Ipa07-G-091-The Paleogene Basin Within the Kendeng Zone

IPA07-G-091

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Thirty-first Annual Convention & Exhibition, May 2007 THE PALEOGENE BASIN WITHIN THE KENDENG ZONE, CENTRAL JAVA ISLAND, AND IMPLICATIONS TO HYDROCARBON PROSPECTIVITY Eddy A. Subroto* Dardji Noeradi* Awali Priyono* Handoyo E. Wahono** Eddy Hermanto** Praptisih*** Kuwat Santoso****

ABSTRACT

A large part of the E-W trending Kendeng Zone of Central Java, is covered by Quaternary volcanic  products with some windows of late Tertiary outcrops, especially in the northern part. The NESW structure found in the area is generally associated with a Paleogene basin and has been  proven as the locus of hydrocarbon source-rock deposition (e.g. Central Deep). The regional gravity map of Central Java shows that the zone is a deep  basin. The gravity map also indicates that the Kendeng Zone is dissected by a NE-SW structural lineament that can be interpreted as the southward  prolongation of the Paleogene structural trend. Field observations have proven that in the southern part of Java, Paleogene outcrops are located along these lineaments (eg. Cimandiri, Luk Ulo, Nanggulan, and Bayat). Stratigraphic studies in some Paleogene outcrops in the southern part of Java revealed that a Paleogene basin was present in the southern part of the island. A series of sediment samples comprising Paleogene and Neogene intervals have been collected during this study. Detailed geochemical analyses have been  performed on some selected sediments. The characteristics of the sediments from both stratigraphic intervals have therefore been defined. Several oil samples were available for this study. Most of the oil samples have experienced severe  biodegradation. Even though the samples are  biodegraded some detailed geochemical analyses, such as gas chromatography – mass spectrometry (GC-MS) and stable isotope analyses, were still conducted. The results indicate that there is relatively good correlation of the biomarker * ** *** ****

Institute of Technology Bandung BPMIGAS Indonesian Institute of Research, LIPI UPN “Veteran”

characteristics of the crude oils with those of the Mio-Pliocene Halang Formation. However, as the Halang Formation is not sufficiently mature to  produce liquid hydrocarbons, the source of the crude oils is most possibly the Wungkal Formation (Paleogene) that has similar depositional environment to the Halang Formation but occurred in a deeper stratigraphic position. INTRODUCTION

Central Java (Figure 1), except the northeast part that is very close to the East Java Basin, is a mysterious area. Some hydrocarbon seeps have  been found within the area but problems with regard to the petroleum system are still unsolved. Papers reporting about the geology and petroleum system of the East and West Java basins are relatively abundant, but only a few papers have discussed the geology in relation to the petroleum system of the Central Java area (e.g. Muchsin et al., 2002;  Noeradi et al., 2006; Satyana, 2006; Subroto et al., 2006). One of the reasons why workers are less interested in exploring this area might be the occurrence of thick volcanic rocks covering the Central Java area. Other reasons are the lack of good potential hydrocarbon source rock outcrops in the region, the limited number of well sections where the Paleogene sediments have been  penetrated, and the lack of reliable geochemical data from such well sections. Regarding the source rocks of the Central Java area, it is generally accepted that they occur in two intervals: Paleogene and Neogene. The Paleogene is thought to contain two potential sources, i.e. the Paleogene coal and clastics intervals, while the  Neogene source is contained within the Miocene  prodelta marine clastics. The main problem in defining the source rock is that the biomarker

distributions of both the Paleogene and Neogene intervals are relatively similar. The aim of this  paper is to first establish a regional Paleogene and  Neogene stratigraphic framework in the Central Java Basin and then to correlate between the crude oils and source rocks in order to try establishing the source rock that is responsible for the occurrence of the crude oils in the area. GEOLOGICAL SETTING a. Tectonics

Asikin (1974), based on field observations of a mélange complex in the Luk Ulo area, Central Java,  proposed two tectonic models for the evolution of Java during the Tertiary. In the Late Cretaceous to Paleocene, a subduction complex trending NE-SW occurred in the area, extending from the Luk Ulo area to the Meratus Mountains in South Kalimantan. According to this model, Central and East Java are considered as part of a non-subducted, oceanic  basin. The subduction trend changed to an E-W direction since the Neogene and therefore Central and East Java changed their status from an oceanic  basin to an island-arc system. It appears that the model needs some revisions, especially in the Paleogene since some new evidence indicates that the basement affinity of Central and East Java is more continental than oceanic.  New data from oil and gas exploration (onshore and offshore) of East Java indicate that two rift basin systems developed during Eo-Oligocene times (Sribudiyani et al., 2003). The first rift system, striking NE-SW follows the Meratus structural trend of Kalimantan and the second system, trending in E-W direction is parallel with the RMK (Rembang-Madura-Kangean) structural trend. Both rift systems are prolific hydrocarbon basins (Figure 2). Sribudiyani et al. (2003) compiled surface and subsurface data in the eastern part of East Java (Figure 2). The results indicate that the  NE-SW structural trend is still present in the middle of the island and extends to the southern part. This fact may imply that the Paleogene rift basin observed in the offshore area of East Java is also  present in middle and southern parts of Java Island, following the NE-SW structural trend. The Neogene basin is another system. The main controlling factor of basin development during this  period was the emplacement of the Java magmatic arc that is striking in a E-W direction. In the East Java region this structural trend is an overprint of the RMK lineament. The Kendeng Deep was one of

the E-W trending deep basins that developed in the  back-arc setting during this period (Smyth et al., 2003).

b. Stratigraphy

The eastern part of Java Island, in terms of its  physiographic characters, can be divided into four zones, i.e. from south to north, respectively: the Southern Mountain zone, Solo zone, Kendeng zone, and the Randublatung-Rembang zone (van Bemmelen, 1949). However, based on the more recent gravity data, only three geological provinces can be observed. They are the Southern Mountains, Kendeng zone and the Rembang zone (Smyth et al., 2005). The three geological provinces reflect more the Neogene to present geological conditions rather than the Paleogene geology. Three areas in the Southern Mountain zone where Paleogene sections crop out have been used as reference; they are the Karangsambung Luk Ulo area of northern Kebumen, the Nanggulan area of the Kulon Progo high, and the Bayat area of southern Yogyakarta. Figure 3 shows the stratigraphic column of the three areas. In the Karangsambung Luk Ulo area, the Paleogene section consists of broken formations with tectonic mélange at the base followed by an olistostrome unit comprising deformed marine shales with some  blocks of conglomerate, limestones, sandstones and  pillow lavas. Tuffaceous sandstones and breccias were deposited over the broken formation in the upper part of the Paleogene section. Lithostratigraphically, the Paleogene section  belongs to the Karangsambung and Totogan Formations of Eocene to Oligocene age. The  Neogene section is marked by a thick interval of gravity mass-flow breccias with fragments dominantly of volcanic rocks (Waturanda Formation). The Waturanda Formation is a manifestation of early Neogene volcanism in southern Java. Higher up, the Middle Miocene section is characterised by marine sediments, including a volcano-calci-turbidite sequence named the Penosogan Formation. The volcano-calciturbidite deposition continued until Pliocene times with the deposition of Halang Formation. The basin was deformed and uplifted during the PlioPleistocene tectonic event. The Paleogene stratigraphy of the Kulon Progo high is characterised by shallow marine to deltaic sediments of the Songo Beds. The formation consists of interbedded quartz sandstones, shales

with thin coal streaks at the base, followed by marine shales with tuffaceous sand intervals. The  base of this formation does not outcrop in Kulon Progo area; however the quartz sandstones that is assign being the Kulon Progo Formation (equivalent with Songo Beds) found in the Nanggulan area indicate that the basement is composed of “continental” rocks (metamorphic?). The Neogene section in the Kulon Progo area is occupied by the Watu Puru Beds of Late Oligocene to Early Miocene age and consists of volcanic breccias and lava flows. The volcanic rock characteristics indicate that Kulon Progo area was a centre of volcanic activity during Late Paleogene to Early  Neogene times.

rocks were deposited in a fluvial to deltaic setting at the foot of a volcanic cone. The Paleogene stratigraphy of the Rembang Zone is characterised by rift-related sedimentation. The synrift sediments correspond to the lower Ngimbang unit deposited in a lacustrine to marine setting during the Middle Eocene to Early Oligocene. The rift-sagging period is represented by the Kujung Formation which is composed of limestones and shales of Late Oligocene age. The Neogene interval consists of shallow marine to beach sediments of marls and limestones and sandstones of the Tuban,  Ngrayong, Wonocolo, Ledok, and Mundu formations.

The Paleogene sediments in the Bayat area nonconformably overly the basement rocks which consist of metamorphic rocks, phyllite, marble and schist. The Paleogene sediments belong to the Wungkal Formation which is composed of conglomerates, quartz sandstones, shales and  Nummulitic limestones of Eocene age. The rock series were deposited in shallow marine conditions occupying the littoral to neritic zone. The basement rock characters as well as the clastic Paleogene sediments show that the tectonic setting during Paleogene was continental. The Neogene section in this area is occupied by the Kebobutak Formation of late Oligocene to Early Miocene age and consists of volcanic breccias and lava flows. Overlying the Kebobutak Formation is the Wonosari Formation of late Miocene age , consisting of mainly reefal limestones.

c. Reconstruction of the Paleogene Basin

Figure 4 illustrates the stratigraphic column of the Kendeng Zone according to Pringgoprawiro (1983). The Paleogene rocks do not crop out in this zone. The oldest outcrop in this area is the Pelang Formation of Late Oligocene to Early Miocene age. The formation consists of deep-water marls and claystones with calcarenite intercalations containing large forams. Gravity data modelling in this zone indicates that the deepest part of the zone is about 8 km. The model might indicate that the Paleogene section could also be present in this zone. The  Neogene interval consists of deep-water sediments of marls and claystones with turbiditic intervals of tuffaceous calcareous sandstones in the lower section (Pelang, Kerek and lower Kalibeng Formations) and gradually changes to shallow marine in the upper section (upper Kalibeng). The Pleistocene interval consists of thickly-bedded sandstones and breccias with predominantly tuffaceous material of the Pucangan, Kabuh, and  Notopuro Formations (Figure 4). The Pleistocene

The second reconstruction indicates that the Paleogene basin in the Southern Mountain, as evidenced from the stratigraphic data of the Southern Mountain, is the prolongation of the NESW Meratus Trend that is observed clearly in the offshore area (Figure 6). Two lineaments are also found to coincide with Paleogene outcrops in the southern part of Java (Luk Ulo and Bayat). The two scenarios are possible and regardless whichever model is used, it is most likely that the Paleogene  basin must exist below the Kendeng Zone.

Based on the results of this study, two basin reconstructions of the Paleogene structural trend and stratigraphy are proposed. The first reconstruction is that the Paleogene basin in the northern part of East Java follows the RMK structural trend (E-W), extending to the south in the Kendeng Zone, then it curves to a S-W direction. The interpretation is based on the gravity map of the Kendeng Zone that is segmented by NE-SW lineaments (Figure 5). Two lineaments are apparent and they coincide with the Paleogene outcrops. Moreover, the geology of the Kulonprogo and Bayat areas support this interpretation. The form of Paleogene basin in this model resembles a horsetail structure related to sinistral strike-slip movement along the RMK Zone.

GEOCHEMICAL ANALYSIS Samples

During this study, a series of sedimentary rocks comprising sections from Paleogene and Neogene intervals have been collected. The samples represent some formations, i.e. Halang, Rambatan, Pemali, Lawak, Totogan, and Karangsambung. The

samples were partly weathered. This condition may influence the source richness. The variation of carbon organic content in the samples is between  poor and fair (0.2 to 0.9%) andthe samples containing fair organic carbon contents were then subjected to detailed geochemical analyses such as stable isotope and GC-MS to assess the biomarker distribution. These samples are in fact from the Halang (HL), Rambatan (RB), Pemali (PM), and Karangsambung (KS) Formations (Figure 7). Please note that Pemali Formation is found in northern Serayu that is equivalent to Pelang and Kerek Formations in the Kendeng Zone.

saturated fraction carbon-isotope analyses were  performed for potential source rock extracts and, therefore, they are indicated by a vertical line on the Sofer (1984) plot in Figure 9. One sample of the Karangsambung Formation and two samples of the Halang Formation plot very close to the crude oils whereas the positions of other samples of these formations are relatively far from the crude oils. Therefore, it cannot be interpreted that the two formations have a good correlation with the crude oils. It is apparent that these techniques cannot be effectively used to separate the source rocks and crude oils.

Figure 7 also indicates the geographical position of the crude oil samples. Four crude oil seeps were involved in this study. They were collected from the Karangnangka-1 (KRN-1), and Gunung Wetan-1 (GNW-1) wells, Kali Panjatan, Lawen (KP-L) and Karangkobar (LW-05A). These crude oil samples were also subjected to detailed geochemical analyses. Two samples (KRN-1 and GNW-1) have experienced biodegradation, so that they could not  be fully used in the correlation with the source rocks. The other two samples are relatively unbiodegraded.

Another technique that can be applied relatively well to indicate the source rock within the area is the correlation between source rocks and crude oils using triterpanes and steranes. On the basis of  biomarkers and stable isotopes, it is apparent that the crude oils belong to one family. These crude oils, in terms of their triterpanes biomarkers, are characterised by a relatively high abundance of oleanane material. Figure 10 shows a comparison of triterpane distribution in the crude oil and in the source rocks, based on mass chromatograms of m/z 191. It is clear that the Pemali Formation does not contain oleanane; the Karangsambung sample indicates a small concentration of oleanane, and the Halang and Rambatan Formations show a relatively high concentration of oleanane (labeled as OL in the mass chromatograms). In this case, the Pemali Formation can be disregarded as the source rock of the crude oils in the area, whereas the other three formations are still possible sources.

Source Rock to Oil Correlation

Correlation between source rocks and crude oils was based on some techniques, such as the stable isotope and biomarkers distribution. It should be noted, that in general, the geochemical characteristics of the four sedimentary formations and the crude oils are relatively similar. Such a similarity might be due to the relatively similar conditions of the paleoenvironment, i.e., fluviodeltaic with organic matter contribution from marine and higher plants. Therefore, the distribution of n-alkanes and isoprenoids (pristane/n-C 17,  phytane/n-C18, pristane/phytane ratios), some steranes, and bicyclic biomarkers (drimanes and rearrange drimanes) cannot be used to differ and separate between the source rocks and crude oils. Figure 8 shows the traditional sterane triangle diagram of Huang and Meinschein (1979). It is apparent that the plot indicates a unity in the middle of the area suggesting a mixture between marine and terrestrial organic matter. A plot of the stable isotopes is given in Figure 9. Again, in general, the isotope distribution of the crude oils (represented only by two samples) is very close one to another. The isotope distribution for the source rocks spreads around that of crude oils. It is to note that only

Figure 11 illustrates a comparison of extended steranes shown by mass chromatograms of m/z 217. On the basis of the concentration of the short steranes (C21 and C 22) and diasteranes (labeled as A and B), it is apparent that only Halang Formation has the best match with the crude oil. However, the geological age of Halang Formation is MioPliocene and it is not likely that its thermal maturity has reached a sufficient level to generate hydrocarbons. If this is the case, the most possible source rock in the Central Java is a formation whose  biomarker distributions resemble those of the Halang Formation but with much higher maturity. Since, the other formations such as the Rambatan, Pemali, and Karangsambung are unlikely formations for source rocks, one of the possible sources of the crude oils is the Middle to Late Eocene Wungkal Formation deposited in a shallow marine environment, because this formation, in many aspects, has similar characters with Halang Formation.

CONCLUSIONS

There are two proposed basin reconstructions for the Kendeng Zone, based on the Paleogene structural trends and stratigraphy. The first reconstruction is that the Paleogene basin in the northern part of East Java follows the RMK structural trend (E-W), turning to the south in the Kendeng Zone then curving in a S-W direction. The second reconstruction indicates that the Paleogene  basin in the Southern Mountains, as seen from analyses of the stratigraphic data of the Southern Mountains, is the prolongation of the NE-SW Meratus Trend of southern Kalimantan that is observed clearly in the offshore area. Correlation studies suggest that the crude oils found as seeps in Central Java might be derived from source rocks that have biomarker characters like that in the Halang Formation, but with higher maturation. The most possible candidate is the Eocene Wungkal Formation. ACKNOWLEDGEMENTS

This work is part of the joint project between BPMIGAS and the Bandung Institute of Technology (ITB) in 2005-2006. Therefore, we would like to acknowledge the management of BPMIGAS for financial support and permission to  publish this work. We also thank Messrs. John A. Bates and Bruce Shapiro for their valuable comments and suggestions on the earlier version of this manuscript.

hydrocarbon system of the southern Central Java region. Proceedings the 31 st  Annual Convention of Indonesian Association of Geologists, 58-67.  Noeradi, D., Subroto, E.A., Wahono, H.E., Hermanto, E., and Zaim, Y., 2006, Basin evolution and hydrocarbon potential of Majalengka-Bumiayu transpression basin, Java Island, Indonesia. Proceedings AAPG International Conference and Exhibition, Perth. Soft file CD version.

Pringgoprawiro, H., 1983, Biostratigrafi dan Paleogeografi Cekungan Jawa Timur Utara. Suatu Pendekatan Baru (Biostratigraphy and Paleogeography of the north East Java Basin. A  New Approach). Doctor Dissertation, Bandung Institute of Technology (ITB), Indonesia. Rahardjo, W., Sukandarrumidi, and Rosidi, H.M.D., 1995, Geological Map of the Yogyakarta Sheet, Java. Pusat Penelitian dan Pengembangan Geologi, Bandung, Indonesia. Satyana, A.H., 2006, New insight of Tectonics of Central Java, Indonesia and its petroleum implications. Proceedings AAPG International Conference and Exhibition, Perth. Soft file CD version.

Smyth, H., Hall, R., Hamilton, J., and Kinny, P., 2005, East Java: Cenozoic basins, volcanoes and ancient basin. Proceedings Indonesian Petroleum Association 30th  Annual Convention & Exhibition. Soft file: IPA05-G-045.

REFERENCES CITED

Asikin, S., 1974, Evolusi Geologi Jawa Tengah dan Sekitarnya Ditinjau dari Segi Teori Tektonik Dunia yang Baru (Geological Evolution of Central Java and Surrounding in a Perspective of a New World Tectonic Theory). Doctor Dissertation, Bandung Institute of Technology (ITB), Indonesia. Asikin, S., Handoyo, A., Prstistho, B., and Gafoer, S., 1992, Geologic Map of Banyumas Quadrangle, Jawa. Pusat Penelitian dan Pengembangan Geologi, Bandung, Indonesia.

Huang, W.-Y. and Meinschein, W.G., 1979, Sterols as ecological indicators. Geochimica et Cosmochimica Acta, 43, 739-745. Muchsin, N., Ryacudu, R., Kunto, T.W., Sribudiyani, Yulihanto, B., Wiyanto, B., Nurjayadi, A., Rahardjo, K., and Riandra, F., 2002, Miocene

Sofer, Z., 1984, Stable carbon isotope compositions of crude oils: application to source depositional environments and petroleum alteration. Bull. AAPG, 68, 31-49. Sribudiyani, Muchsin, N., Ryacudu, R., Kunto, T., Astono, P., Prasetya, I., Sapiie, B., Asikin, S., Harsolumakso, A.H., and Yulianto, I., 2003, The collision of the East Java microplate and its implication for hydrocarbon occurrences in the East Java Basin. Proceedings Indonesian Petroleum

Association 30th  Annual Convention Exhibition. Soft file: IPA03-G-085.

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Subroto, E.A., Wahono, H.E., Hermanto, E.,  Noeradi, D., and Zaim, Y., 2006, Re-evaluation of the petroleum potential in Central Java Province, Indonesia: Innovative approach using geochemical inversion and modelling. Proceedings AAPG

International Conference and Exhibition, Perth. Soft file CD version.

van Bemmelen, R.W., 1949, The Geology of Indonesia. Government Printing Office. 2 volumes.

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Figure 6 - Reconstruction of the Paleogene basin in Central Java based on structural trend that is parallel with the Meratus trend. Other legends can be seen in Figure 3.

HL-2 PM-1

HL-1 PM-2

HL-5

RB-3

HL-3 HL-4 RB-2 RB-1 KP-L

KRN-1

LW-05A

KS-2 KS-1

Cipari-1

KS-3 G. Wetan-1

Sample location Oil seeps

Figure 7 - Map of Central Java showing locations of outcrop and crude oil seep samples.

100% C28

Planktonic

Open Marine or  Deep Lacustrine

Estuarine or

Shallow Lacustrine

100% C27

Legend:

Terrestrial Higher  Plant

100% C29

Halang Fm. (5 samples) Rambatan Fm. (3 samples) Pemali Fm. (2 samples)

Crude oils (4 samples)

Karangsambung Fm. (3 samples)

Figure 8 - C27-29 steranes triangle diagram of Huang and Meinschein (1979) for crude oil and sediment samples collected from the Central Java Basin.

Isotopic Characterisation of Sediments and Crude Oil

Terrigenous KRN-1

GNW-1

KS HL RB PM

Algal (marine or non-marine)

Figure 9 - Sofer’s plot of carbon stable isotope for sediment and crude oil samples collected from Central Java. Line and dotted symbols indicate that only saturate isotope available.

Figure 10 - Partial mass chromatogram m/z 191 showing distribution of tricyclic and pentacyclic terpanes in sediments and crude oil collected from Central Java.

Figure 11 - Partial mass chromatogram m/z 217 showing distribution of steranes in sediments and crude oil collected from Central Java.