evolution of the South China Sea: Geol. SOC.Malaysia Bull., p19-58. Hamilton, W.G., 1979, Tectonics of the Indonesian Region: U.S.Geol. Survey Prof. Paper 1078,345~. Harding, T.P., 1974, Petroleum Traps Associated with Wrench Faults: Am. Assoc. Petroleum Geologists Bull., V. 58, NO. 7, ~ 1 2 90 1304. ------, and JamesDLoweIl, 1979,Structural Styles, Their Plate-Tectonic Habitats and Hydrocarbon Traps in Petroleum Provinces: Am. Assoc. Petroleum Geolgists Bull., V. 63,No. 7, p 1016 - 1058.

165 Lowell, JamesD., 1980,Wrench vs. Compressional Structure with Application to Southeast Asia: Southeast Asia Petrol. Explor. Soc. Proc., V. 5,p63 - 70.

Geology and Well Data: Indonesian Petrol. Assoc., ROC. 2nd @ual Conv., p223 - 241.

Mattes, EM., 1979, Udang Field A New Indonesian Development: Indonesian PetroL Assoc., Roc. 8th Annual Conv., p177 - 184.

White, J.U and Wing, RS.,1978, Structural Development of the South China Sea with Particular Reference to Indonesia; Indonesian Petrol. Assoc. Proc. 7th Annual Conv., p159 - 177.

Moody, J.D. and Hill, IW., 1956,Wrench-FaultTectonics Ge01. Soc. Am. Bun., V. 67,p1207 - 1246. ------, 1973, Petroleum Exploration Aspects of Wrench-Faults Tectonics: Am. Assoc. Petroleum Geologjsts Bull., V. 57,No. 3, p449 - 476. Pupilli, M.,1973, Geological Evolution of South China Sea Area -- Tentative ReconstNction from Borderland

Wilcox, Ronald E., TP. Harding, and D R . Seely, 1973, Basic Wrench Tectonics Am. Assoc, Petroleum Geologists BUH., V. 57, NO. 1,p74 - 96. Tapponier, P.,G. Peltzer, A.Y. Le M, R Armgo, and P. Cobbold, 1982, Propagating Extrusion Tectonics in Asia: New Insights from Simple Experiments with Plasticine: Geology,V. 10,p611- 616.

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