IPA08-SG-082

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Thirty-Second Annual Convention & Exhibition, May 2008

HYDROCARBON PLAY IN KETUNGAU-MELAWI BASINS

Willy Brodus Bossu Passe*

Handra Rasfi Eko Nugraha* Mirza Alfikri Wijaya*

Liswanto Sitio* Yulia Febriyeni*

ABSTRACT Ketungau - Melawi Basins is located in westside of Kalimantan Island (West Kalimantan Province, Indonesia), and separated by Highland of Semintau Ridge Complex, categorized as intra-continental basin. The basin was controlled by tectonic activities during Early Cretaceous and Paleocene. Pre-Tertiary tectonics activities lead to basin configuration, which is bounded by basement high of schist and phyllite from Semintau Formation (Triassic-Jurassic age). The next Neogene tectonic activity caused Tertiary rocks folding of Silat and Tebidah Formations and created Ketungau, Silat and Melawi synclines, and Sintang anticline. Source of sediment (provenance) was came from granitic mountain belt of Schwaner and also come from High of Semintau Ridge. Stratigraphies in Ketungau - Melawi are divided in to 2 phases, PreTertiary stratigraphy and Tertiary stratigraphy. Pre-Tertiary stratigraphy consist of basement rocks with age of Triassic and Jurassic recognized as Semintau Complex, which unconformable underlies with clastic sedimentary rocks Pendawan Formation with age of Cretaceous and was deposited in open marine environment (Heriyanto et al, 1993). This formation is estimated as source rock. The Tertiary stratigraphics mostly is composed of fine-coarse grained terrigeneous clastic sediment to open marine deposits rocks. The source rock was recognized in Silat Formation and Sekayak Formation; meanwhile reservoir rock had been recognized as deltaic sandstone of Early-Middle Miocene Haloq Formation. Based on paleocurrent and provenance analysis indicate that the source sequence in Ketungau-Melawi Basins was came from North particularly

* University of Padjadjaran

coming from uplift of orogenic material from Boyan mlange and Lubok Antu. Another indication is maturity level of organic which influenced by Pre-Tertiary subduction and overburden from Tertiary sedimentary. Hydrocarbon migration controlled by a Pre-Tertiary tectonic event. The trap of this basin was caused by tertiary basin inversion which causing thrust-faults. Therefore, from this data we can conclude that Ketungau - Melawi Basins have a good potential for hydrocarbon. INTRODUCTION Ketungau and Melawi Basins located in West Kalimantan Province, Indonesia which separated by High of Semintau Ridge Complex and categorized as intra-continental basin. The coordinates of these basins are 110 46 18 113 East Longitude 25 28 and 0 41 58 South Latitude 1 02 21 North Latitude. Tectonically, these basins are a part of Sunda Land that spreading from Malaya Peninsula. On the north side is bordered by basement high that consist of schist and phyllite ( Triassic Jurassic ), while on the central and the south sides are bordered by basement high (granite) in age of Cretaceous. These basins were controlled by tectonic event during early Cretaceous until Paleocene. Ketungau basin is about 20.000 km2 wide including Sanggau regency, Sintang and Kapuas Hulu. The sediment in Ketungau Basin was started deposition since Upper Eocene until Upper Oligocene with 7000 m thickness. The Melawi Basin is about 36.330 km2 wide including Sanggau regency, Sintang, and Ketapang. Sediment in Melawi basin started deposition since Upper Eosen until Lower Oligosen. The thickness of Melawi Basin is predicted about 4500-9700 m. GENERAL TECTONIC SETTING The tectonic event of this area was influenced by Eurasia crust movement to the Southeast. The

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Eurasia crust moved during Cretaceous until early Tertiary caused the Ketungau and Melawi Basins have a complex tectonic. The formations of Ketungau and Melawi Basins are controlled by Pre-tertiary tectonic event. The activity of Pre-tertiary tectonics is causing a forming of the basin that bordered by basement high. It consists of schist and phyllite from Semintau Formation that attain the age of Trias-Jura. Sedimentary rock which attain the age of Cretaceous from Sekayan and Selangkai Formation was deposited on top of the basement rocks. The activity of tectonics during last Cretaceous is causing high land and low land part forming. The Cretaceous Granit has formed almost all of the high land. This high land was bordering Melawi basin to the South and bordered Ketungau basin to the North. The neogene tectonic activities have been caused Tertiary rocks in Silat and Tebidah Formation was folded. This fold was causing Ketungau, Silat and Melawi syncline and Sintang anticline. After the evolution of the basin which caused an accumulated sedimentary structural occurs, the next process is material filling of Ketungau - Melawi Basins. Provenance in this material filling process was came from Kucing Highland or from Rajang Embaluh Accretion Complex which dominating on the northern side. On the southern side, the provenance was dominated by Schwarner Mountain zone. The next process in Late Eocene -Early Oligocene was a basin modified by tectonic phase which caused a basin structural change and divided basin into two small basins, Ketungau Basin on the northern part and Melawi Basin in the southern part. Ketungau Basin changed from active basin into inactive basin, but in Melawi Basin the sedimentation process was still happened until Late Oligocene time. BASIN STRATIGRAPHY

1. Pre-tertiary Stratigraphy

Pre-tertiary stratigraphy consists of basement rocks (Triassic and Jurassic), with an unconformity of a clastic sedimentary rock lithology (Cretaceous); they are such as:

a. Semintau Complex

Semintau Complex is a basement rock in Melawi basin which consists of green schist, amphibole, phyllite, quartz and serpentine. At

several places, rock unit in Semintau Complex changed into Melange Complex with tectonic contact in each unit. The age of these units are Perm Trias (Heriyanto et al, 1993).

b. Pendawan Formation

Pendawan Formation is the oldest clastic sedimentary rock in the surface. This formation consist of shale, slate-shale, carbonaceous mudstone, claystone and sandstone. Some of the sandstone contained chalk substance, tuff and fossil. This formation estimated as turbidite deposit because the sequence from the sandstone shows a fining upwards intercalation with flute cast and slump structure. The age of this formation is Late Cretaceous which was deposited in open marine environment.

c. Selangkai Formation

Selangkai Formation is Pre-Tertiary clastic sedimentary rock that disclosed in Melawi Basin. This formation consists of the intercalation of mudstone, calcareous siltstone, non calcareous siltstone, sandstone, polimic conglomerate and limestone. Based on the interpretation and sequence analysis at several places, they have been showing that this formation was deposited as a turbidite marine deposit. The age of this formation is Early Cretaceous - Late Cretaceous. This formation was deposited in transition open marine environment (Heriyanto et al, 1993).

2. Tertiary Stratigraphy The Tertiary sediments were deposited unconformity at basement rock. Generally, the Tertiary rocks consist of fine - coarse clastic sedimentary rocks that are the terrigenous - open marine deposits. The Tertiary sediments from the oldest to the youngest are:

a. Haloq Formation

Haloq Formation is the oldest tertiary rock that showed in Melawi Basin. This formation consists of the unit of quartz sandstone of pebble conglomerate sandstone and mudstone with massive bed, cross bedded and unfossiliferous. The age of this formation is unknown but based on the statigraphy the age of this formation is older from Ingar Formation.

b. Ingar Formation

Ingar Formation consists of calcareous mudstone, thin silt and soft sandstone lamination. This rock consists of foraminifera and radiolarian. On the southern part this formation was closed by arcose sandstone unit. The estimation age of this formation is lately Miocene. And this formation was deposited at shallow-deep marine (Heryanto, 1993).

c. Dangkan Formation

Dangkan Formation was deposited unconformable above Ingar Formation. This formation consists of quartz intercalation until lithic sandstone, pebble sandstone and conglomerate in base part of the sequence. The unit of the rock has a total thickness about 1000 m and shows the massive structure until thin bed and cross bedding until graded bedding. The age of this formation is Late Miocene. According to sediment structure, this formation was deposited in braided stream environment.

d. Silat Formation

As deposited conformable above Dangkan Formation, sediment sequence of Silat Formation shows black deposit carbonaceous mudstone, slate shale and claystone with dark grey colour, sandstone with intermediate fine grain, calcilutide where in some places have a coal bed and pelecypoda fossil. This formation was spreaded widely in the basin as referred to the area of the Silat River. This formation age is Upper Miocene -Lower Oligocene. The composition and variation from lithology show that this formation deposited in transition-restricted marine environment.

e. Payak Formation

Payak formation was deposited unconformable above Silat Formation. This formation consists of sandstone intercalation, mudstone, siltstone and in some places rich of fossils. This formation was distributed in the southern part of the basin. It was deposited in delta area until shallow marine environment with the thickness about 15000 m and the age is Late Oligocene. f. Tebidah Formation

Tebidah Formation was deposited conformable above Payak Formation. At the lower part of

this consists of mudstone intercalation and siltstone, and at the upper part consists of sandstone intercalation and mudstone with grey, red, and green colour with thin bed coal. This formation has a thickness about 1000m and deposited in delta area-shallow marine.

g. Sekayam Formation

Sekayam Formation was deposited conformable above Tebidah Formation and distributed in the western part of the basin with the thickness about 300-500 m. This formation consists of moderate-coarse lithic sandstone intercalation, mudstone and in some place has grain sandstone. The age of Sekayam Formation is Middle Oligocene Upper Oligocene

h. Landau Formation

Landau Formation was deposited unconformable above basement and has a similarity with Sekayam Formation, with Upper Miocene age. This formation consists of sandstone intercalation, sandstone conglomerate and thin bed siltstone. This formation is interpreted as a deposited river.

PETROLEUM SYSTEM Source Rock Source rock in Melawi Basin is predicted coming from clastic sediment in Pendawan Formation (Cretaceous), Silat Formation (Eocene) and Sekayam Formation (Oligocene). Another source rock can be found as shale in Silat Formation (Eocene) and in the bottom of Payak Formation (Oligocene). Maturity level is about 0, 50% Ro. Reservoir Rock Main reservoir in Ketungau and Melawi Basins are deltaic sandstone in Haloq Formation (Early Miocene - Middle Miocene), sandstone in Ingar Formation (Early Miocene - Early Oligocene), sandstone in Payak Formation (Oligocene) and shallow marine sandstone in Tebidah Formation.

Trap and Migration

The primary hydrocarbon migration in Ketungau and Melawi Basins is a vertical migration from source rock in Pendawan Formation and Selangkai Formation, it was also happened from shale in a bottom part of Silat Formation, this migration was came to the reservoir through fault zone. The

secondary migration is a lateral migration which came from primary reservoir to secondary reservoir. In this basin, migration was occurred from northern to southern which is a local migration. Traps which developed in this basin are stratigraphic trap and structural trap such as fold. HYDROCARBON PLAY 1. Sandstone Play of the Late Eocene Ingar

Formation Transgressive sandstone in Ingar Formation (Late Miocene) was deposited in deltaic transition environment consist of lithic-arenite-sub-arcoses (porosity 10% - 25%). Reservoir was distributed in whole basin, especially in the eastside. The cap rock is black shale in Silat Formation. The hydrocarbons trap is faults blocks and also the combination of faults and folds. 2. Sandstone Play of the Early Oligocene Silat

Formation At early Oligocene, sandstone in Silat formation which consist of lithic-arenite-quartz arenite was deposited in shoreline-shallow marine environment. This sandstone was distributed mainly in an eastern part of Melawi Basin. The claystone in upper part of Silat formation act as a cap rock. It has an anticline trap system. 3. Sandstone Play of the Late Oligocene Payak-

Tebidah Formations A late Oligocene of Payak/Tebidah Formation contains lithic arenite sandstone from deltaic-fluviatile environment which has porosity from 10.6% to 13.5%. The reservoir was distributed into all part of the basin, and it has claystones from Payak and Tebidah Formation as a cap rock. CONCLUSIONS AND IDEAS Both of Pendawan and Selangkai Formation are predicted as sedimentary basement and also as source rocks. We also predicted that Tertiary formations in Ketungau-Melawi can be act as good reservoirs. The hydrocarbon maturity processes happened during Tertiary tectonic regime, it was started from Eocene when rifting happened until Oligocene time where compression tectonic processes happened. The migration processes happened in same time with maturity processes and

hydrocarbon migrated to upper formation such as Haloq, Payak, and Tebidah Formation. If we connected this basin with tectonic process, we can conclude that maturity from source rocks will be better in Pre-Tertiary sedimentary rocks rather than maturity in Tertiary sedimentary rocks. Besides that, we can conclude that the thickness from Tertiary sedimentary rocks is not enough to give proportional burden because it just has a thin sedimentary rock (Eocene-Oligocene). If we do not have enough burdens, subsidence would not happen and it affected to the temperature and pressure as we need to make the hydrocarbon in Tertiary sedimentary rocks become mature. The first drilling in Ketungau-Melawi Basins was penetrated until Silat Formation only, it has not reached Ingar and Haloq Formations. If we see Haloq formation, we conclude that this formation consist of quartz sandstone, pebble conglomerate sandstone, and mudstone which has better porosity and permeability. This formation also has many fractures because of the effect from geological structures. Haloq Formation has an unconformity contact with upper formation (Ingar Formation), so we can conclude that this unconformity contact can be a trap. Sedimentation processes in Ketungau Basin was stopped in Early Oligocene, but sedimentation in Melawi Basin still happened until Late Oligocene. It will make overburden in Melawi Basin and it also affected the temperature and pressure to become higher than before. We concluded that source rock in Melawi Basin will be more mature than source rock in Ketungau Basin, so the possibility of hydrocarbon prospect will be better in Melawi Basin than in Ketungau Basin. REFERENCES Awang Harun Satyana. Accretion and dispersion of southeast Sundaland the growing and silvering of a continent. Proceeding of joint Convention Jakarta 2003 The 32nd IAGI and The 28th HAGI Annual Convention and Exhibition Bemmelen,Van. The Geology of Indonesia. 1949. Indonesia Darman,Herman et.al. An Outline of The Geology of Indonesia.2000. Ikatan Ahli Geologi Indonesia. Jakarta.

Hall, Robert & Gary Nichols. SE Asia Research Group, University of London, UK

Indonesia Basin Summary, 2006; PT. Patra Nusa Data, Jakarta

IPA Proceeding, Indonesian Petroleum Association. 20th 22nd October 1987. Sixteenth Annual Convention. Vol I Vol II. Jakarta.

Katili, J.A. 1970. Geotectonics of Indonesia, A modern View. Jakarta, Indonesia.

Sutjipto, Rachmat Heryanto. (Sedimentology Of The Melawi and Ketungau Basin, West Kalimantan, Indonesia).1991. University Of Wollongong (Australia). http://www.Indonesian Invesment Online-Whats New. htm http://www.LEMIGAS R&D CENTRE FOR OIL AND GAS TECHNOLOGY. htm.

Figure 1 - Map Location of Ketungau and Melawi Basins in Kalimantan, Indonesia. (Adapted from :

Indonesia Basin Summary, 2006; PT. Patra Nusa Data, Jakarta.)

Figure 2 S - N cross section of SE Sundaland (Adapted from : Awang Harun Satyana, PROCEEDINGS

OF JOINT CONVENTION JAKARTA 2003 The 32nd IAGI and The 28th HAGI Annual Convention and Exhibition)

Figure 3 - Tectonic Setting of Kalimantan Island (Adapted from: Robert Hall & Gary Nichols. SE Asia

Research Group, University of London, UK)

Tertiary Subduction Related Melange and Metamorphosed Sediments

Melange

Tertiary Volcanoes and Volcaniclastic

Granite Pluton

Continental Crust

Mesozoic Indurated and/or Metasediments Section

Potentially Prospective Pre-Tertiary Section

Intermediate Crust of South China Sea

INDIAN OCEAN JAVA

EAST JAVA BASIN

VOLC ARC BAWEAN

ARC MURIAH TROUGH

KARIMUNJAWA ARC

KALIMANTAN

MELAWI BASIN

KETUNGAU BASIN

SOUTH CHINA SEA

NW BORNEO BASIN

NATUNA

Astenosphere Late

Cretaceous

Tertiary

Moho

Top Mantle

S N

ACCRETED CRUST ACCRETED CRUST CONTINENTAL

CORE

Figure 4 Sandstone Play in Ketungau Melawi Basins (Adapted from : Indonesia Basin Summary, 2006;

PT. Patra Nusa Data, Jakarta.)

Figure 5 Stratigraphy Chart of Ketungau Melawi Basins (Adapted from : Indonesia Basin Summary,

2006; PT. Patra Nusa Data, Jakarta.)

Basins Evolution

a. (Pre Carbon Lower Cretaceous) Schwarner zone formed, and then marine sediment, lava and tuff deposited.

b. (Upper Carbon) Hypabisal intrusion happened which formed geosynclines structure at basin

basement.

c. (Eocene) Rifting happened, caused by extensional tension dominating and formed horst and graben at the basement.

Figure 6 Basins Evolution of Ketungau Melawi Basins Source : Original figure from us based on intepretation