The Geology of Indonesia/Natuna

Contributors: Murti, N., Minarwan, Darman, H.

The Natuna area discussed in this article is Natuna Islands and the surrounding area within the Indonesian territory. The area forms the south-westernmost tip of South China Sea, with Malay Basin, Penyu Basin, Sarawak Basin and Sunda Shelf as its immediate geological neighbours. Natuna area itself is presently configured by two Tertiary basins, namely West Natuna Basin and East Natuna Basin, separated by Natuna Arch (Figure 1).

This article is a result of a comprehensive library study of the available published materials and various unpublished internal studies within the writers' reach. The objective of this article is to explain simply and hopefully clearly the geological facts of this Natuna Area and to expose its geological highlights. The more detailed study, investigations and experiments can be found in the bibliography.

Geological information of the Natuna Area mainly comes from petroleum exploration, which high frequency has resulted in many comprehensive geological study of the area. The availability of these studies does not mean that the geological history of this area is clearly understood. There are still many uncertainties, which need to be addressed, as can be seen in the subsequent text.

Tectonic
The West Natuna Basin is bounded at the north by Khorat Swell, which is a south-dipping monocline basement high. To the south it is bounded by Sunda Shelf and to the east by Natuna Arch. At the north-west it opens to the NW-SE elongated Malay Basin, and at the south-western tip it opens to a smaller E-W elongated Penyu Basin. The Malay Basin is separated from the Penyu Basin by Tenggol Arch, a ridge extruding east to the western part of the West Natuna Basin. In the West Natuna Basin itself several main structural elements can be recognised, i.e. Anoa Graben, South Kakap Graben, Northern Central High, Southern Central High and the Anambas Graben (Figure 2).

The predominant structural orientations in the West Natuna Basin are SW-NE and NW-SE (Wongsosantiko & Wirojudo, 1984), including several big NW-SE wrench faults. The faults are typified by the changes in their sense of movements, from reverse in the shallower horizons to normal in the deeper. The fold styles also change from anticlines at the top to half grabens at depth. The West Natuna Basin is classified by Daines (1985) as an intracontinental failed rift basin. The various structural elements in this basin were formed during two distinct deformation period i.e. Late Eocene-Oligocene period of extension which resulted in graben and half graben formation, and a later compressional phase which resulted in the inversions of the structures. This compressional phase re-activated the existing structures with reversed sense of movements.

The Natuna Arch, which separates the West and East Natuna Basins, is a northern protrusion of Sunda Shelf. Along with Khorat Swell, this ridge was emergent periodically during Tertiary (White & Wing, 1978) and thus provided the source of sediments for the two adjacent basins.

The East Natuna Basin is bounded at the west by Natuna Arch, to the east it opens to Sarawak Basin. The south part is bounded by Sunda Shelf and the northern part by Vietnam Basin. Hutchison (1989) divided the East Natuna Basin into Sokang Sub Basin and North East Natuna Basin. They are separated by the Paus-Ranai Ridge, a feature parallel to the eastern side of Natuna Arch, which also forms the north-eastern boundary of Sokang Sub Basin. To the east both basins open to Sarawak Basin (Figure 2). In a Late Cretaceous - Early Eocene reconstruction for the area, White & Wing (1978) concluded that the East Natuna Basin was part of a large fore-arc basin extending from offshore Vietnam, across Natuna waters to Sarawak.

Unlike in the West Natuna Basin, the main structural trend in the East Natuna Basin is NNW-SSE, only in the northern part SW-NE trending structures exist (Wongsosantiko & Wirojudo, 1984). This SW-NE trending structures are extensional faults and half grabens similar to the ones found in West Natuna Basin, but the rift magnitude is generally less than the ones in the West Natuna Basin. In the East Natuna Basin all the structures are extensional, there seems to be no compressional tectonic phase happened in this area. White & Wing (1978) stated that the structural trend seems to follow the underlying basement fabric, which is formed by the eastward migrating subduction zone.

Stratigraphic
The stratigraphic framework of both West Natuna Basin and East Natuna Basin was first compiled by Pupilli (1973), based on lithostratigraphy. Several companies working in different concession blocks in the West Natuna Basin have since developed various stratigraphic columns for the area with different nomenclatures, as can be seen in subsequent works by Armitage & Viotti (1977), Pollock et al. (1984), Daines et al. (1985), Sutoto (1991), Fahman et al. (1991) and Ginger et al. (1993). The one used in this paper, a generalised stratigraphic framework for the whole area, can be seen in Figure 4.

Underlying the sequences of the West Natuna Basin is the Cretaceous basement, which according to Pollock et al. (1984) is comprised of amphibolites. According to a more detailed recent study the basement actually comprises intrusive acidic-type igneous rocks such as quartz-diorite, granite and phyllite, chlorite-schist, gneiss and amphibolite.

The earliest sediment found in the West Natuna Basin is thought to be older than Early Oligocene, based on the dating of the diabase intrusion found in the basal sediment. Lama Formation of Late Eocene (?) to Early Oligocene is found above the basement. Lama Formation consists mainly of fluvio-deltaic, fluvial and alluvial fan sandstones. Benua Formation conformably overlies Lama Formation. This formation is a sequence of shales interpreted as of lacustrine origin. Both Lama and Benua Formation are mostly barren of fauna, except some palynomorphs that indicate the fresh-water depositional environment. Overlying Benua shales are the interbedded sandstones and shales of Lower Gabus. The sandstone is fine to medium grade sands with plant debris typically form thick, blocky or fining upward units and usually massive in appearance. This sequence is interpreted to be of fluvio-deltaic and fluvial origin, and its age determined from palynomorphs is Oligocene.

Above the Lower Gabus Formation, Keras Shale was deposited in lacustrine environment at Mid Oligocene to Late Oligocene time. This Keras Shale was then gradually replaced by the interbedded sandstone and shale of Upper Gabus Formation. The sandstone of Upper Gabus Formation is fine to very fine grained, in blocky or fining upward units similar to the Lower Gabus. This formation was deposited in braid-delta and lacustrine plain environment at Late Oligocene-Early Miocene time. Palynological data shows that the Oligocene/Miocene boundary is within the topmost part of Gabus interval. Barat Shale was then deposited above Upper Gabus in lacustrine depositional environment with the influences of marine condition at some places in Early Miocene time. This assigned age is based on the non-existence of Oligocene and older markers, and the possible occurrence of Florschuetzia levipoli. After the deposition of Barat Shale, Arang Formation was conformably deposited above it. Palynological evidence gives an Early to Middle Miocene age for this formation. The entire Arang Formation was deposited in shallow marine environment with fluctuations to coal-swamp dominated coastal plain related to basin inversion and relative sea level changes.

Above Arang Formation, Muda Formation was deposited unconformably in shallow marine environment. The Base Muda unconformity is widely recognised at West Natuna basin. The Muda Formation consists of mudstone, shales and sands. This formation is formed from Late Miocene until the present time.

Geological map by Hakim & Suryono (1994) shows that the eastern part of Natuna islands, situated on the Natuna Arch, is comprised of Mesozoic (Early – Mid Cretaceous) melange sediments (Bunguran Formation) and Late Cretaceous granites. Older igneous rocks, mafic and ultramafic rocks from Jurassic time, outcropped in several parts of the islands. In the western and northern part of Natuna islands, sediment sequence consisting of conglomerate, massive sandstones and siltstone outcropped. This sediment is interpreted as the result of fluviatil processes and interpreted to be the equivalent of Oligo-Miocene sediments of the West Natuna Basin. Granites of Late Cretaceous age can also be found in the Anambas Islands, in the Sunda Shelf to the south of the West Natuna Basin (Samodra, 1995).

The stratigraphic framework for the East Natuna Basin does not appear much in published literature, the one used in this paper is a generalised one taken from several published and unpublished work (Figure 4). The oldest sediment found in East Natuna Basin is of Late Oligocene to Early Miocene age (Pertamina & BEICIP, 1985). This sediment comprises a lower sandstone unit followed by a shale unit. The sandstone is the equivalent of Upper Gabus Formation in the West Natuna Basin. This deposit is interpreted to be of alluvial plain and delta plain origin. The shale that overlies it is the equivalent of the Barat Formation of the West Natuna Basin, and consists of greenish and grey marine shales. This sequence represents the maximum transgression of the area. These sequences cannot be seen in the Sokang Sub Basin because they were not reached by any well. The sediments overlying this Barat shale are the sandstones of Lower Arang Formation, which is formed in the regressive phase, terminating the transgression. The regressive phase was immediately followed by a transgression. Both the regressive and transgressive sandstone sequence form the Lower Arang sandstones. The transgression ended by the forming of Lower Arang shale which consists of grey and brown marine shales. In the south, the Lower Arang shale was then followed by the forming of Sokang sandstones. These formations are deposited in Early Miocene through Middle Miocene time.

In the north part, overlying conformably the Lower Arang shales is the Terumbu Formation. Pupilli (1973) divided the formation into a lower member and an upper member. The lower member is termed the platform member, and is characterised by stratified wackestone, packstone and boundstones. The upper member is termed the reef member which consists of highly fossiliferous diagenetic dolomite. The boundary between the lower member and the upper member is unconformable in certain places. This unconformity marked the end of the regression phase. The upper member of this Terumbu Formation is formed of many reefal build-ups, with marine shale and marls filling the depressions between the build-ups. During the Middle Miocene to Late Miocene forming of the Terumbu Formation in the North, in the South a transgression phase deposited the Upper Arang shale (Pertamina & BEICIP, 1985). This shale was followed by an Upper Arang sandstone. The sandstone extends also in the northern part, overlying the Terumbu Formation. The youngest formation Muda overlies unconformably all the oldest sediment, and formed as a transgressive sedimentary section.

Geological History
Natuna Area is part of the Sundaland, consequently the evolution of this area can be traced back to the forming of Sundaland. According to Hutchison (1989) Sundaland is the stable continental core of South East Asia, and it was formed and cratonised in Late Triassic time. Taylor and Hayes (1983) reconstructed that during Mid Jurassic to Mid Cretaceous in the east part of Sundaland a subduction zone was formed by Pacific plate subducting from the east. In the area west of the subduction zone a volcanism activity occurred along with granitic instrusions. The rocks formed by these activities later were metamorphosed and became the basement of the Tertiary basins in Natuna Area as proven by wells (Pupilli, 1973). The subduction activity also resulted in the forming of melange complexes, as is found in parts of Natuna Islands in the Natuna Arch (Hamilton, 1979).

During Late Cretaceous this subduction activity was still continuing and it continuously moving eastward, as postulated by Pupilli (1973) and Hamilton (1979). At this time the subduction zone occurred along a line from SW Borneo through the east of Natuna Island and up north to offshore Vietnam. This subduction still continued in the earliest Paleocene and through Eocene, as according to Taylor & Hayes (1983) the volcanism along Natuna Arch and SW Borneo continued into the earliest Paleocene, and according to White & Wing (1978) it was during Eocene that a melange facies was formed in Paus-Ranai Ridge. To the west of the subduction zone, a shelf deposit was accumulated, forming the basal formations of South Sokang Sub Basin (White and Wing, 1978). Sedimentary record shows that by Oligocene this subduction zone has stopped completely (White & Wing, 1978).

The cessation of the subduction and the forming and subsequent evolution of the West and East Natuna Basins may be explained by several theories on the relative motions of the India, Asia and the Pacific plates. Regional Tertiary tectonic framework proposed by Tapponnier et al. (1982) and by Daly et al. (1991) have been commonly used as the models to explain the tectonic evolution of South China Sea area, because they seem to fit fairly well with developed structural elements on this area. Both papers stated that the indentation of Indian plate into Asia has led into the rotation of Indochina. However, Tapponnier et al. (1982) suggested that the basin evolution of SE Asia is mainly due to major lateral extrusion of Indochina and Eastern China when India and Asia collided each other. Later in 1991 Daly et al. suggested that the major effect of this collision is just in term of clockwise rotation of Indochina and extension along the Sumatran active margin, not in the term of basin evolution of SE Asia.

Using Tapponier et al. (1982) "extrusion" model, Wongsosantiko and Wirojudo (1984) and Daines (1985) explained the structural development in the Natuna area. The south-eastward movement of the Malay Peninsula and Sunda Shelf relative to the Asia mainland resulted in rifts and pull-aparts in the Gulf of Thailand and West Natuna. In Thai and Malay basin, NW-SE orientated grabens were developed as a result of left-lateral movement associated with rifting. Meanwhile, SW-NE grabens which dominate the West Natuna basin were formed by pull-aparts in response to the separation of Sunda Shelf from Indochina (Wongsosantiko and Wirojudo, 1984). This feature, which occurred during Early Oligocene time was called by Daines (1985) as graben development phase in West Natuna basin. However, Daines (1985) suggested that the graben (especially the SW-NE orientated Boundary graben) were developed as a direct result of the collision between India and Asia, not due to a pull-apart.

Using a different approach, Daly et al. (1991) explained that the forming of the West Natuna Basin is connected to the opening of the South China Sea. The Pacific plate, subducting northward below the Eurasia continental plate, stopped the movement in Eocene. This cessation was followed by a gravitational collapse of the crust, due to the rollback of the subducting oceanic plate. The collapse was then followed by a rifting which opened the South China Sea. The change of Pacific plate movement stopped the subduction along the line from SW Borneo through east of Natuna to offshore Vietnam, and it was transformed into a strike-slip fault. Thus according to the concept of Daly et al. (1991), the rifting which formed the West Natuna Basin was caused by a right lateral stress regime, with the initial opening of South China Sea. This Early Oligocene-Mid Oligocene rifting phase in West Natuna Basin resulted in the sedimentation of the Lama Formation, Benua Formation and Lower Gabus Formations.

In East Natuna Basin when the subduction along the line from SW Borneo to offshore Vietnam became inactive during Oligocene, the previous fore-arc basin sediment deposition was followed by the deposition of shallow and deep water facies (White & Wing, 1978). However, the oldest deposits confirmed by well in this basin are the Late Oligocene sediments, which are the equivalent of Upper Gabus and Barat Formation of the West Natuna Basin and it is mostly of alluvial plain and delta plain setting (Pertamina & BEICIP, 1985). These sediments are the result of the rifting phase, which according to Wongsosantiko & Wirojudo (1984) also affected the East Natuna Basin, especially the northern part, forming the SW-NE half grabens. During Early to Middle Miocene, the South China Sea spreading continued and caused the forming of the NNW-SSE structures, following the basement weak zone patterns.

In Mid Oligocene to Early Miocene a period of tectonic quiescence, a post rifting phase, happened (Ginger et al., 1993). The deposition of this period resulted in the Keras and Upper Gabus Formations. Barat Formation which was deposited after them is interpreted to be included in both post-rift and syn-inversion megasequence (Ginger et al., 1993). The boundary between post-rift and syn-inversion megasequence is marked by an onlap surface to lower part of Barat seen on the upper part of the Barat Formation.

After the post rifting phase, a period of compressional stresses during Early Miocene to Mid Miocene caused the inversion of the grabens and the wrench faulting in the West Natuna Basin. In some inverted grabens the Miocene sediments were entirely eroded, causing the Oligocene sediment to be in direct contact with Muda Formation (Figure 5). Using the concept of Daly et al. (1991) the compressional stresses can be explained by the collision of the continental fragments which form the southern tail of South China spreading with Borneo. According to Daines (1985) the compressional stresses, which mostly form SW-NE trending anticline in West Natuna Basin, came from the north-northeastward penetration of Indian plates against Asia. The deposits formed during this inversion period is the Arang Formation.

The effect of these stresses to East Natuna basin is not too significant to form any compressional structures. According to Pertamina & BEICIP (1985) the compressional phases only caused regional uplifting in the East Natuna Basin. During Early Miocene to Late Miocene several cycles of sedimentation in the South Sokang Sub Basin resulted in the Arang Formation and the Sokang Sandstone. In the North East Natuna Basin the formations formed are the Arang and Terumbu Formations.

The period of uplift was ended by a general unconformity in the Late Miocene (Ginger et al., 1993), which formed the boundary between the syn-inversion and the post-inversion products. The formation formed during this post-inversion phase is the Muda Formation. In the East Natuna Basin the last cycle of sedimentation formed the equivalent of the Muda Formation.

Economical Value
The interest people put on Natuna Area is mainly due to its strategic location and its economical value, i.e. its hydrocarbon resources. It is here that the biggest gas field in Indonesia was found (Esso's giant "L" Field), which is unfortunately not developed yet. The value of the area is increasing lately with the advent of the West Natuna Gas Project, which is a project selling gas produced from gas fields in the West Natuna Area owned by Gulf, Premier Oil and Conoco to Singapore. Exploration history of the Natuna Area was started in 1968 when Natuna Sea Block "A" (covering the north part of Natuna Area) and Natuna Sea Block "B" (the south part) respectively were awarded to AGIP and Conoco. In 1974 Conoco succeeded to discover the Udang oil field and AGIP discovered Anoa oil field. Belanak field was then discovered by Conoco in 1975. In 1980s, Conoco discovered some more significant oil fields such as Ikan Pari (1983), Forel (1984) and Belida (1989), and also gas fields such as Bawal (1979) and Tembang (1981). Several other discoveries were made by other companies in the Kakap Block and Natuna Sea Block "A" during the period of 1980s up to 1990s. The most recent one is the Pelikan gas field, which was drilled by Premier Oil in 1999.

Reservoir rocks in the area are mainly the sandstones of Gabus and Arang, and the limestones of Terumbu Formations. Oil is usually found in Gabus and gas in Arang sandstones. The source rock for oil is mainly from the shales of Benua Formation. The gas is mainly biogenic, generated in the Arang Formation. The Keras and Barat Formation act as excellent seals over the Lower and Upper Gabus Formations respectively. In the Arang Formation, the seals are intraformational. The traps in the West Natuna Basin are mostly structural, in the form of inverted anticlines (Figure 5) and wrench-faulted structures. In the East Natuna Basin traps are mainly present in the form of stratigraphic traps such as reefal buildups (Figure 6) or structural traps such as drapes over basement high.

Anoa, Belida and Kakap Fields are the three most prolific oil producer in the West Natuna Basin. The extraordinary thing about the oil produced from these fields is its high quality, which can reach 46-49API, fetching good prices in the market.

Acknowledgement
We would like to thank Pertamina BPPKA and Premier Oil for the permission to use data and publish this paper, and IPA for the permission in using references. Our gratefulness also extends to our colleagues in Premier Oil for their support in the form of encouragement and discussions and to Mr. Wartono Rahardjo and the librarians of Geology Dept. Gadjah Mada University for their willingness to help us in researching.