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PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Twenty-Ninth Annual Convention & Exhibition, October 2003 THE COLLISION OF THE EAST JAVA MICROPLATE AND ITS IMPLICATION FOR HYDROCARBON OCCURRENCES IN THE EAST JAVA BASIN Sribudiyani* Nanang Muchsin* Rudy Ryacudu* Triwidiyo Kunto* Puji Astono*
ABSTRACT The East Java basin is one of the major petroliferous basins in Indonesia. However, understanding of the tectonic development of the area is still subject to ongoing debate. Detailed regional geologic investigation was carried out to study and evaluate tectonic history and basin development in relation to hydrocarbon potential of the area. Recently acquired seismic data and hydrocarbon discovery in the sandstone unit of the Ngimbang Fm. in the EJ-1 well at East Java triggered a new interpretation of tectonism and basin development, especially during Paleogene time. From the end of Cretaceous to Early Eocene, a continental fragment, possibly detached from the Gondwana super-continent to the south, drifted northeastward approaching the Late Cretaceous to early Tertiary subduction complex (Lok Ulo-Meratus belt). The collision of this micro-continent with the eastern margin of the Sunda Microplate caused the Eocene magmatic activity to cease and uplifted the subduction complex, creating the Meratus Mountains in the eastern part of Kalimantan and The Lok Ulo mélange complex in the central Java. The grain of the continental basement influenced basin trends. In eastern part of East Java Basin the dominant basement grain is E-W, as can be particularly well observed controlling the Kendeng and Madura Troughs. At the very close of this period, compressional tectonism due to the northward convergence between the Australian plate and the Sundaland margin reactivated these pre-existing E-W basement faults into strike-slip movement (e.g. along * **
Pertamina Institute Technology Bandung
Indra Prasetya * Benyamin Sapiie** Sukendar Asikin** Agus H. Harsolumakso** Ivan Yulianto**
the Sakala Fault Zone). Another type of basin configuration developed at the collision zone oriented NE-SW, parallel to the direction of the collisional suture along Lok Ulo – Meratus Complexes. As a result, it is proposed that the Muria Trough in the north is a foreland basin which probably can be traced further south through Kendal Sub-basin to Kebumen Sub-basin. The result of this study reveals a new target for hydrocarbon exploration in the area. INTRODUCTION The complex processes resulting in basin formation of Indonesia owe their origin to the interaction of plate movements between the Indian, Eurasian, Australian and Pacific plates (Audley-Charles, 1988; Hamilton, 1979; Hall, 2002). Within the framework of these major plate movements, a large number of rigid microplates may be incorporated, the boundaries of which influence basin formation and deformation. Such basins should not be considered as existing in a tectonically steady state, but they evolve and change complexly in response to the speed and direction of plate movement and type of interactions (Hall, 2002). A better understanding of the characteristics of a basin is most important before the execution of intensive exploration programs (Dickinson, 1974). This is generally true for exploration conducted in areas where data are limited, or in frontier areas. However, the same situation may also be applicable to mature basins, in which case re-evaluation of existing data by applying new theories and concepts may highlight new plays and new targets. This is the case in the East Java Sea basin. Re-interpretation of old data (surface and sub-surface), and assessment of hydrocarbon plays using newly developed concepts,
may result in a new understanding of the geologic history of the basin and previously unrecognized hydrocarbon systems can be deduced. Hence, new leads and prospect areas previously overlooked may be exposed. The purpose of this paper is to evaluate the tectonic development of Java Island, particularly in the east basinal area. METHOD OF STUDY The present tectonic development and framework of Java Island cannot be considered in isolation, as they are the product of the tectonic history of Southeast Asia, extending from Late Mesozoic and Tertiary. Therefore, the main portion of the study focused on the understanding of the complex plate tectonic history of Southeast Asia. Understanding the tectonic history of this region is critical to the acceptance of the concept of geodynamic evolution of all Tertiary basins in Indonesia. The geologic setting of the region is created by forces acting on the crust, which is controlled by relative motion between interacting plates. This will lead to the conclusion that to understand the tectonic evolution of a region, it is critical to define the timing and condition of plates (type of plate interaction), which has taken place during the deformation. It is necessary to use the principles and characteristics of plate movements since the tectonic setting of a particular area is the product of plate interaction whether it was convergent, divergent or transform. The resulting tectonic process, which is responsible for the formation and deformation of the basins, is ultimately dependent on the behavior and mechanical properties of plates involved. Geologic maps, geophysical data (seismic, gravity and magnetic) and wells penetrating basement provide information to estimate these mechanical properties. In this study, the evaluation was conducted using existing published data and newly acquired exploration data such as 3D and 2D seismic data, wireline log and well data. In addition, selected detailed surface geological data were used to verify the subsurface interpretation and model. RESULTS In this study, identification of the successive plate interactions which resulted in the tectonic events affecting the Sunda Microplate allowed analysis and
interpretation of the tectonic evolution of Java and its surrounds (Figure 1A, B, C and D). Late Cretaceous – Early Tertiary (70 – 35 Ma) During this period, the contemporaneous northeastward movement of the Australian plate resulted in its subduction under the Sunda Microplate along Java-Meratus suture. Late Cretaceous magmatic activity can be traced continuously from northeast Sumatra through Java to southeast Kalimantan (Figure 2). In West Java the Jatibarang Formation represents a volcanic series deposited in a north-south oriented graben-like depression associated with an extensional regime within the uplifted magmatic arc. Constructive fore-arc basins developed in the southern part of west Java and the South Serayu Mountains in Central Java. Ciletuh and Karangsambung Formations in the Lok – Ulo complex represent sediments deposited within this unstable fore-arc basin (Figure 3). Basin fill consists of tightly folded mudstone with intercalation of sandstone, conglomeratic sandstone and limestone. Turbidites and mass-flow deposits are common (Martodjojo, 1998). Based on field work in Central Java, middle to upper Eocene quartz clastic sandstones named Nanggulan Fm. are found scattered in the southeast and west parts of Luk-Ulo complex. In both onshore and offshore of east Java, a similar age quartz clastic unit known as Ngimbang Fm. is found in both NE-SW (along Meratus trend) and E-W graben known as the Sakala trend. Ngimbang Fm. is distributed further east in the East Kangean-Pagerungan area. This formation was penetrated near TD of EJ-1 well. Seismic section in the vicinity of the well shows strong reflector below Ngimbang Fm., suggesting the presence of deeper sedimentary units grouped as PreNgimbang Fm. It has been reported that PreNgimbang Fm. was penetrated in several wells in East Kangean area. From limited data, this formation occurs along E-W trending depression and is unconformably overlain by Ngimbang Fm. This evidence suggests the most likely presence of a paleohigh consisting of continental source at least during Cretaceous – Eocene time. This continental fragment would have to extend to the west as far as central Java and to the east as far as East Kangean area. Close to the end of Cretaceous – Early Eocene, a continental fragment, possibly detached from
Gondwana super-continent to the south, drifted northeastward approaching the subduction zone. The presence of allochthonous micro-continents in Southeast Asian region has been observed and reported by many authors (e.g. Audley-Charles, et al., 1988; Bergman, et al., 1996; Metcalfe, 1994; Parkinson, et al., 1998). Continental basement located east of the subduction zone underlying Makassar Strait has been identified in well Rubah-1 drilled by Conoco in January 1977, which encountered granitic basement at 5056 ft, overlain by sandstone containing fragments of granitic rocks. The sandstone is calcareous, angular, poorly sorted, granule to fine sand sized, in a carbonate matrix. The terrigenous constituents are most likely derived from granitic basement. The Taka-talu well, located further southeast, penetrated basement consisting of diorite. However, no radio metric dating is available. The docking of this micro-continent with the eastern margin of the Sunda Microplate caused Eocene magmatic activity to cease and uplifted the subduction zone, creating the Meratus complex (see Figure 1B). Rifting took place away from the plate boundary and extension was associated with movement along the pre-existing regional faults within the continental fragment. The grain of the continental basement influenced basin trends. Global plate reorganizations in Southeast Asia region occurred in middle to late Eocene, including the collision of greater India with Asia and the change in velocity of Pacific plate motion (Tapponnier, et. al., 1986). The collision of India with Asia resulted in east and southeast extrusion of eastern Eurasian margin along large strike-slip faults. Strike-slip and block faulting during this time resulted in the development of the Thai, Malay and West Natuna basins. The basin systems of Sumatra and Java were also initiated during this period (see Figure 1A, B). Basin formation commenced with the transtensional phase along strike-slip movements (Red River Fault Zone, Thai-Burma Shear, Bangka Shear and Sumatra Fault Zone) in which predominantly north-south extensional troughs, grabens and half-grabens were formed. Continental clastics were then deposited in these basins as the initial fill. Major strike-slip movements along large shear zones might have been compensated by counter-clockwise rotation of the Sunda Microplate (Tapponnier, et. al., 1986).
Oligocene – Early Miocene period (35 – 20 Ma) It is generally accepted that at the beginning of Oligocene a widespread rapid reduction of plate movement took place (Hall, 2002). The northward Australian Plate slowed from 18 cm/year to only 3 cm. A direct consequence of this reduction of plate movement will have been an increase in the angle of dip of subduction. It is proposed that during this period, due to the deceleration of plate movement, a general uplift took place within the whole southeast region of Sundaland. Erosion and local subsidence along existing fault traces resulted in deposition of terrestrial and transitional marine deposits (Figure 4). During this period, the South China Sea became an active sea-floor spreading centre. Northward convergence of the Indian Plate resulted in compressional tectonic regime in Sumatra and Java "Fore-Arc" region causing basin inversion. The convergence rate of the Indian Plate with Sunda Microplate had now stabilized at 5 to 6 cm/year (Hall, 2002). On the contrary, the back-arc basins were subject to dominantly north-south strike-slip movements along existing north-south faults (Figure 5). Middle Miocene – Late Miocene period (20 – 5 Ma) A southward shift of Indian-Australian Plate boundary took place, followed by continuous magmatic activities occupying almost the whole of the island of Java. In the northern part, back-arc basins developed, subdivided into several sub-basins, which were separated by basement highs, controlled by basement block faulting. In northern West Java the orientation of basement faulting remains north-south, whereas in the Southwest and Central Java a significant NE-SW and NW-SE fault system seems more pronounced (Figure 6). These patterns may have been initiated as strike-slip pair faulting during the process of northward subduction along the southern part of Java. It is here suggested that basement grain strongly controlled the basement configuration of the basin, and consequently also the Miocene sedimentation. Cimandiri Trough, Citanduy Sub-basin and Western Deep are considered to be examples of this type. Reactivation along those faults resulted in transtension and transpression mechanisms associated
with turbidite sedimentation in the subsiding part. In easternmost of East Java, however, the dominant basement grain is east-west, as can be particularly well observed controlling the Kendeng and Madura Troughs. E-W grain forms part of the underlying continental fragment previously transported from the south and which collided wit h Sundaland along the Meratus Suture (NE-SW structural grain). At the very close of this period, compressional tectonism due to the northward subduction has changed these E-W basement faults into a strike-slip movement (Manur and Barraclough, 1994). Another type of basin configuration developed at the collision zone oriented NE-SW, parallel to the direction of the suture. This orientation is dominant structural grain at the western half of East Java. It is proposed that the Muria Trough in the north is a foreland basin of this type, which probably can be traced further south through Kendal Sub-basin to Kebumen Sub-basin. Basins in the north of Central Java are considered to be transitional between NWSE and NE-SW grain (Figure 6). Figure 7 summarizes the relationship between tectonics and stratigraphy units in East Java Basin. Many previous models have been proposed to explain the geological complexity of the East Java Basin. However, most of them are regional models and they do not incorporate actual geologic field evidence such as stratigraphic ages and local structures. The proposed model is aimed to answer some of the uncertain geologic evidence observed in the field at East Java. The model was developed based on actual geologic data from both subsurface and surface mapping. However, this model is not the only answer for the geologic complexity found in the East Java Basin. Two main proposed ideas resulted from this study. The first: most of basement at the East Java Basin including onshore is made of continental crust, namely East Java micro-continent. The second: distribution of Ngimbang Fm. is controlled by both NE-SW and E-W depressions. This evidence could suggest the possibility of two facies of Ngimbang Fm., deposited in two different environmental systems. These results will lead new thinking in designing hydrocarbon exploration program in East Java Basin. IMPLICATION FOR HYDROCARBON PROSPECTIVITY OF EAST JAVA The East Java Basin area is well known as one of the
most prolific hydrocarbon potential areas in Indonesia since the early exploration history from 1887. However, the major exploration activities in the onshore and offshore area just have taken for the last 20 years. Recent discoveries include Banyu Urip in the Cepu area and Wunut gas field in the Madura Strait Sub-basin. From an exploration standpoint, the East Java Basin is considered as a mature basin. Hydrocarbons occur and are produced from different stratigraphic levels, and widely distributed from west to east. Several good sources are present within the east Java area are largely restricted to organic rich of shales and coals of the Ngimbang and Kujung Fms. and Cepu Member of Tawun Fm. In the Northern Platform they can be found in western part of the platform inclu ding Muria Trough, East Bawean Trough, Tuban-Camar Central Depression, Masalembo Basin and to the east of this platform including Pagerungan; in the Central High they cover most of the area which was previously a depocenter; in the Southern Basin they cover the whole southern basinal area. It is likely that the East Java Basin comprises multiple source rocks and each local area shows distinct source rock characteristics. The PreNgimbang Clastics consist of interbedded thin sand and shales, with minor coal, the thickness varies from 118 to 445 ft. Results from integrated geological and geochemical study of the Northern Platform and Central High indicates that hydrocarbons in these areas originated from the Pre-Eocene to Eocene age source rocks, as it is observed in the JS-53 well and in the Pagerungan gas and condensate field. The Cepu Area contains the major produc ing oil fields in the East Java Basin. The producing horizons range in age from Oligocene to Miocene. In the offshore Madura Island, the producing horizons range in age from Eocene to Pliocene. In the Pagerungan Field, the producing horizons are from Eocene limestone and sandstone. The hydrocarbon occurrence and distribution of major oil fields in East Java Basin are presented in Figure 8. One of the new concepts arising from this study is the presence of the proposed East Java micro-continent. This new concept allows a very different interpretation of the tectonic evolution of Java, particularly the East Java basin. This proposed tectonic concept also gives us greater opportunity to develop new play concepts in this block. Traditionally, the Ngimbang Formation has been considered as both source and reservoir rocks in this region. Carbonates of the Kujung Formation are
another target for reservoir in most East Java prospects. However, geologic information is scarce and understanding of the Ngimbang Formation is poorly developed. This is partly because there are no Ngimbang outcrops present anywhere in East Java. In addition, both seismic and well data are limited. Therefore, it is very difficult to study this formation in more detail. With the new tectonic concept from this study, we can have a better understanding of the facies distributions of Ngimbang Formation. Our tectonic model suggests that Ngimbang Formation was deposited at the margin of the East Java micro-continent and its depositional history can be divided into pre-, syn- and post-collision. The Ngimbang Formation section in EJ-1 well includes a very well-rounded polymict conglomeratic layer composed mostly of quartz fragments. This evidence suggests initiation of collisional event and this conglomerate is interpreted as a molasse deposit. Therefore, the stratigraphic unit below the conglomerate should be considered as pre-collision or pre-Ngimbang Formation. This unit is unexplored and it may be important as source or reservoir. Based on seismic interpretation at EJ-1 well, these precollisional units are present and very thick. Pre-collisional structures (e.g. E-W Sakala Fault Zone) can play very important role in hydrocarbon migration and trapping, and also in controlling the distribution of post-collision Ngimbang Fm. Intersection of these older features with younger structures such as Meratus trend can generate a very complex structural pattern which may control the distribution of Ngimbang Fm. (Figure 9). In addition, the distribution of the E-W structural trend to the south is unknown. Therefore, this play is mostly unexplored in the southern part of East Java, and its potential is unknown. Based on our new tectonic concept, two main new play types can be explored in the eastern part of Java, particularly in the East Java Basin area, these are: 1. Looking for pre-collision structures and lithology (pre-Ngimbang Fm.) in the eastern part of the East Java Basin (e.g. NE Madura and South Madura). This can be done by re-interpretation of subsurface data using new objectives. 2. Looking for the intersection of NE-SW and E-W structures in onshore East Java, particularly
following the Cretaceous – Tertiary suture zone (trend of Meratus complex). CONCLUSIONS The regional tectonic study of East Java supports evidence of the collision between a micro-plate (drifted continent) and Sundaland, starting approximately at late Cretaceous to middle Eocene. Two major structural trends are found in the East Java area: the E-W Sakala trend and the NE-SW Meratus trend. Based on the stratigraphy and age of the oldest sedimentary unit deposited in the basin, the Sakala structural trend is slightly older than the Meratus trend. The oldest sediment fill in the Sakala trend in the EJ-1 is dated Eocene in age. The unit consists of conglomerate with as interbedded quartz sandstone, limestone and shale, and is known as the PreNgimbang Fm. Thus far the evidence shows that this unit is only found along the E-W trends. This evidence supports the interpretation in which the E-W trend is inherited from the pre-existing structural trend of the micro-continent. This model creates significant exploration opportunity in the eastern part of the East Java Basin particularly to the south of the Sakala Fault Zone. Further studies will have to concentrate on conducting subsurface and surface mapping in the southern part of the East Java province and continuing toward the eastern part of the island, particularly along the southern part of Sakala Fault Zone. ACKNOWLEDGEMENTS This study is part of the tectonic study of Java supported by a grant from PERTAMINA Directorate Hulu. We would like to give thanks for the support and help from all geologic staff at the PERTAMINA Kwarnas office. We also thank the faculty and staff of the Geodynamic laboratory for their help and support during the study. A lot of others are not listed and they have contributed in some way to our knowledge and ideas although they may not share the same views. A challenging review of an earlier version of the manuscript by Colin Ford stimulated revision of the text to give better explanation and clarification. REFERENCES Audley-Charles, M.G., 1988. Evolution of the Southern Margin of Tethys (North Australian Region)
for Early Permian to Late Cretaceous, in: M.G. Audley-Charles and A. Hallam (eds.), Gondwana and Tethys, Geol. Soc. London, Spec. Pub. 37.
Hamilton, W.R., 1979. Tectonics of the Indonesian Region, US Geol. Survey Professional Paper 1078, US Government Printing Office.
Beicip, 1985. Hydrocarbon Potential of Western Indonesia, Pertamina.
Manur, H., and Barraclough, R., 1994. Structural control on hydrocarbon habitat in the Bawean Area, East Java Sea, IPA Proceedings 23rd Annual Convention, p. 129-144.
Bergman, S. C., Coffield, D. Q., Talbot, J. P., Garrard, R. J., 1996. Tertiary tectonic and magmatic evolution of western Sulawesi and the Makassar Strait, Indonesia: evidence for a Miocene continentcontinent collision. In: Hall, R., Blundell, D. J., (Eds), Tectonic Evolution of SE Asia, Geological Society of London Special Publication, 106, p. 391430. Dickinson, W.R., 1974. Plate Tectonics and Sedimentation in Tectonic and Sedimentation, Society Economic Paleontologist Mineralogist, Spec. Pub. 22. Dickinson W.R., and Seely D.R., 1979. Structure and stratigraphy of forearc regions: Am. Assoc. Petrol. Geol. Bull, v. 63, p. 2-31. Hall, R., 2002. Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computerbased reconstructions, model and animations, Journal of Asian Earth Sciences 20, p. 353-432.
Martodjojo S., 1998. The Geology and Sedimentation of West and Central Java, Lembaga Pengabdian Kepada Masyarakat, Institut Teknologi Bandung. Metcalfe, I, 1994. Gondwanaland origin, dispersion and accretion of East and South Asian continental terranes, Journal of South American Earth Sciences 7, p. 333-347. Parkinson, C. D., Miyazaki, K., Wakita, K., Barber, A. J., Carswell, D. A., 1998. An overview and tectonic synthesis of the pre-Tertiary very-high pressure metamorphic and associated rocks of Java, Sulawesi and Kalimantan, Indonesia, Island Arc 7, 184-200. Tapponnier P., Peltzer G. and Armijo R., 1986. On the mechanics of collision between India and Asia. In Coward, M.P. and Ries, A. (eds.), Collision Tectonics, Spec. Publ. geol. Soc. London, 19, 115157.
Figure 1 - Tectonic evolution of western Indonesia.
Figure 2 - Tectonic framework of Java Island at 70 – 35 Ma.
Figure 3 - Schematic cross-section of Java Island at 70 – 35 Ma.
Figure 4 - Tectonic framework of Java Island at 35 – 20 Ma.
Figure 5 - Schematic cross-section of Java Island at 35 – 20 Ma.
Figure 6 - Tectonic framework of Java Island at 20 – 5 Ma.
Figure 7 - Chonostratigraphic chart of tectonic evolution and distribution of known hydrocarbon occurrences at East Java Basin.
Figure 8 - Tectonic elements and the distribution of known hydrocarbon occurrences at East Java Basin.
Figure 9 - Block diagram showing schematic model of deposition of the Ngimbang Fm. in East Java.
