coals source rocks and hydrocarbons in the south palembang sub-b

8/21/2019 Coals Source Rocks and Hydrocarbons in the South Palembang Sub-b

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University of Wollongong

Research Online

University of Wollongong Tesis Collection University of Wollongong Tesis Collections

1991

Coals, source rocks and hydrocarbons in the SouthPalembang sub-basin, south Sumatra, Indonesia

Rubianto Indrayudha AmierUniversity of Wollongong

Research Online is the open access institutional repository for the

University of Wollongong. For further information contact the UOW

Library: [email protected]

Recommended CitationAmier, Rubianto Indrayudha, Coals, source rocks and hydrocarbons in the South Palembang sub-basin, south Sumatra, Indonesia,Master of Science (Hons.) thesis, Department of Geology, University of Wollongong, 1991. hp://ro.uow.edu.au/theses/2828
http://ro.uow.edu.au/http://ro.uow.edu.au/theseshttp://ro.uow.edu.au/thesesuowhttp://ro.uow.edu.au/http://ro.uow.edu.au/thesesuowhttp://ro.uow.edu.au/theseshttp://ro.uow.edu.au/http://ro.uow.edu.au/http://ro.uow.edu.au/


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COALS,SOURCE ROCKS AND HYDROCARBONS

IN THE SOUTH PALEMBANG SUB-BASIN, SOUTH SUMATRA,

INDONESIA

A thesis submitted in

partial)

fulfilment of the

requirements for the award of the degree of

MASTER OF SCIENCE

HONS.)

from

THE UNIVERSITY OF

WOLLONGONG

by

RUBIANTO INDRAYUDHA MIER

B.Sc. AGP BANDUNG)

DepartmentofGeology

1991


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13657


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I certify that the substance of this thesis is

original and has not already been submitted for any

degree and is not being currently submitted for any

other degree.

Rubianto IndrayudhaAmier


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TABLE OF CONTENTS

ABSTRACT

ACKNOWLEDGEMENTS

LIST OF FIGURES

LIST OF TABLES

LIST OF PLATES

PAGE

CHAPTER ONE-INTRODUCTION

1.1

AIM OF THE STUDY

1.2 PREVIOUS STUDIES

1.3 HISTORICAL BACKGROUND OF SOUTH SUMATRA BASIN

1.4 LOCATION AND ACCESS

1.5 .MORPHOLOGY

CHAPTER TWO TERMINOLOGY AND ANALYTICAL METHODS

2.1

TERMINOLOGY

2.2 ANALYTICAL METHODS

2.2.1 Sampling

2.2.2 Sample Preparation

2.2.3 Microscopy

2.2.3.1 Reflected white light microscopy

and determination of vitrinite

reflectance

2.2.3.2 Fluorescence-mode microscopy

2.2.3.3 Maceral analysis

1

2

3

4

7

8

10

10

14

14

15

15

15

17

18


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CHAPTER THREE REGIONAL GEOLOGY AND TECTONIC 9

SETTING

3.1 REGIONAL GEOLOGY 19

3.2 STRATIGRAPHY

21

3.2.1 The pre-Tertiary rocks 22

3.2.2 Lahat Formation (LAF) 23

3.2.3 Talang Akar Formation (TAF) 24

3.2.4 Baturaja Formation (BRF) 26

3.2.5 Gumai Formation (GUF) 28

3.2.6 Air Benakat Formation (ABF) 29

3.2.7 Muara Enim Formation (MEF) 30

3.2.8 Kasai Formation (KAF) 32

3.3 DEPOSITIONAL HISTORY OF THE TERTIARY SEDIMENTS 33

CHAPTER FOUR ORGANIC MATTER TYPE OF TERTIARY 38

SEQUENCES

4.1 INTRODUCTION 38

4.2 TYPE AND ABUNDANCE 39

4.2.1 Lahat Formation 39

4.2.2 Talang Akar Formation 41

4.2.2.1 DOM 41

4.2.2.2 Coal and shaly coal 42

4.2.3 Baturaja Formation 43

4,2.4 Gumai Formation 44

4.2.5 Air Benakat Formation 45

4.2.6 Muara Enim Formation 46


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4.3 RELATIONSHIP BETWEEN RANK AND MACERAL TEXTURES 5 0

AND FLUORESCENCE INTENSITY

CHAPTER FIVE - ORGANIC MATURATION AND THERMAL HISTORY 54

5.1 INTRODUCTION 54

5.2 RANK VARIATION AND DISTRIBUTION 55

5.3 THERMAL HISTORY 61

5.4 SOURCE ROCKS AND GENERATION HYDROCARBONS 66

5.4.1 Source rocks for hydrocarbons 66

5.4.2 Hydrocarbon generation 74

5.4.2.1 Timing of hydrocarbon generation 77

using Lopatin Method

5.5 POTENTIAL RESERVOIRS 82

CHAPTER SIX - CRUDE OIL AND SOURCE ROCKS GEOCHEMISTRY 85

6.1 INTRODUCTION 85

6.2 OIL GEOCHEMISTRY 86

6.2.1 Experimental Methods 86

6.2.2 Sample fractionation 86

6.2.3 Gas chromatography analysis 86

6.2.4 Preparation of b/c fraction 87

6.2.5 Gas chromatography-mass spectrometry 88

analysis

6.2.6 Results 88

6.2.6.1 Gas chromatography 89

6.2.6.1 Gas chromatography-mass 92

spectrometry


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6.3 SOURCE ROCK GEOCHEMISTRY

6.3.1 Experimental Section 9 5

6.3.1.1 Sample extraction 96

6.3.2 Results 96

CHAPTER SEVEN COAL POTENTIAL OF SOUTH PALEMBANG100

SUB BASIN

7.1 INTRODUCTION 100

7.2 COAL DIVISIONS IN THE MUARA ENIM FORMATION 101

7.3 DISTRIBUTION OF MUARA ENIM COALS 103

7.3.1 Enim Prospect Areas 104

7.3.2 Pendopo Areas 105

7.4 COAL QUALITY 106

7.5 ASH COMPOSITION 108

7.6 STRUCTURES 109

7.7 COAL RESERVES HI

7.8 BUKIT ASAM COAL MINES 112

7.8.1 Stratigraphy 113

7.8.1.1 Quarternary succession 113

7.8.1.2 Tertiary succession 113

7.8.1.2.1- Coal seams 113

7.8.1.2.2 Overburden and 114

Intercalations

7.8.2 Coal Quality

115

7.8.3 Coal Reserves 116

7.9 BUKIT KENDI COALS

117

7.10 BUKIT BUNIAN COALS


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CHAPTER EIGHT - COAL UTILIZATION 120

8.1 INTRODUCTION 120

8.2 COMBUSTION 121

8.3 GASIFICATION 124

8.4 CARBONISATION 125

CHAPTER NINE - SUMMARY AND CONCLUSIONS 128

9.1 SUMMARY 128

9.1.1 Type 128

9.1.2 Rank 131

9.1.3 Thermal History 132

9.1.4 Source rock and hydrocarbon generation 133

potential

9.1.5 Coal potential and utilization 136

9.2 CONCLUSIONS 137

REFERENCES 143

APPENDIX 1 Short descriptions of lithologies and

organic matter type, abundance and

maceral composition from wells studied.

APPENDIX 2 Summary of the composition of maceral groups

in the Tertiary sequences from wells studied.


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ABSTRACT

The South

Palembang

Sub-basin, in the southern part of

the South Sumatra Basin, is an important area for coal and

oil production. In order to develop the economy of this

region, an understanding of the coal and source rock

potential of the Tertiary sequences within the South

Palembang Sub-basin is essential.

Collisions between theIndo-Australianand the Eurasian

Plates formed the South Sumatra Basin and particularly

influenced the development of the South Palembang Sub-basin

since the Middle Mesozoic toPlio-Pleistocene.

The Tertiary sequences comprise from oldest to youngest

unit;the Lahat, Talang Akar, Baturaja, Gumai, Air Benakat,

Muara Enim and Kasai Formations. These sequences were

developed on the pre-Tertiary rocks which consist of a

complex of Mesozoic igneous rocks and of Palaeozoic and

Mesozoic metamorphics and carbonates.

Coals occur in the Muara Enim, Talang Akar and Lahat

Formations. The main workable coal measures are

concentrated in the Muara Enim Formation. The Muara Enim

coals are brown coal to sub-bituminous coal in rank, while

the Lahat and Talang Akar coals are sub-bituminous to high

volatile bituminous coals in rank. From the viewpoint of

economically mineable coalreserves, the M2 Subdivision is

locally the most important coal unit. Thicknesses of the M2

coals range from 2 to 20 metres. The coals can be utilized


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for electric power generation, gasification but are

generally unsuitable as blends for coke manufacture. They

have some potential for the manufacture of activated

carbons.

In general, coals and DOM of the Tertiary sequences are

dominated by vitrinite with detrovitrinite and telovitrinite

as the main macerals. Liptinite is the second most abundant

maceral group of the coals and DOM and comprises mainly

liptodetrinite, sporinite and cutinite.

The Lahat, Talang Akar, Air Benakat and Muara Enim

Formations have good to very good hydrocarbon generation

potential. The Baturaja and Gumai Formations have less

significant source potential as this unit contains little

organic matter but in some places these formations are

considered to have good potential to generategas.

The vitrinite reflectance data and studies using the

Lopatin model indicate that the onset of oil generation in

the South Palembang Sub-basin occurs below 1500 metres. In

general the Gumai Formation lies within the onset of oil

generation zone, but in some places, the lower part of Air

Benakat and Muara Enim Formations occur within this zone.

Crude oil geochemistry shows that the oils are

characterized'

by high ratios of pristane to phytane

indicating a source from land-derived organic matter. The

presence of bicadinane-type resin and oleanane in the oils

is further evidence of a terrestrial source. The

biomarker

and thermal maturity of the source rocks and coals from the

Talang Akar Formation are similar to those of the oils

studied.


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ACKNOWLEDGEMENTS

This study was carried out at the Department of

Geology, University of Wollongong under the tenure of a

Colombo Plan funded by the Australian International

Development Assistance Bureau (A.I.D.A.B). I am thankful to

Associate Professor A.J. Wright, the Chairman of the

Department for his support and for allowing me to use the

Department facilities during my study. This study was

carried out under the supervision of ProfessorA.C.Cook and

Associate Professor B.G. Jones. I would like to thank to

Associate Professor B.G Jones for his suggestion and

guidance during the

finishing

of this

thesis.

I am also

grateful especially to Professor A.C. Cook for introducing

me to the field of organic petrology and also for his

assistance, patience guidance and suggestions throughout

this study. I wish to record my deep appreciation to Dr.

A.C.Hutton for his suggestion, encouragement, help and

general assistance during the finishing of this thesis. I

also wish to thank all members of the staff of the Geology

Department, University of Wollongong, for their help,

including Mrs R. Varga, Mr Aivars Depers and Mrs B.R.

McGoldrick who gave general assistance and helped in

numerousways.

I thank the Government of Indonesia, particularly the

Ministry of Mines and Energy for selecting me to accept the

Colombo Plan Award. The author also wishes to specially


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thank the management and staff of PERTAMINA, particularly

Ir

M. Anwar, Ir L. Samuel, Ir L.

Gultom,

Ir. H.Hatuwe and Ir A.

Pribadi for allowing me to collect and to use the samples

and technical data from various wells of the South Palembang

Sub-basin. I am also grateful toIrBusono SE, Director of

Directorate of Coal and to his predecessor, Drs Johannas for

permitting me to study in the Geology Department, University

of Wollongong. I would like also to thank the staff of the

Directorate of Coal who helped and supported me in this

study.

Special appreciation is given to Dr R.E. Summons, Mrs

J.M. Hope and P. Fletcher from the Bureau of Mineral

Resources in Canberra, for carrying out oil analyses and

Rock-Eval

pyrolysis of the source rocks samples. The

assistance and guidance of Dr R.E. Summons particularly, is

gratefully acknowledged.

The author wishes to express his. gratitude to the

A.I.D.A.B staff particularly to the Training Liaison

Officers such as Mr B. Rush, Mrs G. Ward, Dr D.Engeland Mr

B. O'Brien, and I would like also to thank Mr K. Passmore,

Ms N. Limand Ms Lisa

Huff,

for the assistances given

during this study.

I am thankful to all my colleagues particularly H.

Panggabean, S.M. Tobing, N.

Ningrum,T. Ratkolo, B. Daulay,

Susilohadi, Y. Kusumabrata, K. Sutisna, R. Heryanto, B.

Hartoyo, Herudiyanto, A.Sutrisman and A. Perwira K. for

their help, support and suggestions during this study.


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These contributions of all these people are gratefully

appreciated.

Finally, I am forever grateful to my wife Ida and

daughters,IndriandEmilwho gave me endless support, love

and encouragement during this study.


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LIST OF FIGURES

Figure 1.1

Figure 1.2

Figure 1.3

Figure 1.4

Figure 1.5

Figure 2.1

Figure 2.2

Figure 2.3

Figure 2.4

Figure 3.1

Figure 3.2

Figure 3.3

Figure 4.1

Figure 4.2

Figure 4.3

SouthSumatra

coal province and its

demonstrated

coal resources (after Kendarsi,1984).

Location map of Suraatran back-arc basins.

Tectonic elements of South Sumatra Basin (afte

Purnomo,1984).

Oil well locations and their relationship with

major tectonic elements of the South Palembang

Sub-basin (after Pulunggono,1983).

Geological features of the Bukit Asam and

-surrounding areas, and locations of boreholes

studied (after Kendarsi,1984).

Flow diagram showing the method for polishing

and mounting samples (after Hutton,1984).

Alteration of the macerals during coalificatio

stage (after Smith and Cook,1980).

Diagram showing optical configuration for

reflectedwhite light andfluorescence-mode

observation used in this study (from AS.2856,

1986).

Visual aid to assist in the assessment of

volumetericabundance of dispersed organic

matter in sediments.

Lineaments of subduction zones in western

Indonesia (after Katili,1984).

Pre-Tertiary rocks underlying the Tertiary in

the South Sumatra Basin (after De Coster,

1974).

Distribution of Talang Akar Formation within t

South Palembang Sub-basin (after Pulunggono,

1983).

Abundance range and average abundance by volum

and maceral group composition of DOM,shalycoal

and coal in the Lahat Formation at

five

well

locations in the South Palembang Sub-basin.


and maceral group composition of DON, shaly coal

and coal in the Talang Akar Formation at ten

well locations in the South Palembang Sub-basin.


and maceral group composition of DOM in the

Baturaja Formation at six well locations in the

South Palembang Sub-basin.


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Figure 4.4 Abundance range and average abundance by volume

and maceral group composition of DOM in the

Gumai

Formation at ten well locations in the


Figure 4.5 Abundance range and average abundance by vol

and maceral

group

composition of DOM in the Air

Benakat Formation at ten well locations in the


Figure 4.6 Abundance range and average abundance by vol

and maceral group composition of DOM and coal in

the Muara Enim Formation at ten well locations

in the South Palembang Sub-basin.

Figure 5.1 Plot of reflectance against depth for sample

from the MBU-2

well.


from the PMN-2well.


from theGM-14well.


from theKG-10well.


from the

KD-01

well.


from the BRG-3well.


from the TMT-3

well.


from the L5A-22well.


from the BL-2

well.

Figure 5.10 Plot of reflectance against depth for sampl

from theBN-10well.

Figure 5.11 Schematic cross-section A-B through the Mua

Enimarea showing isoreflectance surfaces.

Figure 5.12 Schematic cross-section C-D through

Limau-Pendopo

area showing

isoreflectance

surfaces.

Figure 5.13 Plot of reflectance against depth for sampl

f

romSouth Palembang Sub-basin.

Figure 5.14 Pre-tectonic

coalification


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Figure 5.15 Syn-tectonic

coalification

Figure 5.16 Post-tectonic

coalification

Figure 5.17

Figure 5.18

Figure 5.19

Figure 5.20

Figure 5.21

Figure 5.22

Figure 5.23

Figure 5.24

The relationship between

coalification

and

tectonicsmas proposed byTeichraullerand

Teichrauller

(1967).

Karweil Diagram showing relationship of time

(Ma),temperature ( C) and rank scales (after

Bostick,

1973).

Scale H is used for

calculating thermal history of Table 5.11 and

5.12.

Hydrocarbon generation model for oil and

condensate from source rocks containing

terrestrial organic matter (after Snowdon and

Powell,1982).

Pyrolisis data S2/Org.C Index, which is

indicative of the amounts of already generated

hydrocarbons, show the contribution of

inertinites to generation of hydrocarbons. The

Tmaxdata showing the maximum decomposition of

inertinite-rich kerogens occurs at higher

activation energies compared to inertinite-poor

kerogens (after Khorasani,1989).

The relationship between S1+S2 values and the

Score A for samples studied from the Muara Enim

Formation and the Talang Akar Formation (after

Struckroeyer

(1988).

Generalized zones of petroleum generation and

approximate correlation withmaximum

palaeotemperaturesand reflectance of

vitrinite, exinite and inertinite (from Smith

and Cook,1984).

Maturation model for the main organic matter

groups and sub-groups (from Smith and Cook,

1984).

Lopatin-type model for the coalification

history of the Muara Enim area. Assumptions:

no compaction effect, present geothermal

gradient assumed to have operated since the

Eocene, erosion approximately 250 metres.

Lopatin-type reconstruction of coalification

for the Pendopo area. Assumptions: no

compaction effect, present geothermal gradient

assuramed

to have operated since the Eocene,

erosion approximately 623 metres.


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Figure 6.11 N-alkanedistribution profile in the saturated

fractions in the extracts from the Muara Enim

Formation (sample5383).

Figure 6.12 N-alkane distribution profile in the saturat

fractions in the extracts from the Muara Enim

Formation (sample

5384).


fractions in the extracts

from

the Talang Akar

Formation (sample

5385).


fractionsin theextracts fromthe Talang Akar

Formation (sample5386).

Figure 6.15 The determination of petroleum formation zon

by using Tmax. (after Espitalie et al.,

1985).

Figure 6.16 Modified Van Krevelen diagram using

conventional whole-rock pyrolisis data (after

Katz et al.,

1990).

Figure 7.1 General stratigraphy of the Bukit Asam mining

area (after Von Schwartzenberg,

1986).

Figure 8.1 The transportation net of the Bukit Asam coal,

South Sumatra (after Kendarsi,1984).

Figure 8.2 Generalized relationship of coke strength and

coal rank, indicated by vitrinite reflectance

and carbon content of vitrinite, at constant

type (after Edwards and Cook,1972).


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LIST OF TABLES

Table 1.1

Table 2.1

Table 2.2

Table 2.3

Table 2.4

Table 2.5

Table 3.1

Table 3.2

Table 3.3

Table 5.1

Table 5.2

Table 5.3

Table 5.4

Table 5.5

Table 5.6

Table 5.7

Table 5.8

Oil fields in South Sumatra and their

cummulativeproduction until 1966 (after

Koesoeraadinata,

1978).

Generalized classification of coal rank (from

Cook,1982).

Summary of the macerals of hard coals (from

I.C.C.P.Handbook,1963).

Maceral Groups (Stopes-Heerlen system of

nomenclature).

Summary of the macerals of brown coals (from

I.C.C.P.Handbook,1971).

Proposed coal maceral classification system for

coals (Smith,

1981).

Stratigraphy of South Sumatra Basin according t

some authors.

Stratigraphy of South Sumatra Basin used in the

present study based onSpruyt'sNomenclature

(1956).

Stratigraphic column of Muara Enim Formation

according to Shell Mijnbouw, 1978.

Reflectance values and temperature data

against depth in the MBU-2well.


against depth in the PMN-2well.


against depth in the

GM-14

well.


against depth in theKG-10well.


against depth in theKD-01well.


against depth in the BRG-3well.


against depth in the TMT-3

well.


against depth in the L5A-22well.


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Table 5.9 Reflectance values and temperature data

against depth in the BL-2well.

Table 5.10 Reflectance values and temperature data

against depth in theBN-10well.

Table 5.10A Vitrinite reflectance values of Muara Enim coa

measured from core samples.

Table 5.11 Thermal history data from selected wells in the

Muara Enim area.

Table 5.12 Thermal history data from selected wells in the

Pendopo-Limauarea.

Table 5.13 Summary of petrographic features and their

significance

in relation to oil generation and

migration (from Cook andStruckmeyer,1986).

Table 6.1 Locations of crude oil and cutting samples.

Table 6.2 The composition of the oils in terras of the

polarity classes of saturated, aromatic

hydrocarbons and combinedNSO-asphaltene

fraction.

Table 6.2A Peak assignments for triterpanes present in

Figure 6.6.

Table 6.3 The composition of saturated normal hydrocarbons

determined by GC analysis. The data is presented

quantitatively and this is related to the peak of

the internal standard3-raethylheneicosane

(anteiso C22)

giving quantities in

ug/mg.

Table 6.4 The composition of isoprenoid and bicadinane

hydrocarbons determined by GC analysis. The data

is also presented quantitatively in relation to

the peak of the internal standard

3-methylheneicosane(anteiso C22) giving

quantities inug/mg(ppt).

Table 6.5 The composition of the triterpenoid hydrocarbons

determined byGCMS.

Table 6.6 The composition of the steroid hydrocarbons and

four of the bicadinanes determined byGCMS.

Table 6.7 The composition of the steroid and triterpenoid

hydrocarbons and

four

of the bicadinanes of whole

oil determined byGCMS.

Table 6.8 The total organic carbon (TOO, rock eval data

and the bulk composition of the SouthSumatran

shales/coals extract.


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Table5.9

Table 6.10

Table 6.11

Table 7.1

Table 7.2

Table 7.3

Table 7.4

Table 7.5

Table 7.6

Table 7.7

Table 7.8

Table 7.9

Table 8.1

Table 8.2

The composition of saturated hydrocarbons of

SouthSumatran shales/coalsdetermined by gas

chromatography analysis.

South Sumatran coals/shales GC results:

isoprenoids.

South Sumatran coals/shales GC results:

Isoprenoidsug/mgSaturates.

Coal qualities of the Enim Area (after KOG,

1987).

Coal qualities of the Muara Lakitan Area (afte

Shell,

1978).

Coal qualities of the Langaran Area (after

Shell,

1978).

Coal qualities of the Sigoyang Benuang Area

(after Shell,1978).

Coal qualities of the Air Benakat Area (after

Shell,

1978).

Coal qualities of the Prabumulih Area (after

Shell,1978).

Sodium oxide in Ash from the Muara Enim coals

(after KOG,1987).

Summary of coal resources in the Enim area

(maximum overburden thickness 100 metres to top

of the uppermost mineable seam; after KOG,

1987).

Coal qualities of the Kabau Seam from the Buki

Kendi Area{afterShell,1978).

The differences in calorific value among the

three main maceral groupsforfour German coals

determined by Kroger et al., 1957 (after Bustin

et al.,1983).

Comparison of the chemical composition between

Lurgi semi cokes and Bukit Asam semi-anthracite

coals (after Tobing,1980).,


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H PTERONE

INTRODU TION

In the South Sumatra Basin, coal occurs in the Muara

Enim Formation, Talang Akar Formation and Lahat Formation.

The main workable coal measures are concentrated at two

horizons within the Muara Enim Formation. Ziegler (1918)

recognized that the lower horizon comprised (from top to

bottom),the Mangus, Suban, Petai, Merapi and Keladi seams,

and the upper horizon comprized a composite set of coal

.seamscalled the Hanging seam. The seams are in the range

of some metres to more than 10 metres in thickness.

The South Sumatra Basin also plays a role as an

important: oil producing area. Recently there has been

considerable discussion on the oil generation potential of

coals. The Talang Akar Formation has been postulated as a

source rock for oil because of the close association of coal

measures and many of the oil pools in areas such as South

Palembang Sub-basin.

Oil production in South Sumatra was established in the

late 19th century from the Air Benakat Formation. In 1922,

he petroleum company Stanvac established production from

the Talang Akar Formation. The South Palembang Sub-basin is

one of the oil and gas producing areas in South Sumatra.

In the present study, organic petrography was used to

determine

the coal type and rank, and to define the poten

tial of source rocks and maturation level of the organic


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2

matter in the Tertiary sequences of the South Palembang

Sub-basin.

1.1 AIM OF THE STUDY

In general, the aim of the present study is to assess

the rank and abundance of coal and dispersed organic matter

in the Tertiary sequences of the South Palembang Sub-basin.

The study is based on petrological research both on macerals

in the coals and the relatively abundant dispersed organic

matter in the clastic sedimentary sequences. The scope of

this study is to :

1. describe and interpret coal type and rank trends in

the South Palembang Sub-basin;

2.

assess the abundance and composition of organic

matter contained in the stratigraphic sequences;

3. determine the maturity of the organic matter and to

evaluate the lateral and vertical rank variations

within the South Palembang Sub-basin;

4.relate coal rank variation tocoalification

histories;

5 define hydrocarbon source potential of the various

stratigraphic units and lithologies;

6. attempt correlations of potential source rocks with

reservoired hydrocarbons; and

7.drawinferencesconcerning thefutureeconomic

potential of coal and hydrocarbons in the South


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^alembang 5ub-basin.

1,2PREVIOUS STUDIES

The geology of the South Sumatra Basin is relatively

well known from numerous publications (Wenneckers, 1958;

Jackson, 1960; Pulunggono, 1969; Todd and Pulunggono, 1971;

De Coster, 1974; Harsa,

1975; Pulunggono,

1983),

especially

the general geology of this area, primarily in connection

with the search for oil and gas.

Many authors have also described the potential of the

coal measures of South Sumatra including Ziegler (1918),

Koesoemadinata (1978) and Kendarsi

(1984).

Furthermore, a

large exploration campaign was run from 1973 to 1979 by

Shell Mijnbouw N.V covering an area of 71,450 sq km in South

Sumatra(Fig.1.1).

In general, the earliest attempts to examine the

organic matter in sedimentary rocks were made by oil

companies to define the maturation level of the source

rocks (Shell, 1978a; Total Indonesie, 1988; Sarjono and

Sardjito, 1989. Daulay (1985) in his Masters thesis,

studied the petrology of South Sumatra coals, especially the

Muara Enim coals from the Bukit Asam coal mines and from

other places surrounding the mine area.

In the framework of the execution of REPELITA III (Five

Year Development Plan) 1979-1984, the Lahat Geological


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Quadrangle (1012) was mapped by the Geological Research and

Development Center in co-operation with PERTAMINA, an

Indonesian state-owned oil company. The geological map is

at a scale of1:250,000 and covers an area of about 18,700

sqkm.

1. HISTORICAL BACKGROUND OF SOUTH SUMATRA BASIN

South Sumatra Basin is one of the most important oil

and coalproducing areas on the island of Sumatra. South

Sumatra's oil production started as early as 1898 from the

regressive sands of the Air Benakat Formation. The first

fields were small and shallow and close to surface seeps on

exposed anticlines. Surface structure has for many years

guided most of the exploration.

In 1922, Stanvac established production from the

transgressive sands of the Talang Akar Formation, which have

subsequently been the main exploration objective in South

Sumatra. Between 1938-1941, Kuang-1, Ogan-3 and Musi-1

wells were drilled by BPM. In these wells, gas had been

encountered in the Baturaja Formation. Moreover, in 1959,

BPM completed well Limau-5A.144 as the first oil producer

from the Baturaja Limestone reservoir in South Sumatra.

In the South Sumatra Basin, many oil companies are

operating at the present time under production sharing

agreements with PERTAMINA. They include Jarobi Oil,Jambi

Shell, Trend Sumatra Ltd, Caltex, British Petroleum,Asamera


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and Stanvac. PERTAMINA also operates in its own right.

In general, the oil fields are clustered into three

structural sub-basins; the Jambi Sub-basin, the Central

Palembang Sub-basin and the South Palembang Sub-basin

(Fig.1.3). According to PERTAMINA

(1986),

there are 57 oil

fields within the South Sumatra Basin.

The maximum oil production capacity from the basin was

62,200 BOPD and the cumulative production was 1,680 MMBO, on

1-1-1985. The occurrences of oil and gas in the South

Sumatra Basin, largely occur in the Talang Akar Formation

(93%) with 3% in the Air Benakat Formation and a few

occurrences in reefs of the Baturaja Formation, the Gumai

Formation and from sandstones in the Muara Enim Formation

(Anwar Suseno,1988).

The Talang Akar Formation generally produces a paraffin

based oil ranging from 35 to 37 API

(Koesoemadinata,

1978),

but the gravity ranges between 21 and 51 API. The Baturaja

Formation typically produces oil which has an API gravity of

37.3 . Oil is also produced from the Air Benakat Formation

and this is a low to mediumparaffin-basedoil, 45-54 API.

However, from the same producing formation, an

asphaltic-based oil, 22-25 API, is produced in Jambi and

these low gravity oils are biodegraded. Table 1 shows some

of oil fields in South Sumatra and their cumulative

production until 1966 (Koesoemadinata,1978).

About 6 billion tons of coal reserves have been

demonstrated in the South Sumatra Basin. These consist


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6

mainly of hard brown coal and are clustered into several

areas. Figure 1.1 also shows the coal potential of South

Sumatra Basin.

In

Tanjung

Enim area, coal has been mined since 1919 in

underground as well as open pit mines; the underground

workings were abandoned in 1942. These coal mines are

situated inMuaraenimRegency, about 180 kilometres west of

Palembang and production comes mainly from the Mangus, Suban

and Petai seams of the Muara Enim Formation. These coals

are mainly hard brown coals, but in the immediate vicinity

of some andesite intrusions, the coals reach anthracitic

rank. According to Schwartzenberg (1986), the Bukit Asam

Coal Mine has potential reserves of about 112 million tons

which comprise about 1 million tons of anthracite, 45

million tons of bituminous coals and 66 million tons of

subbituminous

coal.

The open pit was restricted to small areas with very

favorable stripping ratios and draglines were used to remove

part of the overburden until the late fifties. From 1940 to

1982 the open pit mine was operated by means of power

shovels and trucks and a small belt conveyor system for coal

haulage.

Development of the Bukit Asam Coal Mine began in

1985 when a modern system of bucket wheel excavator

operations with belt conveyors and spreaders was installed.

The mine is operated by thestate-owned Indonesian company

PT.Persero Batubara Bukit Asam.


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7

1 4 LOCATION AND ACCESS

Geologically, the study area is located in the South

Palembang Sub-basin which lies in the southern part of the

South Sumatra Basin (Figure 1.2). This sub-basin is bounded

to the south by the Lampung High and to the north by the

Pendopo High. Eastward, the South Palembang Sub-basin is

bounded by the Iliran High and to the west by the Barisan

Mountains (Figure 1.3). The Palembang Sub-basin covers

approximately 125 x 150 kilometres (Pulunggono, 1983).

The samples studied were collected from oil exploration

wells and coal exploration boreholes which are situated in

various oil and coal fields within the South Palembang

Sub-basin. The oil fields are as follows; Prabumenang,

Meraksa, Kuang, Kedatoh, Beringin, Tanjung Miring,

Limau and Belimbing (Figure 1.4). Mostly, the oil

exploration wells were drilled by PERTAMINA but some old

exploration wells were drilled by Stanvac/BPM.

In general, the oil exploration wells used in this

study penetrated a high proportion of the Tertiary sequences

and some reached basement. The initials, depth and year of

drilling of exploration wells drilled by PERTAMINA are as

follows; GM-14, 1398 m, 1969; KG-10, 1575.8 m, 1970; PMN-2,

1959.6 m, 1972; KD-1, 1858.5 m, 1976; BN-10, 2565 m, 1977;

BRG-3, 2300 m, 1987; MBU-2, 2200 m, 1988. The three wells

drilled by Stanvac/BPM are L-5A.22, 2287 m, 1954; ETM-3,

1633 m, 1959; and BL-2, 1675 m, 1965.


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8

Coal core samples were collected from seven

exploration boreholes, drilled between 1986-1988 by the

Directorate of

Coal,

in several coal fields such as Suban

Jerigi,

Banko, Tanjung Enim, Muara Tiga, Arahan and

Kungkilan (Figure 1.5). The maximum depths reached by these

coal exploration boreholes range from 100 to 200 metres and

all were drilled within the Muara Enim Formation section.

These boreholes are annotated as KLB-03, AU-04, AS-12,

BT-01, KL-03,MTS-06,andSN-04.

Administratively, the study area falls under

Lematang

Ilir Ogan Tengah Regency and Lahat Regency which are

situated in the western part of the South Sumatra Province.

The study area includes the Lahat Quadrangle which is

bounded by the longitudes 103

30'-105 00'E

and latitudes

0300'-04 00'S (Gafoer et al.,1986).

The population of this area is sparse with 40.4

inhabitants per square kilometres (Central Bureau of

Statistics, 1978). Principally, the population is

concentrated in various towns such asPrabumulih, Muaraenim

and Lahat.

In general, the area is covered by dense vegetation,

particularly the hills and swamps. Irregular clearings are

also found in some places for agriculture and cash-crop

cultivation, such as rubber, coffee and pineapples.

Wildlife such as tigers, bears, crocodiles, elephants and

monkey roam the jungle in this area but their numbers are

dwindling. In order to save these species, they are now


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9

protected by law.

Transport to other areas is by car, rail and boat.

Pendopo, especially, can be also reached by air transport

with regular services run by PT. Stanvac Indonesia. A

railroad connects Palembang with Prabumulih, Baturaja,

Muaraenim, Lahat and Tanjung Enim. The roads in this area

are partly

unsurfaced

and therefore are muddy when wet at

which time they are passable only by four-wheel drive

vehicles.

1.5 MORPHOLOGY

Morphologically, this area can be divided into three

units;

the mountainous area, the rolling country and the

plain.

The mountainous area occupies the western corner of

the Quadrangle with summits such as Bukit Besar (735 m) and

Bukit Serelo (670

m ) .

The slopes in this area are generally

steep,

the valleys narrow and locally cascades occur in the

rivers. Braided streams develop in the foothill areas.

The rolling country occupies half of the western

portion of the quadrangle with summits reaching heights of

some 250 metres. The slopes are generally gentle. The

rivers have wide valleys, are meandering, and have deeps on

many

bends.

The drainage pattern is dendritic.

The

low-lying

plain area occupies the eastern

portion

of the quadrangle and is characterized by meandering streams

and dendritic drainage patterns. Elevations on the plain

range from 0 to about 50 meters.


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10

CHAPTER TWO

TERMINOLOGY AND ANALYTICAL METHODS

2.1 TERMINOLOGY

According to the International Committee for Coal

Petrology (1963), coal can be defined as a combustible

sedimentary rock formed from plant remains in various stages

of preservation by processes which involved the compaction

.of the material buried in the basins, initially at moderate

depth. These basins are broadly divided into limnic (or

intra-continental) basins, and paralic basins which were

open to marine incursions. As the underlying strata

subsided progressively, and more or less regularly but

sometimes to great depths, the vegetable debris was

subjected to the classical factors of general metamorphism,

in particular those of temperature and pressure .

Based on this definition, generally it can be concluded

that there are two basic factors involved in the formation

of coal; firstly the type of peat-forming flora and

depositional environment, and secondly the degree of

alteration which is a function of time, temperature and

pressure. In coal petrology, these factors determine the

variables termed type and rank.

According to Cook (1982), these variables can

essentially be considered as independent because the type of

a coal has no influence upon its rank and the reverse is


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11

also true. Cook (1982) also considered that in coal

petrography or more broadly in organic petrography, the term

type is related to the nature of the organic matter found in

a coal or sedimentary rock.

In addition, Hutton (1984) stated that type is a

function of both the type of precursor organic matter that

was deposited as peat and the nature and degree of

alteration that peat components underwent during the early

stages of diagenesis which is a response to the first

(biochemical) stage of coalification (Stach, 1968; Cook,

1982).

Rank generally refers to the stage of coalification

that has been reached by organic matter. In coal

particularly, rank can be defined as the relative position

of a coal in the coalification series of peat through the

stages of the different brown coals (lignite),

sub-bituminous and bituminous coals to anthracites and

finally meta-anthracites, semi-graphite and graphite.

The term rank has been accepted as an international

scientific term. The International Committee for Coal

Petrology (ICCP),in the second edition of the International

Handbook of Coal Petrography (1963) suggested degree of

coalification as a synonym for rank. In coal petrology,

the rank of coal is measured by the reflectance of

vitrinite. The reflectance of vitrinite increases as the

rank of coal increases (Table 2.1).

Petrographic variation of coal can be assessed in

tei

jrms


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12

maceral groups (Stopes, 1935), microlithotypes (Seyler,

1954),or lithotypes (Stopes, 1919; Seyler,

1954).

Macerals

are the microscopically recognizable components of coal and

are predominantly defined by morphology, color and

reflectance in reflected light. Macerals are analogous with

the minerals of rocks. The ICCP (1963) concepts for

macerals are most closely applicable toCarboniferousblack

coals because they were based on these coals (Table 2.2).

However, Smith (1981) showed that the basic concepts of

macerals can be also applied to coals of Tertiary age.

The termmicrolithotype was proposed by Seyler (1954)

to describe typical maceralassociationsas seen under the

microscope (minimum band width 0.05 mm). Lithotypes are

macroscopically recognizable bands visible within a coal

seam.

On the basis of morphology, optical properties and

origin, macerals can be divided into three main maceral

groups;

vitrinite, inertinite and liptinite. The origin,

properties and subdivision of these three groups are shown

in Table 2.3.

Brown et al. (1964) divided vitrinite into two groups;

vitrinite A and vitrinite B. Furthermore, Hutton (1981) and

Cook et al. (1981) proposed additional terms for alginite

within the liptinite group.

The International Committee for Coal Petrology in the

International Handbook of Coal Petrography

(1971,

1975) has

classified macerals of brown coal as shown in Table 2.4.


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^3

This classification has been modified by Smith (1981) as

shown in Table 2.5. He recognized that thehuminitemaceral

group of the ICCP

classification

represents the same

material as the vitrinite maceral group, but at an earlier

stage of maturation. The system proposed by Smith (1981)

has been adapted in its basic form as the system used in the

Australian Standard for Coal-Maceral Analysis (AS

2856-1986).

In addition, Cook (1982) also discussed the term

bitumen which was termed eubitumen by Potonie

(1950).

In

the International Handbook of Coal Petrography (1963,1971),

bitumen is still described as resinite which has a very low

melting point. Bitumen can be mainly recognized at the

sub-bituminous/bituminous coal boundary (Teichmuller,1982).

Teichmuller

(1982) also noted that bitumen develops from

lipid constituents of liptinites andhuminitesand generally

occurs in vein-form or as fillings of bedding plane joints

but sometimes it fills in empty cell lumens. Furthermore

Cook (1982) stated that bitumen is the term applied to all

natural substances of variable color, hardness and

volatility which are composed of a mixture of hydrocarbons

substantiallyfree from oxygenated bodies . He added that

bitumens are generally formed from the degradation of

natural crudes by processes such as microbial attack,

inspissation or

water-washing.

Asphalts, natural mineral

waxes, asphaltines and petroleum are all considered to be

bitumens. Cook

(1990,

pers.coram)

also


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14

considered that some bitumens. (including the maceral

exsudatinite) represent primary generation products.

Impsonitic bitumens generally result from the alteration of

reservoired oil, probably dominantly, but not exclusively,

during the process of deasphalting.

The coal petrographic terms used in the present study

follow those described by the Australian Standard for Coal

Maceral Analysis

(1986).

2 2 ANALYTICAL METHODS

2 2 1 SAMPLING

As mentioned in the previous chapter, the core and

cuttings samples studied were

cbllected from various coal

fields and oil fields in the South Palembang Subbasin area

(Table 2.5). Sampling has mainly focused on the Muara Enim

Formation, the Talang Akar Formation and the Lahat

Formation.

Samples were taken to give as wide a lateral and

vertical coverage of the sequences which are rich in organic

matter (coal-rich or coal) as possible. However, samples

were also collected from other formations to examine the

degree of coalification and the origin of organic matter

occurring in these sequences. Composite samples which were

taken through the entire thickness of a coal seam have been

obtained from cores from shallow boreholes. Cuttings

samples were collected from oil exploration wells over


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intervals ranging between 20 to 50 metres for coal-bearing

sequences and 50 to 200 metres for non coal-bearing

sequences. Sampling was based on the procedure of the

Standards Association of Australia (1975). In addition,

four oil samples were also collected from BRG-3 well (2

samples) and MBU-2 well

(2

samples). These samples were

recovered from the Baturaja Formation ( both MBU-2

samples),

Talang Akar Formation and Lahat Formation (BRG-3samples).

2 2 2 SAMPLE PREPARATION

The method of preparation of polished particulate coal

mounts for microscopic analysis is shown in Figure 2.1. All

samples examined are listed in the University of Wollongong

grain mount catalogue and where blocks are cited in this

study, the catalogue numbers areused.

2 2 3 MICROSCOPY

2 2 3 1 Reflected white light microscopy and determination

of vitrinite reflectance

Vitrinite reflectance measurements on the samples were

made under normal incident white light using a Leitz

Ortholux microscope fitted with a Leitz MPV-1

microphotometer. All measurements were taken using

monochromatic light of 546nm wavelength, in immersion oil


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16

(DIN 58884) having a refractive index of 1.518 at 23 -lc.

In order to calibrate the microphotometer, synthetic

garnet standards of 0.917%, and 1.726% reflectance and a

synthetic spinel standard of reflectance 0.413% were used.

The maximum vitrinite reflectance was obtained by rotating

the stage of the microscope to yield a maximum reading and

then the stage was rotated again through approximately 180

for the second maximum reading. The results of these

measurements were averaged and the mean calculated to give

the mean maximum vitrinite reflectance in oil immersion

(R

v

max).

ICCP (1971, 1975) and Stach et al. (1982) recommended

that one hundred measurements should be taken to obtain a

precise mean value. Determination of R max standard

deviations for a number coals showed that the standard error

of the mean approaches the precision of the measurement

standards, where twenty readings have been taken.

Therefore, in the present study thirty to forty readings

were taken on the coal.

Brown et al. (1964) also recommended that the most

accurate method of reflectance measurement is achieved by

measuring vitrinite A (Telinite + Telocollinite). However,

selective measurement of one vitrinite type is generally not

possible with dispersed organic matter. In general,

vitrinite macerals give the best measurements in relation to

rank assessment because they undergo changes

consistenly with rank (Smith and Cook, 1980) and show less


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17

inherent variability in reflectance according to type (Brown

et al. 1964) (Figure 2.2) compared to liptinite and

inertinite.

2.2.3.2 Fluorescence-mode Microscopy

In order to provide information on organic matter type,

liptinite abundance and maturity, fluorescence-mode

examination was carried out on all samples by using a Leitz

Orthoplan microscope with a TK40 0 dichroic beam splitting

mirror fitted in an Opak vertical illuminator. The

fluorescence-mode filter system comprised BG3 and BG38

excitation filters and a K490 suppression filter. Figure

2.3 shows the optical system for reflected and fluorescence

microscopy used in this study (modified from AS2856, 1986).

A Leitz Vario-Orthomat automatic camera system which is

fitted to the Leitz Orthoplan microscope, was used to take

photographs of the samples. The camera system has a 5 to

12.5X zoom which provided a wide range of magnification.

Kodak Ektachrome 400ASA/21DIN reversal film was used for all

color photographs. Fluorescence-mode photographs were taken

in oil immersion using the BG3/BG38/TK400/K490 filter

system. Photographs were also taken in normal incident

white light with the same type of film used for fluorescence

mode.


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18

2.2.3.3 Maceral Analysis

Conventional point count techniques for maceral

analysis in coal and coal-rich block samples were carried

out using an automatic point counter and stage The traverses

were made on the surface of the samples. The total surface

area of theblocksample traversed was 2 cm x 2 cm and the

yraindensity was about 50%. Approximately 300 points were

counted for each maceral analysis under reflected white

light and fluorescence mode. The volumetric abundance of

various maceral groups was expressed as a percentage of the

total points recorded.

Visual approximations of the abundance of dispersed

organic matter in each grain mount sample were also made by

assessing volumetric abundances as illustrated in Figure

2.4. The total dispersed organic matter (DOM) abundance was

visually estimated in approximately 50 grains from several

traverses across each block. This method was first

described by Padmasiri (1984) and later modified by

Struckmeyer (1988). The method used in this study is based

on the Struckmeyer modification (1988). The total dispersed

organic matter abundance is calculated using the equation :

2 (y x a)

V = , where V = volume of a specific maceral

n occurring as dispersed organic matter,

y = number of grains containing the maceral in a given

abundance category; n = number of grains counted.


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>n

CHAPTER THREE

REGIONAL GEOLOGY AND TECTONIC SETTING

3 1 REGIONAL GEOLOGY

South Sumatra Basin is one of the Sumatran back-arc

basins located along the island of Sumatra. These basins

came into existence as a consequence of the interaction

between the Sunda Shield as part of the Eurasian plate and

the Indo-Australian plate (Katili, 1973; 1980; De Coster,

1974;

Koesoemadinata and Pulunggono, 1975; Pulunggono, 1976;

Hamilton, 1979; Pulunggono,

1983).

Oblique collision and

subduction has occurred along this margin since the Late

Cretaceous (Figure 3.1).

The South Sumatra Basin is an asymmetric basin bounded

to the west and south by faults and uplifted exposures of

pre-Tertiary rocks along the Barisan Mountains, to the north

east by the sedimentary or depositional boundaries of the

Sunda Shelf. The south-east boundary is represented by the

Lampung High; the northern boundary, however, is poorly

defined as the South Sumatra Basin is connected to the

Central Sumatra Basin by a series of Tertiary grabens,

although the TigaPuluhMountains are generally taken to be

the boundary between the two basins (Figure 1.2). The South

Sumatra Basin occupies an area of roughly 250 by 400 km (De

Coster,1974).

The tectonic features present in the South Sumatra


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20

Basin are the result of Middle Mesozoic to

Plio-Pleistocene

orogenic activity (Katili, 1973, 1980; De Coster, 1974;

Koesoemadinata and Pulunggono, 1975; Pulunggono, 1976;

Hamilton, 1979; Pulunggono,

1983).

These orogenic

activities were primarily related to the collision and

subduction of theIndo-Australianplate underneath the

Sumatra portion of the Eurasian plate.

The Middle Mesozoic orogeny was the main cause of the

metamorphism

affecting Palaeozoic and Mesozoic strata.

These strata were faulted and folded into large structural

blocks and subsequently intruded by granite batholiths, with

postulated extensions in the subsurface parts of the basins.

Pre-Tertiary features combine to form the basic northwest to

southeast structural grain of Sumatra.

In Late Cretaceous to Early Tertiary time, the second

significant tectonic event occurred when major tensional

structures,

including grabens and fault blocks, were formed

in Sumatra and the adjoining Sunda Basin. The general trend

of these faults and grabens is north to south and

north-northwest to south-southeast.

The last tectonic phase was the Plio-Pleistocene

orogeny which caused the uplift of the Barisan Mountains and

the development of major right lateral wrenching through the

length of these mountains. The most prominent structural

features within this Tertiary sedimentary basin are

northwest trending folds and faults.

Structurally, the South Sumatra Basin is subdivided


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21

into four sub-basins, as seen inFig.1.2;

- Jambi Sub-basin;

- North Palembang Sub-basin;

- Central Palembang Sub-basin; and

- South Palembang Sub-basin.

3.2 STRATIGRAPHY

Regional stratigraphic terminologies for the South

Sumatra Basin have been proposed by several authors such as

Musper (1937), Marks (1956), Spruyt (1956), De Coster

(1974), Pulunggono (1983) and Gafoer et al. (1986), as shown

in Table 3.1. The stratigraphic nomenclature used in this

thesis is based primarily on that of Spruyt (1956), because

Spruyt's nomenclature has been widely accepted as the basis

for rock stratigraphic subdivisions, but alternative

nomenclature has also been developed (Table 3.2).

All these authors considered that two phases of

sedimentation took place in the South Sumatra Basin; they

were the Paleogene and Neogene cycles. With the onset of

clastic deposition in the Paleogene, basement depressions

and fault grabens became filled. Harsa (1975) pointed out

that the whole sequence of basin fill represents one major

transgressive-regressive sedimentary cycle which was

accompanied by periodic volcanic activity and periodic

movement along lines of basement faults.

The Tertiary sequences were developed on the


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22

pre-Tertiary surface of eroded igneous and metamorphic

rocks. The pre-Tertiary rocks are generally considered as

economic basement for the basin in terms of oil exploration.

3.2.1 THE PRE-TERTIARY ROCKS

Pre-Tertiary rocks crop out extensively both on the

Sunda Shield and in the Barisan Range. Minor outcrops also

occur in uplifts within the Tertiary retro-arc

basins.

These rocks generally consist of a complex of Mesozoic

igneous rocks and of Paleozoic and Mesozoic metamorphic

rocks and carbonates (Adiwidjajaand De Coster,197

3).

Adiwidjaya and De Coster (197 3)have also distinguished

the basement rocks in the South Sumatra Basin as shown in

Figure 3.2. They mapped the subcrop of the pre-Tertiary

rocks in broad zones termed Zone A, B, C, D and E.

Zone A consists of Permo-Carboniferous metamorphic

rocks including phyllites, slates, argillites, quartzites

and gneisses and occasional limestones. These rocks were

intruded by diorite and granite batholiths.

Zone B consists of Mesozoic metamorphic rocks including

phyllites, quartzite, slates. These rocks are locally

intruded by granite. In Bangka Island and other islands

northeast of Sumatra, Triassic metamorphic rocks crop out

extensively and they are intruded by granite batholiths of

possible Jurassic age.

Zone C consists of Mesozoicmetasedimentaryrocks and


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limestones associated with mafic igneous rocks such as

diabase, serpentine, andesite and tuffs. The limestones

have been dated as Early Cretaceous or possibly Late

Jurassic age.

Zone D consists of micritic limestone which is

interpreted as possibly Cretaceous age.

Zone E consists of a band of irregular width of

granite,

syenite and diorite.

The main structural trends shown in the basement rocks

are NW-SE and NE-SW. According toAdiwidjajaand De Coster

(1973),the structural features of the pre-Tertiary roctes

probably formed during the folding of the Palaeozoic and

Mesozoic strata by the Mesozoic orogeny.

3.2.2 LAHAT FORMATION (LAF)

The name Lahat Series was proposed firstly by Musper

(1937) for a sequence of andesitic tuffs and andesitic

breccias which crop out upstream of Air

Kikim.

The type

locality is situated in the western part of the town of

Lahat,

about 150 kilometres southwest of Palembang City. At

this location, the Lahat Formation liesunconformably upon

the pre-Tertiary basement rocks which are indicated as

Cretaceous.

Sediments of the Lahat Formation show angular grains of

coarse sand to pebble size, mainly comprising volcanic

fragments and unstable minerals. In the central part of the


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basin, the Lahat Formation comprises grey-brown to dark grey

shales interbedded with light green-grey to light blue-grey

tuffaceous

shales, siltstones and some

tuffaceous

sandstones

and coals. Thin limestone and dolomite stringers and

glauconite are occasionally present (De Coster,1974).

Based on the lithology of this formation, it is thought

to represent a continental phase of deposition in fresh

water to brackishlimnicenvironments. This interpretation

has also been supported by the discovery of fish remains,

fresh water molluscs and pyrite from the Kepayang-1 well

(Pulunggono,

1983).

The thickness of the Lahat Formation is strongly

controlled by the palaeotopography and fault blocks. In the

south part of the basin, the thickness of the Lahat

Formation is typically more than 765 metres, whereas about

1070 metres was found in the central part of the basin

(Adiwijaya and De Coster,1973). At the type locality, the

formation reaches about 800 metres in thickness (Pulunggono,

1983).

The age of the Lahat Formation is interpreted to be

Eocene to Early Oligocene based on the spore-pollen analysis

and K/Ar radiometric dating methods (De Coster,

1974).

3.2.3 TALANG AKAR FORMATION TAF)

The Talang Akar Formation represents the second phase

of Tertiary deposition in the South Sumatra Basin and


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25

contains a continental fluviatile sequence composed of

thickly bedded, very coarse to coarse sandstones,

alternating with thin shales and some coals. The grit-sand

facies was firstly recognized by Martin

(19 52)

from the

borehole data of theLimau5A-3 well and was also named the

Talang Akar Stage.

The lower part of the sequence generally consists of

coarse to very coarse-grained sandstone alternating with

thin layers of brown to dark grey shale andcoal. Fossils

are not found in this lower sequence. The upper part is

dominated by alternations of sandstone and non-marine shale

with some coalseams. The shales are grey to dark grey in

colour and the sequence becomes more marine upwards as

indicated by the presence of glauconite and carbonate and

the absence of coal

layers.

Some fossils of molluscs, crustaceans, fish remains and

Foraminiferaare found in the upper part of the sequence;

unfortunately they are not diagnostic fossils in terms of

stratigraphic age.

Based on these features, the Talang Akar Stage was

further divided by Spruyt (1956) into two members; the

Gritsand Member (the lower part) and the Transition Member

(the upper part). Jackson (1960) reported that the

Gritsand Member varies considerably in thickness from zero

to at least 610 metres, whereas the Transition Member

ranges between 61 to 360 metres. Figure 3.3 shows the

distribution of the Talang Akar Formation in the South


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2fi

Palembang Sub-basin, in terms of thickness.

Lithologys

and fauna of the Talang Akar Formation

indicate afluvio-deltaicenvironments passing upwards into

paralic then into a marine environments (De Coster, 1974;

Pulunggono,1983).

On the basis of some palaeontological and palynological

studies,andalso bystratigraphicposition, the Talang Akar

Formation has been dated as Late Oligocene to Early Miocene

(De Coster,1974). Pulunggono (1983) reported that the age

of the Talang Akar Formation can be dated using the

PlanktonicForaminiferalZones of Blow (1969) as N3 to lower

N5 (Late Oligocene to lower part of EarlyMiocene).

3.2.4 BATURAJA FORMATION BRF)

The Baturaja Formation was formerly known as Baturaja

Stage. This term was introduced by VanBemmelen (1932) to

distinguish the carbonate facies of the lower part of Telisa

Layer as proposed by Tobler (1912). He recognized firstly

the Baturaja sequence at Air Ogan, near Baturaja town, about

180 kilometres south of Palembang City.

In most areas of the basin, the Baturaja Formation lies

conformably upon the Talang Akar Formation. In general, the

Baturaja Formation isa platform carbonate, including some

coral reefs which were developed onpalaeo-highsespecially

at the edge of the basin. Towards the basin margins, the

limestones grade into calcareous clays and fine to medium

sands.


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According to Simbolon

(1974),

in Air Ogan the Baturaja

Formation can be subdivided into two divisions; a lower

bedded and an upper massive unit separated by calcareous

shales. The bedded unit consists of lime

mudstones

and lime

wackestones intercalated withmarls,while the massive unit

consists of mudstones, wackestones/packstones and

boundstones with abundant largeForaminifera in the upper

part.

The Baturaja Formation occurs only on the broad shelf

and platform areas of the basin. In some areas, this

formation was not deposited. In structural high areas, the

Baturaja Formation was deposited directly upon the*

pre-Tertiary basement rocks.

The thickness of the Baturaja Formation is strongly

variable, depending on the palaeotopography, from about 60

to as

much

as 200 metres thick. In the

Limau Anticlinorium

area, the Baturaja Formation reaches 60 to 75 metres in

thickness, while well data from Benuang, Raja, Pagardewa and

Prabumenangshow the maximum thickness reached is about 200

metres (Pulunggono,1983).

Based on the presence of Spiroclypeus, especially

Spiroclypeus orbitoideus and Spiroclypeus tidoenganensis,

the lower part of the Baturaja Formation is dated as

Aquitanian (lower part of Early Miocene), while the upper

part is dated as Burdigalian (middle to upper part of the

Early Miocene) to Lower Langhian (lower part of Middle

Miocene) on the basis of the presence of Eulepidina and the


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28

absence of Spiroclypeus fauna (Adiwidjaya and De Coster,

1973).

Pulunggono (1983) inferred that on the basis of the

PlanktonicForaminiferal Zonation (Blow, 1969), the age of

the Baturaja Formation is probably N5-N8 (lower part of

Early Miocene-lower part of Middle

Miocene).

3.2.5 GUMAI FORMATION GUF)

The most widespread rock sequence occurring in the

Tertiary is the Gumai Formation which was deposited during

the maximum phase of the marine transgression. Formerly,

this formation was named by Tobler (1906) as Gumai Schiefer

for the shale sequence which crops out at Gumai Mountain,

near Lahat town. During the fifties, oil companies termed

this sequence the Upper Telisa, but then the name was

changed to Gumai Formation.

In general, the Gumai Formation is characterized by

fossiliferous, typically globigerinal marine shale,

including minor intercalations of limestones and sandstones

(De Coster,

1974).

At the type locality, it comprises

tuffaceous marl layers alternating with some marly limestone

layers (Pulunggono, 1983). In Limau area, a dark grey

shale,

bituminous and containing thin layers of marl and

marly sandstone from the Gumai Formation was penetrated by

some boreholes.

Faunas such Bolivina and Uvigerina are common in the

Gumai Formation. De Coster (1974) believed the Gumai


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29

Formation was deposited in warm neritic conditions which

were indicated by the presence of these faunas, combined

with the widespread occurrence of glauconitic

foraminiferal

limestone.

The thickness of the Gumai Formation varies greatly

with basin position. In the Palembang Sub-basin, the

thickness of the Gumai Formation varies from about

15 0 to

500 metres, but in theLematangDepression it reaches about

2500 metres (Pulunggono,

1983).

The age of the Gumai Formation can be dated by using

the Planktonic Foraminiferal Zonation from Blow (1969) as-N9

to N12 ( lower part of Middle Miocene to middle part of

Middle Miocene; Pulunggono,1983).

3.2.6 AIR BENAKAT FORMATION (ABF)

The Air Benakat Formation corresponds with the onset of

the regional regressive phase. In general, this formation

comprises-

shale with glauconitic sandstones and some

limestones deposited in a neritic to shallow marine

environment.

Formerly, the Air Benakat Formation was named by Tobler

(1906) as the Onder Palembang but this name was changed by

Spruyt (1956) to the Air Benakat Formation. The upper part

of this formation is dominated by tuffaceous sandstones

alternating with marl or glauconitic sandstones. Tuffaceous

claystones and sandstones are dominant in the middle part,


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3

n

while the lower part consists mostly of claystone.

According to Pulunggono (1983), the thickness of Air

Benakat Formation ranges from 100 to 1100 metres. In the

Limau

area, about 600 metres of Air Benakat Formation was

penetrated by Limau 5A-156 well (Pulunggono,1983).

The age of the Air Benakat Formation can be interpreted

using the Planktonic Foraminiferal Zonation from Blow as

Nll/12to N16 (middle part of Middle Miocene to lower part

of Late Miocene; Pulunggono,1983). In most reports, it has

been interpreted to be mostly Late Miocene in age (De

Coster,1974).

3.2.7 MUARA ENIM FORMATION MEF)

The Muara Enim Formation was first described as the

Midden Palembang Series by Tobler in 1906 at the type

locality, Kampung Minyak near Muara Enim town. At this type

locality, the formation comprises three lithological

sequences; coal units, claystone units and sandstone units.

This formation lies conformably upon the Air Benakat

Formation. Haan (1976) further divided the Muara Enim

Formation into two members; Member A and Member B. During

the ShellMijnbouw

Coal exploration program in 1978, the

stratigraphic column of the Muara Enim Formation was further

modified and the members have been divided into four

divisions;

- M4 comprises an upper coal division corresponding to


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31

the HangingCoals.

- M3 comprises the middle clay, sand and coal division.

- M2 comprises the middle coal division corresponding

to the Mangus/Pangadang coals.

- Ml comprises the lower clastic and coal division.

Table 3.3 shows the stratigraphic column of the Muara

Enim Formation. These divisions can be recognized

throughout most of the South Sumatra Basin, with apparent

wedging out of the upper and middle coal divisions on the

basin margins. ShellMijnbouw (197 8)reported that the coal

seams of the middle and lower divisions are more widespread

and thinner than the seams of the upper division due to a

shallow marine influence during sedimentation.

The lower boundary of the Muara Enim Formation was

first defined by

Tobler(1906)

at the base of the lowest coal

band in the South Palembang area (theKladicoal) but this

definition could not be applied to the North Palembang and

Jambi areas where the coals are less well developed.

Another criterion used by oil industry geologists to define

the boundary is the top of the continuous marine beds or the

base of the first non-marine beds; the base of the

non-marine beds can be recognized by the presence of

arenaceous units, displaying coal lenses and a lack of

glauconite.

The Mangus seams of the M2 division have good marker

features,

especially a clay marker horizon which can be

recognized over a wide area. This clay marker contains


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32

discoloured biotite which was deposited over a wide area

during a short interval of volcanic activity and it can be

used to correlate the coal seams over most of the South

Sumatra Basin.

Fossils are rare in the Muara Enim Formation.

Therefore, the determination of the Muara Enim Formation age

is mainly based on its regional stratigraphic position

rather than palaeontological data. Baumann

et al.

(197 3)

determined the age of the formation as Late Miocene to

Pliocene on the evidence of its regional stratigraphic

position and the palaeontological data, admittedly rather

poor, oflamellibranchsand arenaceous

Foraminifera.

On the

basis of Planktonic Foraminiferal Zonation from Blow,

Pulunggono (1983) determined the age of the formation as

N16-N17 (lower part of Late Miocene - upper part of Late

Miocene).

The thickness of this formation is about 45 0to 750

metres (De Coster,1974).

3.2.8 KASAI FORMATION KAF)

Conformably overlying the Muara Enim Formation is the

Kasai Formation. This formation is often marked by a

distinct pumice or lapilli horizon containing rounded pumice

fragments of about 1 cm diameter. Light coloured, poorly

bedded tuffaceous sands and gravels, often containing clear

grains of crystalline quartz, are interlayered with light


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33

olo-red

or bluish-green clays (Shell Mijnbouw,

1978).

Rare,thin coal seams are also present.

The Kasai Formation is interpreted to be

Plio-Pleistocene in age based on its association with the

orogeny and associated vulcanicity of that age.

3.3 DEPOSITIONAL HISTORY OF THE TERTIARY SEDIMENTS

In general, deposition of the Tertiary sediments in the

South Sumatra Basin occurred during a period of relative

tectonic quiescence which happened between the periods of

tectonic upheaval in the Late Cretaceous-Early Tertiary and

the Plio-Pleistocene (De Coster, 1974). De Coster (1974)

stated that the tectonic quiescence probably resulted from a

reduction in the rate of sea-floor spreading activity during

that time. Consequently, sedimentation of the Tertiary

sequences was mainly controlled by basin subsidence,

differential erosion of the source areas and eustatic

sea-level changes.

The initial deposition of Tertiary sediments in the

basin occurred in the Late Eocene and Early Oligocene in a

continental environment. These deposits are represented by

the Lahat Formation filling a terrain of substantial

topographic relief which developed as a result of the

orogenic activity during the

mid-Mesozoic, the faulting of

the Late Cretaceous and Early Tertiary and differential

erosion of the exposed pre-Tertiary basement rocks. The


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34

Lahat Formation formed as a set of alluvial fan, braided

stream, valley fill and piedmont deposits and is

characterized by a feldspathic basal unit. Probably, this

unit is an erosional product of nearby granitic hills.

The tuffs occurring in the Lahat Formation were derived

from the intermittentvulcanismand probably from erosion of

earlier-deposited tuffs. Indications of local swamp

conditions can be recognized from the presence of thin coal

layers. In the Late Eocene-Early Oligocene a fresh water to

brackish, lacustrine environment developed in parts of the

South Sumatra Basin and a shale sequence was deposited in

this environment. During this time, the lakes may have had

intermittent connections with the adjacent seas giving rise

to some limestone, dolomite and

glauconite-rich

beds.

According to De Coster

(1974),

probably in the Middle

Oligocene, sedimentation of the Lahat Formation was

interrupted by regional uplift which occurred in the late

Early and Middle Oligocene. This interruption is

represented by the unconformable contact between the Lahat

Formation and the Talang Akar Formation.

Deposition of the Talang Akar Formation began, in the

Late Oligocene in the form of alluvial fan and braided

stream environments filling topographic lows and

depressions. Therefore, the Talang Akar Formation locally

occurs overlying the pre-Tertiary rocks. This sedimentation

continued in Early Miocene in a fluviatile, deltaic and

marginal-shallowmarine environment. During this time, the

connection to open


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3

5

marine conditions became more significant and the sea

gradually encroached into the basin. Topographic relief

became less pronounced as sedimentation continued.

Subsequently, delta plain sediments developed over broad

areas consisting primarily of point bar and braided stream

deposits. These graded into delta front and marginal marine

sands which in turn graded into prodelta shales laid down in

the more distal parts of the basin. As the progradation

continued, delta plain facies such as channel,

crevasse-splay,flood-plain

or marsh deposits were formed.

The Talang Akar Formation has its type area in the/

South Sumatra Basin but the term is also used for similar

sequences in the Sunda Basin and Northwest Java Basin as far

east as Cirebon in Java. The Talang Akar sequence is also

recognized in the Bengkulu Trough, a fore-arc basin to the

southwest of the South Sumatra Basin.

As the sea level rose in the Early Miocene, the sea

started to encroach upon the basement highs and the sediment

input declined leading to deposition of the Baturaja

platform carbonates in reef, back-reef and intertidal

environments. In the early stages, the Baturaja Formation

was deposited onshelfalandplatformportions of the basin

as platform or bank limestone deposits. In the later

stages,

further buildups of detrital,

reefal

and bank


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36

limestones were formed on top of these banks in restricted

localities.

In the central part of the basin the Baturaja

Formation grades laterally into argillaceous limestones or

marl and vertically into shales of the Gumai Formation.

Deep marine conditions became more widespread in the

early part of the Middle Miocene as basin subsidence

exceeded sedimentation and the deposition of Gumai shale

continued. In some areas, the deposition of Gumai Formation

was directly after the Talang Akar Formation. During this

time,the basin experienced the maximum marine incursion and

the most widespread phase of deposition. According to De

Coster (1974),

the South Sumatra Basin was probably

connected with the Sunda Basin when sea covered most of the

remaining topographic highs in the basin.

In the Middle Miocene, the sea became shallower and

environments of deposition gradually changed from neritic to

continental. This event may be related to the regional

uplift

accompanied by vulcanism and by intrusion of diapiric

masses and batholiths (De Coster, 1974). The Air Benakat

and Muara Enim Formations were deposited during this time in

shallow-inner neritic to paludal-delta plain environments.

During the deposition of the Muara Enim Formation,

widespread areas of swampland and marsh were present

throughout the basin and extensive, thick coals were formed

at this time.

The last of the major tectonic events in the South


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Sumatra Basin was the Plio-Pleistocene orogeny. This

orogeny was probably the direct result of renewed collision

betwween the Indo-Australian Plate against the Sumatra

part of the Eurasian plate. Sedimentation occurred in the

basin during that time resulting in deposition of the Kasai

Formation. The Kasai Formation consists mostly of erosional

products derived from the uplifted Barisan and Tigapuluh

Mountains and from the uplifted folds being formed in the

basin during the orogeny.


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CHAPTER FOUR

ORGANIC MATTERIN TH TERTIARY SEQUENCES

4 1 INTRODUCTION

Cuttings samples from ten oil exploration wells drilled

in

the

South Palembang Sub-basin were studied with

an

emphasison the organic petrologyand maturation levelof

the organic material. Selection

of

well sections

to be

examined

was

determined

by

availability

of

sample material

and drilling data, as well as preferences given by

PERTAMINA.

The

samples were taken from

the

PERTAMINA core

shed

at Plaju,

Palembang,

and

were examined

for

maceral

contentat theUniversityofWollongong. Theresultsof the

analyses

are

expressed

on a 100%

maceral basis. Cuttings

samples were selectedby theauthorforstudy,on thebasis

of their content

of

coal

and

carbonaceous

or

dark shale

particles. All samples are from Tertiary sedimentary

sequences.

Because

of

poor initial sample collection methods

at

the well site, someof thecuttings samples fromtheolder

oil exploration wells (L5A-22,TMT-3,BL-2,BN-10), contain

vitrinite having oxidation rims

( frypanned

rims).

The well locations

are

given

in

Figure

1.4.

Some coal

samples from theMuara Enim Formation were also collected

from shallow boreholes located around the Bukit Asam coal

mine

as

shown

in

Figure

1.5.

Table

4.1

shows wells sampled


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39

and the total number of samples from each formation. Bar

diagrams and pie diagrams of organic matter type, abundance

and maceral composition are shown in Figures 4.1 to 4.6.

Short descriptions of lithologies and organic matter type,

abundance and maceral composition from each well, are

presented in Appendix 1.

4 2 TYPE

4 2 1 LAHAT FORMATION

The Lahat Formation is largely confined to the deeper

parts of oil well sections studied, such as in the BRG-3,

GM-14, BN-10, MBU-2, L5A-22 and PMN-2 wells. The Lahat

Formation consists mainly of sandstone, shale, siltstone and

thin coal, but in the MBU-2 well, it consists of volcanic

breccia.

Organic matter is predominantly terrestrial in origin.

DOM content in the samples ranges from 0.09%-16.99% (average

= 8.5%) by volume. DOM on mineral matter free basis

comprises 21% to 99% (average = 84%) vitrinite, trace to 9%

(average = 2%) inertinite and trace to 55% (average = 14%)

liptinite.

Several thin coal seams occur in the Lahat Formation.

The coal content of the samples from this formation ranges

from 2% to 34% (average = 18%) by volume. The coal

comprises (m.m.f. basis) 73%-99% (average = 86%) vitrinite,


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40

0.14%-7% (average = 4%) ine

coals source rocks and hydrocarbons in the south palembang sub-b

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