BS27-Sumatera_Final.pdf

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Berita Sedimentologi SUMATERA

Number 27 –August 2013

Published by

The Indonesian Sedimentologists Forum (FOSI) The Sedimentology Commission - The Indonesian Association of Geologists (IAGI)

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Editorial Board

Herman DarmanChief Editor

Shell International Exploration and Production B.V.P.O. Box 162, 2501 AN, The Hague – The NetherlandsFax: +31-70 377 4978E-mail: [email protected]

MinarwanDeputy Chief EditorMubadala Petroleum (Thailand) Ltd.31st Floor, Shinawatra Tower 3, 1010 ViphavadiRangsit Rd.Chatuchak, Bangkok 10900, ThailandE-mail: [email protected]

Fuad Ahmadin Nasution Total E&P Indonesie

Jl. Yos Sudarso, Balikpapan 76123

E-mail: [email protected]

Fatrial BahestiPT. Pertamina E&PNAD-North Sumatra AssetsStandard Chartered Building 23rd Floor Jl Prof Dr Satrio No 164, Jakarta 12950 - IndonesiaE-mail: [email protected]

Wayan Heru YoungUniversity Link coordinator

Legian Kaja, Kuta, Bali 80361, IndonesiaE-mail: [email protected]

Visitasi FemantTreasurerPertamina Hulu EnergiKwarnas Building 6th Floor Jl. Medan Merdeka Timur No.6, Jakarta 10110E-mail: [email protected]

Rahmat UtomoMubadala Petroleum (Thailand) Ltd.31st Floor, Shinawatra Tower 3, 1010 Viphavadi

Rangsit Rd.

Chatuchak, Bangkok 10900, ThailandE-mail: [email protected]

Advisory Board

Prof. Yahdi ZaimQuaternary Geology

Institute of Technology, Bandung

Prof. R. P. KoesoemadinataEmeritus Professor

Institute of Technology, Bandung

Wartono RahardjoUniversity of Gajah Mada, Yogyakarta, Indonesia

Ukat Sukanta ENI Indonesia

Mohammad SyaifulExploration Think Tank Indonesia

F. Hasan Sidi

Woodside, Perth, Australia

Prof. Dr. Harry Doust Faculty of Earth and Life Sciences, Vrije UniversiteitDe Boelelaan 10851081 HV Amsterdam, The NetherlandsE-mails: [email protected];[email protected]

Dr. J.T. (Han) van Gorsel6516 Minola St., HOUSTON, TX 77007, USA

www.vangorselslist.comE-mail: [email protected]

Dr. T.J.A. Reijers Geo-Training & Travel

Gevelakkers 11, 9465TV Anderen, The NetherlandsE-mail: [email protected]

Peter M. Barber PhD Principal Sequence StratigrapherIsis Petroleum Consultants P/L47 Colin Street, West Perth, Western Australia 6005

E-mail: [email protected]

• Published 3 times a year by the Indonesian Sedimentologists Forum (Forum Sedimentologiwan Indonesia, FOSI), a commission of the

Indonesian Association of Geologists (Ikatan Ahli Geologi Indonesia, IAGI).

• Cover topics related to sedimentary geology, includes their depositional processes, deformation, minerals, basin fill, etc.

Cover Photograph:

The contact of Sawahlunto

Formation (dark color at the

bottom part) and Sawahtambang

Formation (bright color) outcrop

near Sawahlunto city – West

Sumatera. Taken in June 2012.

Photo courtesy of Kirandra Ferari

and Peri Raudatul Akmal fromIAGI Riau.

mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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Welcome to Berita Sedimentologinumber 27!

We’re very pleased to deliveranother volume of BeritaSedimentologi to you. In this 27th edition of Berita Sedimentologi,we received 5 articles, including 2articles from students to showour commitment to engage theirinterest in publishing their workon sedimentology/stratigraphy.

Berita Sedimentologi No. 27focuses on Sumatra island

however only three articles willdiscuss about Sumatra, while therest of them are on other areas. The articles on Sumatra include ashort communication on latesynrift turbidite systems in theNorth Sumatra Basin written byLawrence (Trey) Meckel, a fieldtrip report to Central Sumatra

prepared by Andrew Carnell and astudent article on fracturedbasement in the Malacca Strait byIgnatius Primadi.

Lee Chai Peng submitted anarticle on stratigraphy of theLangkawi Island, Malaysia; and agroup of students from

Diponegoro University in Central Java wrote a short article ondepositional environment analysisof Kali Banyumeneng (Demak,Central Java).

We would like to remind you ofour plan for near futurepublication themes as follow:

• BS#28 Borneo: to be publishedin November 2013

• BS#29 SE Asia Biostratigraphyto be published in early 2014

Invitations have been sent out topotential contributors to seekarticles for both volumes. Wehope to get enough responses forfuture publications and in themeantime, we hope you willbenefit from the current edition ofBerita Sedimentologi. See youagain in November.

Warm Regards,

Minarwan

Deputy Chief Editor

I N S I D E TH I S I SSUE

Review of the Palaeozoic Stratigraphy ofthe Langkawi Islands, Malaysia – Lee ChaiPeng

5

Student articleEconomic vs Fractured Basement: A CaseStudy from North Sumatra Basin – IgnatiusPrimadi

21

Book Review : The SE Asian Getway:History and Tectonic of the Australian- As ia Col lis ion , editor: Rober t Hall et al –T.J.A. Reijers

56

Short communication

Late Syn-Rift Turbidite Systems in theNorth Sumatra Basin – Lawrence D. Meckel,III

15

Student article

Depositional Environment Analysis of KaliBanyumeneng, Mranggen, KabupatenDemak – Samuel R.N. Simorangkir et al.

26

Book Review - Biodiversity,

Biogeography and Nature Conservationin Wallacea and New Guinea (Volume 1),Edited by D. Telnov, Ph.D. – H. Darman

58

A Field Trip to the Syn-Rift Petr oleu mSystem of Central Sumatera – AndrewCarnell et al.

18

Call for paper

BS #28 – focus in Borneoto be published in November 2013

Berita Sedimentologi

A sedimentological Journal of the Indonesia Sedimentologists Forum

(FOSI), a commission of the Indonesian Association of Geologist (IAGI)

From the ditor


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About FOSI

he forum was founded in1995 as the Indonesian

Sedimentologists Forum(FOSI). This organization is acommu-nication and discussionforum for geologists, especially for

those dealing with sedimentologyand sedimentary geology inIndonesia.

The forum was accepted as thesedimentological commission ofthe Indonesian Association ofGeologists (IAGI) in 1996. About300 members were registered in1999, including industrial andacademic fellows, as well asstudents.

FOSI has close internationalrelations with the Society of

Sedimentary Geology (SEPM) andthe International Association ofSedimentologists (IAS).Fellowship is open to those

holding a recognized degree ingeology or a cognate subject andnon-graduates who have at leasttwo years relevant experience.

FOSI has organized 2international conferences in 1999and 2001, attended by more than150 inter-national participants.

Most of FOSI administrative workwill be handled by the editorial

team. IAGI office in Jakarta willhelp if necessary.

The official website of FOSI is:

http://www.iagi.or.id/fosi/

Any person who has a background in geoscience and/or is engaged in the practising or teaching of geoscienceor its related business may apply for general membership. As the organization has just been restarted, we useLinkedIn (www.linkedin.com) as the main data base platform. We realize that it is not the ideal solution,and we may look for other alternative in the near future. Having said that, for the current situation, LinkedInis fit for purpose. International members and students are welcome to join the organization.

T

FO SI M em b e r s h i p

FOSI Group Memberas of AUGUST 2013

http://www.iagi.or.id/fosi/http://www.iagi.or.id/fosi/http://www.iagi.or.id/fosi/


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Review of the Palaeozoic Stratigraphy of the Langkawi

Islands, Malaysia

Lee Chai PengUniversity of Malaya, Kuala LumpurCorresponding Author: [email protected]

INTRODUCTION

The Langkawi group of 99 islands off thenorthwest coast of Malaysian Peninsula is locatedsome 30 km off the coast of Perlis and 112 kmnorth of Penang. These islands are a paradise forgeologists, including some of the best and mostinteresting exposures of Palaeozoic sedimentaryrocks in Malaysia, ranging in age from Cambrianto Permian. These consist of both clastics andcarbonates, deposited within a range ofdepositional and palaeoclimatic conditions ranging

from shallow marine shoreface to turbidites withdropstones. In addition, these sedimentary rockshad been intruded by younger Mesozoic graniteswith considerable contact metamorphic effects.

Since the initial publication of the Geological Mapof Langkawi and the subsequent Geology and

Mineral Resources Memoir 17 on the States ofPerlis, North Kedah and the Langkawi Islands,both by Jones (1966, 1981) many more recentstudies had been carried out. These studies wereparticularly facilitated by the creation of new

outcrops due to numerous recent developmentprojects on the islands. A helpful compilation ofpublished and unpublished geological researcheson Langkawi was compiled by Sarman et al.(1997).

The spectacular geological heritage of the islandsheralded the formation of the LangkawiGeoforestpark by the Kedah State Government,Malaysia in May 2006 to preserve and display itsunique geological features. The Geoforestpark wasendorsed by the United Nations Educational,

Scientific and Cultural Organization (UNESCO)under the Global Network of National Geoparks in June 2007 (Leman et al., 2007).

REGIONAL GEOLOGICAL SETTING

Langkawi is located within the Northwestern

Domain of the Western Belt of Peninsular Malaysia(Figure 1) which is an integral part of Sundaland,the SE Asian extension of the Eurasian plate.

Bentong-Raub suture

Figure 1. Location map ofLangkawi Islands. The three belts

and northwestern domain within

the Western Belt are after Lee

(2009). Sumatra map is after

Darman & Sidi (2000).


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The Paleozoic sedimentary rocks of Langkawi aremostly shallow-marine shelf type deposits thatextend within a broad linear belt from SouthChina, through Burma, Thailand, and northwestPeninsular Malaysia into northern Sumatra. Thesesediments form part of the Sibumasu Block whichbroke away from northwestern Australia at thenorthern edge of Gondwanaland, to subsequently

collide with the Indochina or East Malaya blockalong the Bentong-Raub Suture during the Triassic (Metcalfe, 2000; Hutchison, 2009a). Theabove-mentioned Northwestern Domain is part ofthe Patani Metamorphic Terrane related to thisplate tectonic suturing episode.

Cenozoic uplift of the Langkawi chain of islands isrelated to the peripheral effects of later Neogene

collision of India with Tibet creating the Himalayasas well extrusion of Sundaland to the southeast(Tapponier et al., 1982). In addition, the Langkawiislands, being geographically located on the

northern cratonic margin of the North Sumatra

Basin, have been affected by Neogene blockfaulting caused by back arc extension associatedwith the continuous northwards push of theAustralian Plate under the Eurasian Plate (Raj etal., 2009).

STRATIGRAPHY OF LANGKAWI

The Palaeozoic sedimentary rocks in Langkawi aretraditionally subdivided into four formations(Jones, 1981), although newer more detailedstratigraphic subdivisions have been proposed(Cocks et al. 2004; Meor and Lee, 2004; Lee,2009). These units are, from the oldest to the youngest, the Machinchang, Setul, Singa andChuping Formations. Their geographical

distribution and stratigraphic relationships aregiven in Figures 2, 3 and Figure 4, respectively. They are mostly shallow marine shelf type depositsand are discussed as follows:

Figure 2. Revised stratigraphy of Palaeozoic rocks up to the Singa Formation in the Northwestern Zone of

Peninsular Malaysia (after Lee, 2009).


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1.

Machinchang Formation

The Machinchang Formation type locality isderived from the conspicuous long serratedquartzite ridge of five peaks aptly called Gunung

Machinchang or Mat Chinchang meaning

“chopped-up mountain” (Figures 5a & 5b).

This formation is composed of a thick successionof predominantly arenaceous rocks with minor

interbeds of conglomerates, phyllitic slates andtuffs exposed in the northwestern corner ofLangkawi. It has been divided into three members(Figure 6) according to Lee (2006) with a coarsersandy middle member sandwiched between twofiner grained members. The overall depositional

environment is that of a high-destructive, wave-dominated delta which had built over an offshoreshelf deposit to produce a series of barrier-beachsands cut by small channels (Lee, 2006).

Figure 3. Geological map of Langkawi Islands showing distribution of the various rock formations in the

islands (after Leman et al., 2007). Common stops on field trip are numbered in circles.


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In the Machinchang area these rocks spectacularly

outcrop as an asymmetrical anticline with arelatively undisturbed western limb that dips andplunge into the waters of the Straits of Malacca(Figure 5b). Up section occurs a gentler moredome-like eastern limb that probably had beenuplifted by underlying Mesozoic granite (Lee,

1983). The base of the formation is not exposed

and the oldest part of the formation is the peliticrocks exposed in the core of the MachinchangAnticline at Teluk Datai. The top of the formationhas several thin calcareous horizons beforepassing conformably up into the succeeding SetulFormation limestone near Teluk Sabong.

a b

Figure 4. Stratigraphy and major events in geological history of Langkawi islands (after Leman et al.,

2007).

Figure 5. a) “Chopped up” Machinchang Range view from the south; b) Machinchang Formation deltaic

sandstone beds in western limb of anticline dipping steeply into Malacca Strait.


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Figure 6. Stratigraphy of the Machinchang Formation (after Lee, 2006).


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Interesting trace fossils including Dictyodora (Lee,

1980) and sedimentary structures such as varioustypes of cross-beddings, convolute bedding andload casts can be seen in the upper parts of theformation at Pasir Tengkorak and Pulau Jemurok.Palaeocurrent studies on the abundant cross-beddings found in the formation gave apredominantly westward downcurrent direction.

The Machinchang Formation is poorly fossiliferousand only poorly preserved fragments of trilobitesand brachiopods had been found at Tg. Buta andPulau Jemurok. Better preserved fossilized saukidtrilobites and orthid brachiopods are found on Tarutao Island just 5 km north of Langkawi in Thailand which give a Late Cambrian to EarlyOrdovician age to the uppermost part of the

Machinchang Formation. This is confirmed byfission track studies on zircon crystals which givean age of 555±37 Ma.

Acid volcanics or tuffs also occur as thin, finegrained, pale green layers in the MachinchangFormation indicative of acid volcanism duringCambrian times.

2. Setul Formation

The Setul Formation ranges in age from EarlyOrdovician to Early Devonian. It is divisible into aLower Setul Formation of Ordovician age,occurring below the Lower Detrital Band on PulauLanggun and a Silurian to Devonian Upper SetulFormation above this band (Figure 4). The top of

the formation is represented by the Upper DetritalBand, a largely arenaceous unit with minorargillaceous beds at Teluk Mempelam on Pulau

Langgun. The formation has been estimated to beabout 1500 m thick and was raised to groupstatus in Cocks et al. (2005) and Lee (2009).

The Setul Formation is made up of predominantlydark coloured shelfal limestone with minor blackdetrital bands. It is largely metamorphosed on themain island but good fossiliferous outcrops of the

limestone are present in the vicinity of PulauLanggun. The impure limestone contains a richfauna of fossils such as gastropods, bivalves,nautiloids, brachiopods, trilobites, crinoids andstromatolites while the hardened black mudstoneshave preserved graptolites, tentaculitids andtrilobites.

The unusual scyphocrinoid, probablyCamarocrinus (Figure 7), with its calcareous

floating lobolith has been discovered (Lee, 2005)from the top part of the Upper Setul limestone at Teluk Mempelam on Pulau Langgun. It is

restricted in age to the uppermost Pridolian-lowermost Lochkovian (Late Silurian to EarlyDevonian) and enables correlation with similarlimestones in Myanmar (Ayeko Aung, pers comm .

2010). Microfossils such as conodonts andostracods are also found in these limestones.

3. Rebanggun Beds

The quartzitic sandstones and red to greymudstones known as the Rebanggun Beds outcropon Pulau Rebak Besar and Pulau Rebak Kecil andhave been grouped together with similar beds

found above the Setul Formation at PulauLanggun.

Figure 7. Upper Setul limestone bed with scyphocrinoid loboliths at Teluk Mempelam,

Pulau Langgun, northeast Langkawi.


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Correlation with better exposed sequences in Perlishas placed the Langgun Redbeds higher up insteadof at the base of the transitional unit (Lee, 2009) tothe Singa / Kubang Pasu Formations. Fossils inthis unit are sparse and consist of smallambocoeliid brachiopods, Posidonomya (Posidonia)bivalves and the trilobite Macrobole kedahensis.

The age of the unit is probably Early Carboniferous

(Visėan) based on Macrobole and its stratigraphic

position above Late Touraisian chert beds. Thesefossiliferous mudstones and associated sandstonesthat are pebbly in places were deposited in a quietprodelta environment with occasional turbiditicinput. These redbeds are quite widespread andcorrelatable with the Khao Chunong Formation insouth Thailand.

4.

Singa Formation

The Singa Formation is best exposed in thesouthwestern part of Langkawi. The basal contactwith the Rebanggun Beds is unexposed and thetop is gradational into the overlying Chuping

Formation limestone that is exposed on PulauSinga Kecil.

The age of the formation is probably Carboniferousto Early Permian. The Singa Formation, namedafter Pulau Singa Besar, is a clastic unit typified

by crudely laminated, dark grey, poorly sortedmudstones with scattered dropstone horizons. Thedropstones range in size from granules to boulders(Figure 8) and are mainly composed of sandstonewith subordinate limestone, vein quartz, granitic,volcanic and metamorphic rocks.

They have been interpreted as glacial marinedropstones derived from peri-Gondwana margin icesheets during the Carboniferous (Stauffer &Mantajit, 1981; Stauffer & Lee, 1986). Fossils areonly common in its uppermost section but burrowsand soft sediment deformation structures can be

found in some parts. Long paired vertical burrowsexceeding a meter in length are found on Pulau Tepor. The brachiopods found in the uppermostpart of the formation associated with dropstonehorizons at Kilim, Batu Asah and Pulau SingaBesar are “cool-water” forms of Permian

(Sakhmarian) age (Waterhouse,1982; Leman,2003). Other associated fossils are bryozoa,bivalves, gastropods, corals and crinoid stems.Sandstone beds are few and often calcareous.

The formation can be correlated with similarglacial marine diamictites found in the Mergui,Martaban and Lebyin Groups of Myanmar, theKaeng Krachan Formation of the Phuket Group in

Thailand and the Bohorok Formation of Sumatra.

5. Chuping Limestone

The Chuping Limestone is a unit of thickly beddedto massive, light coloured limestone (Figure 9) thatsits conformably above the Singa Formation in thesoutheastern part of the island. It can be easilydistinguished from the more clayey and darkercoloured Setul limestone.

The Chuping limestone is fossiliferous in its basalpart with fusulines, brachiopods, bivalves,gastropods, corals, bryozoans and algae rangingfrom a late Early Permian or early Middle Permianage in Langkawi to the Triassic on the mainland(Metcalfe, 1990). Chert nodules are common at its

base in Pulau Singa Besar. The once famousLangkawi marble is a white saccharoidalornamental stone quarried from metamorphosedequivalents of this limestone in southeasternLangkawi and Pulau Dayang Bunting.

Figure 8. Glacial marine dropstones in pebbly mudstone horizon in Singa

Formation at Pulau Ular.


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Mesozoic Igneous Rocks

Two granitic bodies are found in the islands. TheRaya and Tuba granites cover some 113 km2 of theislands. The huge Gunung Raya massif which risesto 878 m forms the highest part of the island. It is

Late Cretaceous in age while the smaller pluton inPulau Tuba is Late Triassic in age. The granites

are petrographically similar and hard todistinguish in the field. Both are porphyritic biotitegranites with tourmaline clots and veins (Figure10).

Figure 9. White coloured marble of Chuping Formation behind former Kedah

Marble factory near Kisap.

Figure 10. Tourmaline-quartz vein in Langkawi granite block.


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Large K-feldspar phenocrysts and spherical quartzphenocrysts are common. Alignment of the largeK-feldspar crystals is common at the margins ofthe plutons. The Raya Granite appears to be themost southerly body of a belt of Late Cretaceousgranite extending from west Thailand.

Metamorphic Rocks

Langkawi occurs within the Patani Metamorphic Terrane which trends northwest from mainlandKedah (Khoo, 1984) Rocks older than mid-Permian within this terrane have suffered severedeformation and low grade regional metamorphismrelated to the collision of the Sibumasu micro-platewith Indochina Rocks of the Machinchang, Setul

and the Singa Formations have developed at leastone set of low angle cleavages.

These rocks occur as slates, phyllites, quartzitesand fine grained marbles where unaffected by later

contact metamorphism while rocks adjacent to thegranites have recrystallized to mica schists, coarsegrained marbles and tough metaquartzites. Smallbodies of skarn have developed at the contactsbetween the igneous intrusions and some of thelimestones.

CONCLUSIONS

Langkawi plays an important role in thepalaeogeographic reconstruction of thenorthwestern part of Peninsular Malaya.

The discovery of Lower Permian (Sakmarian) cool-water shelly fauna particularly brachiopods in theSinga Formation (Waterhouse,1982; Leman, 2003;Shi et al., 1997) in addition to glacial marinedropstones (Stauffer & Mantajit, 1981; Stauffer &Lee, 1986) has enabled correlation with similardeposits in the Canning Basin of Australia(Hutchison, 2009b). This establishes theconnection of Langkawi to Gondwana in pre-Late

Permian times, when it formed part of theSibumasu microcontinent.

Sibumasu subsequently drifted north into lowerpalaeo-latitudes and eventually collided with thewarmer climate provinces of the Indochina Block ofCathaysia, along the Bentong-Raub Suture ofPeninsular Malaysia during the Late Triassic.

SELECTED REFERENCES

Cocks, L. R. M., Fortey, R. A. and Lee, C. P., 2005,A review of Lower and Middle Palaeozoicbiostratigraphy in west peninsular Malaysia andsouthern Thailand in its context within theSibumasu Terrane: Journal of Asian Earth

Sciences, 24. p. 703-717.Hutchison, C.S., 1973, Tectonic evolution of

Sundaland: a Phanerozoic synthesis: GeologicalSociety Malaysia Bulletin, 6, p. 61-68.

Hutchison, C.S., 2009 a, Bentong-Raub Suture. In :

Hutchison, C.S. and Tan, D. N. K. (eds.) Geologyof Peninsular Malaysia: Univ. Malaya &Geological Society of Malaysia. p. 43-53.

Hutchison, C.S., 2009 b, Tectonic Evolution. In :Hutchison, C.S. and Tan, D. N. K. (eds.) Geologyof Peninsular Malaysia: Univ. Malaya &Geological Society of Malaysia. p. 309-330.

Jennings, J. R. and Lee, C. P., 1985, Preliminarynote on the occurrence of Carboniferous-agecoals and in situ plant fossils in EasternPeninsular Malaysia: Geological SocietyMalaysia Newsletter, 11 (3), p. 117-121.

Jones, C.R., 1981, Geology and mineral resourcesof Perlis, North Kedah and the LangkawiIslands: Geological Survey Malaysia DistrictMemoir, 17, 275 pp. (includes a detailed

geological map of Langkawi).Mohammed, K., Ali C.A., and Leman, M. S., 2007,

Ancient Higher Sea Level. In : Mohd Shafeea

Leman, Kamarulzaman Abdul Ghani, Ibrahim

Komoo & Norhayati Ahmad (eds). LangkawiGeopark : LESTARI, Universiti KebangsaanMalaysia. 113 pp.

Khoo, T.T., 1982, Pulau Puchong – a possible newSetul limestone in the Langkawi area, Kedah:Malaysian Journal of Science, 7, p.103-111.

Khoo, T.T., 1984, The terrane of the PataniMetamorphics: Geological Society MalaysiaBulletin, 17, p. 79-95.

Khoo, T.T., and Tan, B.K., 1983, Geologicalevolution of Peninsular Malaysia: Proceedings ofthe Workshop on Stratigraphic Correlation of Thailand and Malaysia, Geological Societies of

Thailand and Malaysia. Vol. 1, p. 253-290.Kimura, T., and Jones, C.R., 1967, Geological

structures in the northern and southern partsof the Langkawi islands, northwest Malaya:Geology and Palaeontology of Southeast Asia, 3,p.123-134.

Koopmans, B. N., 1965, Structural evidence for aPalaeozoic orogeny in northwest Malaya:Geological Magazine, 102, p. 501-520. 34th IGCExcursion MY-1 18

Lee, C. P., 1980, A comparison of the supposedgraptolites of the Tarutao Formation, Ko Tarutao, with Dictyodora trace fossils found in

the Machinchang Formation, Pulau Jemurok,Langkawi: Geological Society Malaysia

Newsletter, 6 (3), p. 69-74.Lee. C. P., 1983, Stratigraphy of the Tarutao and

Machinchang Formation: Proceedings of theWorkshop on Stratigraphic Correlation of Thailand and Malaysia, Geological Societies of Thailand and Malaysia. Vol.1, p. 20-38.

Lee, C. P., 2005, Discovery of plate-typescyphocrinoid loboliths in the uppermostPridolian lowermost Lochkovian Upper Setullimestone of Peninsular Malaysia: Geological

Journal, 40, p. 331-342.Lee, C. P., 2006, The Cambrian of Malaysia:

Palaeoworld,15. p.242-255.Lee, C. P., 2009, Palaeozoic Stratigraphy. In :

Hutchison, C.S. and Tan, D. N. K. (eds.) Geology


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of Peninsular Malaysia: Univ. Malaya &Geological Society of Malaysia. p. 55-86.

Raj, J. K., Tan, D. N. K, and Wan Hasiah Abdullah,2009, Cenozoic Stratigraphy. In : Hutchison, C.

S., and Tan, D. N. K. (eds.) Geology ofPeninsular Malaysia: Univ. Malaya & GeologicalSociety of Malaysia. p. 133-173.

Sarman, M., Komoo I., and Desa. K.M., 1997,

Status Permuliharaan Sumber Geologi DiMalaysia: In Ibrahim Komoo,Mohd Shafeea

Leman, Kadderi Md Desa & Ibrahim Abdullah(eds.) Warisan Geologi Malaysia (GeologicalHeritage of Malaysia): LESTARI UKM, p.13-46.

Meor, H. H., and Lee, C. P., 2005, The Devonian-Lower Carboniferous succession in NorthwestPeninsular Malaysia: Journal of Asian EarthSciences, 24. p. 719-738.

Metcalfe, I., 1990, Stratigraphic and tectonicimplications of Triassic conodonts fromnorthwest Peninsular Malaysia: GeologicalMagazine, 127, p. 567-578.

Metcalfe, I., 2000, The Bentong-Raub suture zone: Journal of Asian Earth Sciences, 18, p. 691-712.

Leman.M.S., 2003, An Early Permian (EarlySakmarian) brachiopod fauna from the SungaiItau Quarry and its relationship to other EarlyPermian brachiopod horizons in Langkawi,Malaysia: Geological Society Malysia Bulletin, 46, p. 155-160.

Leman, M.S., Komoo, I., Mohamed, K.R., Ali C.,and Tanot Unjah. T., 2007, Geology andGeoheritage. In : Mohd Shafeea Leman,Kamarulzaman Abdul Ghani, Ibrahim Komoo &

Norhayati Ahmad (eds). Langkawi Geopark:LESTARI , Universiti Kebangsaan Malaysia, p.

43-86.Leman, M.S., Ghani, K.A., Komoo I., and Ahmad

N., (eds). 2007. Langkawi Geopark: LESTARI ,Universiti Kebangsaan Malaysia. 113 pp.

Shuib. M. K., 2009, Major Faults. In : Hutchison,

C.S., and Tan, D. N. K. (eds.) Geology of

Peninsular Malaysia: Univ. Malaya & GeologicalSociety of Malaysia. p.249-269.

Raj, J.K, Tan, D.N.K, and Wan Hasiah Abdullah.2009, Cenozoic Stratigraphy. In : Hutchison, C.S

& Tan, D. N. K. (eds.) Geology of PeninsularMalaysia: Univ. Malaya & Geological Society ofMalaysia. p. 133-173.

Shi, G. R., Leman M. S.,and Tan, B.K., 1997, Early

Permian Brachiopods from the Singa Formationof Langkawi Island, Northwestern PeninsularMalaysia: Biostratigraphical andBiogeographical Implications. In: PhisitDheeradilok & others (eds): Proceedings of theInternational Conference on Stratigraphy and Tectonic Evolution of Southeast Asia and SouthPacific, p. 67-72.

Stauffer P. H., and Lee, C. P., 1986, Late Paleozoic

glacial marine facies in Southeast Asia and itsimplications: GEOSEA V Proceedings Vol. II,Geological Society Malaysia Bulletin, 20, p. 363-397

Stauffer, P.H., and Snelling, N. J., 1977, APrecambrian trondhjemite boulder in Palaeozoicmudstones of NW Malaya: Geological Magazine,114, p. 479-482.

Tan, B.K., 1981, On the supposed existence of theKisap Thrust in the Langkawi Islands,northwest Peninsular Malaysia: GeologicalSociety Malysia Bulletin, 14, p. 127-133.

Tapponier, P., Peltzer, G., Le Dain, A.Y., Armijo, R.,and Cobbold, P., 1982, Propagating extrusiontectonics in Asia, new insights from simpleexperiments with plasticene: Geology, 10,p.611-616.

Tjia, H.J., 1993, The Kisap Thrust in the KampungKilim area, Pulau Langkawi: Geological Society

Malaysia Newsletter, 19, p. 247-250.Wakita, K., and Metcalfe, I., 2005, Ocean plate

stratigraphy in East and Southeast Asia: Journal of asian Earth Scioences, 24, p. 679-702.

Waterhouse, J.B., 1982, An early Permian cool-water fauna from pebbly mudstones in South Thailand: Geological Magazine, 119, p. 337-354.


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Short communicationLate Syn-Rift Turbidite Systems in the North Sumatra Basin

Lawrence D. Meckel, IIIPexco Energy (Zaratex N. V.), Jakarta, Indonesia

Corresponding Author: [email protected]

The North Sumatra Basin (NSB, Figure 1) inIndonesia is one of the most prolific petroleumprovinces in SE Asia. It has been affected byrifting, transtensional and transpressional shear,and compression from the Paleogene to presentday. This complex tectonic history has creatednumerous opportunities for petroleum exploration,including an inverted deepwater turbidite play(Figures 2 and 3) that has been relatively under-

explored to date.

During the late syn-rift stage of basin evolution

(Late Oligocene-Early Miocene), N-S to NE-SWoriented half-grabens in the central and northernpart of the NSB (Figure 3) were subjected to rapidflooding, as a result of the complimentary effects of

tectonic subsidence and a rise in eustatic sea level.Water depths reached several hundred meters(bathyal paleobathymetries), and slope and basin-floor turbidite systems of the Bampo Formation(Figure 2) filled the newly-created accommodationspace. Time-equivalent basement highs arecharacterized by unconformities or condensed,shallow marine deposits.

Intra-formational, deep marine Bampo shales actas competent seals for individual turbidite sands(Figure 2). The shale-prone stratigraphy of the

stratigraphically younger Baong Formation acts asa regional seal, across which overpressures tend toincrease with depth (Figure 2).

Figure 1. Location map of study area (red box) in southern part of North Sumatra Basin relative to historicalwell database and existing oil (green) and gas (red) discoveries. Despite significant success in the onshoreand on the shelf, fewer than 40 wells have been drilled in water depths greater than 100 m. Inset, upper

right shows ultimate recoverable reserves (URR) for the 13 largest basins in SE Asia. Bubble size is scaled toUR. More than 6 billion barrels of oil equivalent have been discovered in the NSB. Inset, lower left, shows thecreaming curve for the NSB. Note that exploration activity has been negligible in the past decade.


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During the late Miocene to Quaternary, dextral

transpression on NW-SE oriented faults and N-Soriented compression formed a series of invertedstructures, with potential hydrocarbon traps in 4-

way dip-closures at Parapat and Bampo levels(Figure 3). These traps have not been tested in theIndonesian portion of the NSB, but contain

hydrocarbons in the Mergui Basin, the northern

equivalent of the NSB, offshore Thailand.

Bampo marine shales are the most probable

source rocks (Figure 2). The Bampo source rockhas been typed to the gas-condensate at the super-giant Arun gas-condensate field.

Bampo

Bampo

Bampo

Bampo

Bampo

Seal

Reservoir/Trap

Source

Figure 2. Stratigraphy and petroleum system elements of the North Sumatra Basin. Late syn-rift (Bampo)turbidites, highlighted by yellow arrows, are the subject of this paper.

Figure 3. Seismic section illustrating inverted syn-rift structure in the study area. Reservoir presence isdemonstrated in regional wells. All wells have encountered hydrocarbon shows, although none is optimally

placed to test the inverted syn-rift play.


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It is a very lean source rock, with typical TOCvalues less than 1%, but has a demonstratedcapacity to charge multi-TCF gas fields across theNSB, so must be considered to be an ultra-prolific- if not particularly spectacular - source rock.Older, syn-rift lacustrine shales, which could beliquid-prone, may also act as potential source

rocks (Figure 2). Their presence across the basin ismuch more speculative, though, and they have notbeen definitively typed to any hydrocarbonaccumulations. Migration is postulated to occurfrom deep grabens, which thermal models show tobe in oil to post-mature temperature windows, upcarrier beds and faults into closures.

This wildcat inversion play has not been exploredpreviously in the NSB because it only occurs in theoffshore part of the basin, where good-quality 3Dseismic data, only recently acquired, are necessaryto adequately characterize prospects. The play hashigh volumetric potential, and most aspects of the

petroleum system have been proven previously. The play concept is also relatively under-exploredelsewhere in southeast Asia, and openspossibilities in other basins in the region (Figure4).

50 km

N

Basin

Floor

Basin

Floor

Basin

Floor

Basin

Floor Basin

Floor

Basin

Floor

Continental

Shelf

Continental

Shelf

I n c r e a s i n g

T o p o g r a p h y

I n c r e a s i n g

B a t h y m e t r y

Figure 4. Simplified mapof late syn-rift architecture,offshore North SumatraBasin. Map is a smoothedgrid of interpreted topbasement based on 2D and3D seismic, and is

corrected to reflect paleobathymetries inferred from well results (welllocations not shown). Redarrows indicate possibledeepwater sedimentdispersal fairways.Subsequent tectonicinversion of these sand-

prone fairways creates anunder-explored playconcept in the basin.


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A Field Trip to the Syn-Rift Petroleum System of Central

Sumatera

Andrew Carnell1, Chris Atkinson2 and Peter Butterworth31Shell Egypt2Worldwide Petroleum Services3BP Indonesia

In September 2012, a field trip was run by theauthors above into the Ombilin Basin of WestSumatra province to examine outcrop analogs ofthe syn-rift petroleum system of Central Sumatra(Figure 1). This was organized by SEAPEX andAAPG as part of the AAPG International

Conference held in Singapore. The field trip startedand finished in Padang but was conducted in theBarisan mountains amidst the stunning scenery ofWest Sumatra.

Rift basin evolution is a key component of thepetroleum systems of many Southeast Asianbasins (e.g. South Sumatra Basin, Sunda Basin).Syn-rift lacustrine mudstones are prolific oil prone

source rocks and syn-rift and early post-riftclastics sediments can provide excellent reservoirintervals. Rift petroleum systems are, however,geologically complex and hydrocarbon explorationwithin them requires a greater knowledge of thestructures and sedimentological evolution of thebasin than is often the case elsewhere. Localfactors such as provenance and rift related

tectonic activity can have a significant impact onthe quality, quantity and distribution of source,reservoir and seal.

The field trip was conducted over four days witheach day concentrating on a separate aspect of thepetroleum system as follows:

Day 1 Basement and Regional Geology (SYN-RIFT)Day 2 Source Rock (SYN-RIFT)Day 3 Reservoirs (EARLY POST-RIFT)Day 4 Syn-rift Reservoirs (SYN-RIFT)

DAY 1 - BASEMENT AND REGIONALGEOLOGY

The aim of the field trip stops on the first day wasto study the basement geology and to set the

regional setting for the remainder of the field trip.Complex structural aspects were addressed, withreference to the problems associated withdeciphering Oligocene rift related structuressubsequently modified by strike slip deformationand the Plio-Pleistocene Barisan mountain uplift.

Basement in the area is varied with metamorphic,igneous and sedimentary lithologies all exposed.Basement sediments are both clastic and

carbonate in nature and range in age from LowerCarboniferous to Triassic. Sediments are widelymetamorphosed. The igneous component to thebasement is largely of an intrusive nature withgranites dominant. Radiometric dating on a varietyof exposures gives late Permian to early Mioceneages with most basement igneous activityoccurring in the Mesozoic.

Figure 1. West Sumatra,showing location of

principle outcrops visited.

The Karbindo mine is in thehighly fragmented Kiliran

Sub-basin to the east. TheHarau Canyon is on thenorth eastern margin of the

Payakumbuh Basin.


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Unfortunately it was not practical to visit all thebasement lithologies present in the area, althougha representative selection were examined. Thesestudied outcrops give a good indication of how

differing types of basement have a variableinfluence on topography, tectonism, sedimentsupply and water chemistry.

DAY 2 – SOURCE ROCK

The second day of the field trip was spent at the

Karbindo Coal Mine, an open cast coal mine inwhich the Eocene Brown Shale is very well exposed(Figure 2). This is a remarkable exposure since it isone of the few places around the world where bywalking up through the mine it is possible to walkup through the evolution of a syn-rift lacustrinesystem.

The lowest exposures in the mine are a dark greypalaeosol in terrigenous mudstone (Figure 2). Thispalaeosol is at least 25m thick (the base is notexposed) and represents slow sedimentaccumulation and subsidence. Any introduced

sediment was fully reworked by pedogeneticprocesses so that no primary sedimentarystructures remain. The palaeosol is overlain by acoal seam that varies in thickness from 5m to 18malong strike. At its base the coal is black hard andvitreous and represents peat accumulation in aslowly subsiding swamp. Towards the top of thecoal are interbeds of dark brown algal rich coalthat were deposited in standing water. Laterallythese brown coals pass into agal-rich limestoneswhich represent lacustrine margin carbonateaccumulation. This upper part of the coal seamreflects continuing subsidence but at a rate where

ephemeral lakes were able to develop. Coals show TOC and HI values of 36.71% - 71.77% and 237 -350 respectively and are generally mostly gasprone source rocks containing higher-plantmaterial.

The coal seam is capped by a dark brown, highlyorganic rich (TOC 16.3% and HI 582),undercompacted shale of irregular thickness which

is an excellent quality oil prone source rock withalgal organic matter predominant. This isinterpreted to be the deposit of a shallow, algal

rich lake and probably represents the very firstdevelopment of continuous lacustrine conditions.

Overlying the carbonaceous shale are ~100m ofBrown Shale facies. In the Karbindo Coal mine theBrown Shale is broadly divisible into three

intervals, namely: a lower (approximately 40mthick) highly fissile paper-laminated shale, amiddle (approximately 15m thick) gastropod-richred weathering shale and an upper (approximately50m thick) weakly laminated highly bioturbatedshale. At its base the brown shale is dark brown,finely laminated and fissile. The lowermost portion

is strongly calcite replaced. Thin sharp–basedflood-related sandstones are locally developed andbecome more frequent and thicker upwards. High TOC (3.16%-8.91%) and HI (442-717) indicate anexcellent quality oil prone source rock dominatedby algal organic matter. Palynology reveals a

Botrycoccus spp. rich assemblage. Deposition of

the lower Brown Shale interval was in a shallowlake with a strongly stratified water column.

The middle of the studied succession comprisesbeds of Goniobasis -like and Viviparus- like

reworked gastropods. This interval weathers redand shows a slight reduction in organic content(TOC 2.58% - 5.94%) and source quality (HI 424-547). Palynological data indicate a change in algalforms present with abundant Pediastrum spp

recorded. This middle section of the Brown Shaleis interpreted to represent an overall shallowing to

a lacustrine margin setting where gastropodaccumulations mark concentration by wave action.

The upper interval comprises dark brown, faintlylaminated, organic-rich brown shale, which in thinsection is seen to be strongly bioturbated. TOC(3.75%-8.44%) and HI (624-743) values areexceptionally high and confirm the presence of oilprone algal organic matter. Palynofacies analysisreveals a Botrycoccus spp dominated assemblage.

This interval represents a return to deeper

lacustrine conditions.

Coal

Palaeosol

10 km

N

Figure 2. The Karbindo Coal Mine. Palaeosols inthe foreground are overlain by coal, which in turnis overlain by a very thick section of lacustrineBrown Shale. Note the light coloured limestone


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The water column was strongly stratified butsufficiently oxygenated to allow bioturbation tooccur. Overall deposition of the Brown Shalesediments at Karbindo is interpreted to have takenplace in relatively shallow water where subtlevariations in the hydrological regime have resultedin great differences in the rock record.

DAYS 3 AND 4 – RESERVOIRS

The third and fourth days of the field tripconcentrated on sandstone reservoirs in the syn-and post-rift sequence and how their reservoirarchitecture is affected by local tectonic and

provenance factors. In the Central Sumatra area,as with other syn-rift sections from Southeast

Asia, there is considerable variation in sandstonethickness, lateral development and quality. Suchsandstones are exposed in the Ombilin Basin andits environs where, unusually for Southeast Asia,there are numerous excellent quality exposures inriver beds, cliffs, road cuts and coal mines (Figures3 and 4). It is in the last mentioned that much of

the third day is spent since in these minesexposures are extensive and allow for:

• an appreciation of fluvial sandstone reservoirarchitecture, from isolated sand bodiessurrounded by mudstones, to vertically andlaterally stacked sandstone showing good three

dimensional interconnection,• Allow for an interpretation of the evolution of

fluvial systems such that conditions may

become suitable for sandstone aggradation invertical and lateral sense within the sameoverall sequence.

Although the greater part of the outcrops arefluviatile in nature and can be regarded asanalogous to producing reservoirs elsewhere inIndonesia, turbidite and alluvial fan sedimentswere also examined. In the magnificent Harau

canyon it is possible to cross over from basementto alluvial fan sediments and to walk up thealluvial fan observing how the sediments evolvewith time.

Figure 3. Allied Indo Coal Mine (Parambahan-

Rasau Members, Sawahlunto Formation).Fluviatile sands are laterally discontinuous andseparated by thick well developed palaeosols.

Figure 4. Sawahtambang Gorge (Sugar Member, Sawahtambang Formation). Sands are

very well developed and show excellent lateral and vertical continuity.


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Student articleEconomic vs Fractured Basement: A Case Study from NorthSumatra Basin

Ignatius PrimadiTrisakti University, Jakarta, Indonesia


ABSTRACT

North Sumatra basin developed during the Early Tertiary (Eocene-Oligocene) as a result of anoblique subduction of the Indian Oceanic Plate underneath the Sundaland continental block. Thebasin is comprised of Tertiary to Recent sediments that were deposited over Pre-Tertiary basement.

Typically, in many places basement consists of complex igneous and metamorphic rocks, but it isdifferent in the North Sumatra basin. Underneath this basin, there are carbonates (dolomites orlimestones) and sandstones including the Eocene Tampur Formation that have been called economicbasement. Economic basement refers to rocks that have no economic prospectivity whilst the

formation is comprised of the sedimentary deposits. The term economic basement in the NorthSumatra Basin should be reconsidered because some data shows porosity development in poresand fractures, therefore making them potential reservoir that can receive hydrocarbon charge from

proven petroleum system in the basin.

Keywords: Economic Basement, Fractured Basement, North Sumatra Basin

INTRODUCTION

Basement and crystalline basement are defined asthe rocks underneath a sedimentary basin or thosecovered by sedimentary rocks. Nowadays, they arenot merely the rocks which accommodate the

sedimentary basins but also play a significant roleas hydrocarbon reservoirs in several countriesincluding Indonesia. For example, in the BerukNortheast Field, Central Sumatra, which hasproduced about 2 million barrels of oil and inSouth Sumatra, where several gas fields areproducing from the pre-Tertiary basement,collectively have estimated reserves of about 5 TCF(Koning, 2007). Those basements are calledfractured basement because they have fractures

through which hydrocarbons flow. Anotherterminology for describing basement is by usingthe term economic basement. An economic

basement is defined as non-petroleum prospectivesedimentary rocks that underlie a sedimentarybasin.

North Sumatra Basin is one of several prolificbasins in Sumatra, Indonesia. The basin wasformed during Early Tertiary (Eocene-Oligocene) asa result of an oblique subduction. The researcharea is located in Malacca Strait, approximately150km to the NNW of Medan, the capital city of

North Sumatra Province (Figure 1). The pre- Tertiary basement at this location was categorized

as economic basement (Beicip, 1977 in Darmanand Sidi, 2000). Darman and Sidi (2000) describedthe basement as being composed of sandstones,limestones, or dolomites which are dense andfractured, without any metamorphic alteration. They are supposed to be less dense than igneousor metamorphic basement, but they are difficult tobe traced and identified. The fact that thebasement is fractured and possibly connected to a

proven petroleum system means that they can beprospective for hydrocarbon exploration. The main

objective of this study is to reconsider thebasement of North Sumatra Basin as fracturedbasement and not only economic basement basedon data presented in this article.

Figure 1. Location of research area is shown by the red box (map from Google).


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DATA AVAILABILITY & SCOPE OF WORK

The primary data used in this study includeseismic and well data. The seismic data wasinterpreted and used in building a geological crosssection. The well data consist of wireline logs usedin stratigraphic correlation and thin sections for

describing lithology and pore characteristics andestimating the porosity. Secondary data includepublished work on the region by previous authorssuch as Anderson et al. (1993), whoselithostratigraphic subdivision has been used as areference in this study (Figure 2). All of these dataare used to support interpretation for basementprospectivity.

PRE-TERTIARY BASEMENT

The Pre-Tertiary rocks in the North Sumatra Basinconsist of limestones and dolomites which underliethe Tertiary sediments. Previous authors suggested

that within the Pre-Tertiary rocks, there are alsosandstones, metasediments, tuffs, and granites.Drill cuttings from the study area reveal that thesecarbonates are typically wackestone-packstones,bioclasts, skeletal packestones with fracture andvuggy porosity ranging 5-10%. Porosity estimationfrom well data reveals porosity range from 5 to25%. Nevertheless, Caughey and Wahyudi (1993)reported that despite some of the Pre-Tertiarysedimentary rocks were fractured, they are tightly

cemented with calcite and quartz and show noreservoir potential. However, observation of coreplugs and even dating from these “basement”shows that they are not easily recognized asbasement and distinguishing them from the Tampur Formation is difficult.

TAMPUR FORMATION

The name Tampur Formation has been used for allPre-Oligocene carbonates in the North SumatraBasin (Collins et al. 1996), but is commonly viewedas Eocene in age (Caughey and Wahyudi 1994,Ryacudu and Sjahbuddin 1994). However, thefaunas on which this age is reportedly based havenever been documented. Eocene Nummulites

limestone is present in West Sumatra, in theOmbilin Basin. But, on the other hand, 'Basement'carbonate in the Singa Besar 1 well, just acrossthe Malaysian border in Malacca Straits, containsthe Middle-Late Permian foraminifer Shanita and

other Permian fauna (Fontaine et al. 1992).

The Tampur formation was described by Ryacuduand Sjahbuddin (1994) as massive, partlybiocalcarenites and calcilutites, commondolarenites, chert nodules and basal limestoneconglomerates, that indicate deposits of an openmarine sublittoral environment.

Figure 2. Stratigraphic column of North Sumatra Basin (Anderson et al., 1993).


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From the reports of deep wells in the Aru andLangkat areas, it consists of limestones, commonlyfractured and dolomitised (Ryacudu andSjahbuddin, 1994). Collins et al. (1996) also stated

that drill cuttings from the Tampur Formationconsist of mudstones, skeletal wackestones,intraclast, oolite packstones, and grainstones. These descriptions show a slightly similarcharacteristic as those of the Pre-Tertiary

basement rocks.

We may conclude that some of the basallimestones in the North Sumatra Basin are of welldefined Pre-Tertiary (Permian) age, while LateEocene limestones may be present as well, similarto the thin foraminiferal carbonates outcropping inthe southwestern part of the Central Sumatra

basin. If the Late Eocene age interpretation is thecorrect one, this means that the Tampursediments are related to syn-rift phase and mostlikely distinct from the fractured basementcarbonates (Collins et al, 1996).

SOURCE ROCK AND RESERVOIRCONSIDERATION

The Tampur Formation was deposited in an openmarine shelf setting that generally is lean inorganic matter and normally provides poor source

material for large scale hydrocarbon generation(Ryacudu and Sjahbuddin, 1994). This means younger sediments are the only possible source forhydrocarbon generation, such as organic-richshales in the Belumai Formation, Baong Shales,

Bampo black shales and in the MeucampliFormation.

There are no specific permeability and porosity

data taken from Tampur Formation. However, fromcutting description of a well near Aru Bay-1, where85 meters of thick, fractured dolomites that maycorrelate to the Tampur carbonates have beendrilled, the estimated secondary porosity is around

5-10% (Ryacudu and Sjahbuddin, 1996).

INTERPRETATION

Pre-Tertiary basement in the North Sumatra Basinis difficult to be distinguished from TampurFormation, because it is comprised mainly of

wackestone-packstones. At 5990’ and deeperinterval in a well (Figure 3), thin sections mostlyshow that the wacke-packstone is composed ofcalcite and other minerals, with grains size ofaround 0.05-0.2mm, subangular-subrounded ingrain shape, grain supported and poorlysupported, point contact. Matrix consists ofcarbonate. Cements consist of sparry calcite type. The rock contains interparticle and fracture typesporosity totaling 10%. Another thin section alsoshows moldic and vuggy porosity. With theintegration of well and core porosity that ranges 5-20%, the described characteristics are likely

similar to those of Tampur carbonates.

From correlated seismic section (Figure 4) betweenwells, it can be interpreted that the TampurFormation is laterally widely distributed.Structural and stratigraphic traps may exist in the

0 1 mm.

Figure 3. Photo of thin sections in 5990’ interval of basementcarbonates with interparticle and fracture porosity, totaling 10% in

porosity.


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Tampur Formation. Considering reef buildupscould occur along the shelf margin, the fractured Tampur Formation located on the highs as seen onFigure 4 and 5 might contain fair to good porosityand permeability if they had been exposed. This

makes them highly prospective for reservoir. Theshales of Lower Baong Formation could be the caprock for the Tampur carbonates reservoir alongwith its entrapment.

Figure 4. Seismic section of "A" to "E" Well.

Figure 5. Cross section of the study area, red circles indicating the possible

accumulation of hydrocarbon for basement fractured. Red arrow indicating the

migration pathway.


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CONCLUSION

Under certain circumstances, the presence ofcarbonates in the North Sumatra Basin basement,whether Eocene Tampur Formation or older Pre- Tertiary constituents, suggest that the basement is

not an economic basement but fractured basementwith potential to be reservoir. However, this study

only covered a small portion of the basin andfurther work is required to prove the prospectivityof the fractured basement.

ACKNOWLEDGEMENT

The author gratefully acknowledges Mr. TaatPurwanto, as the main mentor of the author, forhis permission to release certain data and topublish this article.

REFERENCES

Anderson, B.L., Bon, J., and Wahono, H. E., 1993,Reassessment of the Miocene Stratigraphy,Paleogeography and Petroleum Geochemistry ofthe Langsa Block in the Offshore NorthSumatra Basin: Indonesian PetroleumAssociation 22nd Annual Convention, v.1, p.170-189.

Caughey, Charles A., and Turcahyo W., 1994, GasReservoirs in the Lower Miocene PeutuFormation, Aceh Timur, Sumatra: IndonesianPetroleum Association 22nd Annual Convention,v.1, p.191-218.

Collins, J.F., Kristanto, A.S., Bon, J., andCaughey, C. A., 1996, Sequence StratigraphicFramework of Oligocene and Miocene

Carbonates, North Sumatra Basin, Indonesia:25th Silver Anniversary Convention Proceedingsof Indonesian Petroleum Association, Vol.1,p.267-279.

Darman, H., and Sidi, F. H. (eds.), 2000, AnOutline of Geology of Indonesia: IAGI, Jakarta.

Fontaine, H., Asiah, M.S., and Sanatul, S.H.,1992, Pre-Tertiary limestones found at thebottom of wells drilled in Malacca Straits: CCOP

Newsl. 17, 4, p.12-17.Koning, T., 2007, Remember Basement in your Oil

and Exploration: Examples of ProducingBasement Reservoirs in Indonesia, Venezuela,

and USA: CSPG CSEG Convention 2007.Ryacudu, R., and Sjahbuddin, E., 1994, Tampur

Formation, The Forgotten Objective in TheNorth Sumatra Basin: Indonesian PetroleumAssociation 23rd Annual Convention, v.1, p.160-179.


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Student articleDepositional Environment Analysis of Kali Banyumeneng,Mranggen, Kabupaten Demak

Samuel R.N. Simorangkir, Fahmi Abdillah, Zul Hayuddin and Galang VirgiawanGeological Engineering Department, Diponegoro University, Semarang


INTRODUCTION

Kali Banyumeneng is located in Mranggen, Demak(Central Java province). The studied outcrop islocated approximately 11.25 km to the SE ofSemarang city and at the geographic coordinate of

100° 28’ 59.65” E and 7° 02’ 43.45” S (Figure 1).

The study area is geologically located in KendengZone, which is also often referred as Kendeng

Mountains, an east-west trending anticlinorium inthe northern part of Java. The northern border ofthe Kendeng Zone is Randublatung Depression,while the southern border comprises a line ofvolcanoes called Solo Zone. Kendeng zone is acontinuation of the Northern Mountains SerayuZone that developed in Central Java. The Salatigasection of the Kendeng zone extends towards theeast to Mojokerto and plunges under the Brantas

river. The continuation of these mountains canstill be tracked under the Madura Strait.

Van Bemmelen (1949) subdivided Kendeng

Mountains into 3 parts that consist of the westernpart, which lies between Mt. Ungaran and Solo(north Ngawi); the central part that lies betweenSolo and Jombang and the eastern part thatextends from east Jombang to Brantas River Deltaand continuously to Madura Bay. The study areaoccurs in the western Kendeng Zone.

Previous work (e.g. van Bemmelen, 1949) showsthat the study area consists of twolithostratigraphic units called Kerek and KalibengFormations. Kerek Formation consists ofinterbedded sandstones, claystones, and sandylimestones with observed sedimentary structuressuch as graded bedding, ripple mark andconvolute. Kalibeng Formation is subdivided intolower and upper parts. The lower part of Kalibeng

Formation consists of sandstones that showsturbiditic nature. The upper part of KalibengFormation is composed of breccias with fragmentsformed by limestone. The Upper KalibengFormation also contains forams, molluscs, coralsand algae and it shows a bedding structure.

Observations were carried out to determine thedepositional environment model of KaliBanyumeneng area based on lithology,sedimentary structures and fossils.

METHODOLOGY

The observation began with data collection in thefield. The field data collection was carried out withstratigraphic section measurement followed by ananalysis to determine the depositional environmentof the outcrops.

Figure 1. Location map of Kali Banyumeneng outcrop discussed in the article

(Google Maps, 2013).


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Figure 2. Sedimentological description of Kali Banyumeneng outcrop (to be continued to the next page).


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Figure 2. Sedimentological description of Kali Banyumeneng outcrop.


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RESULTS

From the results of the 100 meter sectionmeasurement, it can be seen that the constituentlithologies are sandstones, limestones, andsiltstones (Figures 2 & 3). Sandstones in this area

are bright brown in color, with layering structure,and fine to medium in grain size, well sorted and

calcareous. The limestone is characterized by graycolour, laminated structure, with coarser grain,poorly sorted, and composed of Pelecypoda

macrofossil as well as foraminifera fossils. Thesiltstones in this outcrop are characterized bygrayish brown color, no-sedimentary structure,with a silty grain size, well sorted and calcareous.

The sedimentary structures observed in the KaliBanyumeneng area include bedding, lamination,cross stratification, convolute, load casts,hummocky and ripple marks found on sandstones(Figure 4). Hydrodynamic sedimentary structuresare indication of gravitational flow (Boggs, 1987).In addition, the hummocky cross-stratificationindicate that the sequence was deposited understorm influence. Mudcracks and rainmarks are

also visible in younger sediments probably ofQuaternary age.

ANALYSIS

Based on our field observation, the Banyumenengoutcrops are interpreted as having deposited inshallow marine to deep water settings. This isindicated by the presence of sedimentarystructures exposed on the ground in the form ofconvolute (identifier slope areas) and the presenceof Pelecypoda , which usually lives in shallow

marine setting. Other indications include

sedimentary structures such as hummocky cross-stratification that developed in the area, whichhave been described above.

We have also identified the following foraminiferaspecies from the outcrops: Globigerinoidesimmaturus, Globigerina venezuelana, Hastigerinaacquilezeralis, Globigerinoides sicanus, Globigerina

obligus, Globorotalia multicamerata, Globigerinoidesruber, Globigerinoides diminutus, Globigerinoidestrilobus, Globorotalia obesa, Cassigerinellachipolensis, Globigerinoides primodius and

Praerbulina glomerosa . These assemblages offoraminifera species indicate sediments of mostlikely Middle Miocene age.

REFERENCES

Boggs, Sam Jr., 1987, Principles of Sedimentologyand Stratigraphy: Merrill Publishing Company,Columbia.

Staff Asisten Sedimentologi dan Stratigrafi, 2011,Panduan Praktikum Sedimentologi danStratigrafi: Teknik Geologi UNDIP, Semarang.

Van Bemmelen, R., 1949, The Geology of Indonesia

Volume 1A: Government Printing Office, TheHague, Netherlands.

A

B

Figure 3. Interbedded of limestones and

sandstones. Bed thickness increases towards theupstream direction.Figure 4. Sedimentary structures in sandstone,

(A) parallel bedding and (B) ripple marks.


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