Sedimentology of the ngrayong formation and its sandstone reservoir characterization

SEDIMENTOLOGY OF THE NGRAYONG FORMATION AND ITS SANDSTONE RESERVOIR CHARACTERIZATION Along Braholo River, Ngampel Village, Blora District Central Java Province, Indonesia... Key words:

SEDIMENTOLOGY OF THE NGRAYONG FORMATION AND ITS SANDSTONE

RESERVOIR CHARACTERIZATION

Along Braholo River, Ngampel Village, Blora District

Central Java Province, Indonesia

I have the honor of giving my deep gratitude to

my parents for their kindness and love

The basic principle of spiritual life is that our

problems become the very place to discover

wisdom and love Jack Kornfield

ABSTRACT

The Ngrayong Formation has been studied for a long time However, depositional environment of Ngrayong Formation has been determined differently from various author Therefore, Ngrayong Formation was attracted to study deeper about its depositional environment The measure section was carried out along Braholo River from Ngampel to Sendangharjo village for vertical profile lithologic column According to stratigraphic sequence of Rembang zone, the Ngrayong Formation conformably overlies the Tawun Formation Meanwhile, the Bulu Formation unconformably overlies the Ngrayong Formation

Depositional environment was studied in term of sedimentology of Ngrayong Formation and its sandstone reservoir characterization Sedimentology emphasized on foraminifera, trace fossil, grain size analysis, petrography analysis (thin section), provenance Porosity and permeability of the sandstone of Ngrayong Formation were also measured to evaluate reservoir quality Porosity and permeability were measured by consolidation permeameter

The grain size analysis showed that the sandstone is fine to medium grain (2.03 - 3.14 phi), moderately well-sorted to well-sorted (with standard deviation range from 0.36

to 1.0 phi), subrounded to rounded, supermature sediments, whereas, the sandy claystone

is characterized as poorly sorted (with standard deviation range from 1.09 to 1.75 phi), and immature sediments The depositional environment of sandstones could be sand bar environment and lagoon environment for sandy claystones With QFL ternary plot, the provenance of sandstone of Ngrayong Formation came from continental blocks Moreover, the composition of sandstone and heavy mineral (apatite, rutile, kyanite, zircon, hematite, magmatite, and tourmaline) reflected the sediments of Ngrayong Formation came from plutonic igneous rock and metamorphic rock origin Porosities range from 31 percents to 44 percents Permeabilities range from 61 milidarcies to 372 milidarcies These porosities and permeabilities are very good to consider the sandstones

of Ngrayong Formation as reservoir rocks However, permeabilities were higher than

1500 milidarcies by grain-size analysis method Results of the study show that relative sea level changes have affected the processes and products of sedimentation in the Northeast Java Basin during the Middle Miocene

The Ngrayong Formation is composed of six cycles Each cycle consists of a series of sandy limestone - quartz sandstone - sandy claystone or shale layers Base on

foraminifera fossils (Amphistegina lessonii, Elphidium sp, Quinqueculina sp,

Asterorotalia gaimadii, Lepidocyclina sp, Cyclocypues sp) and trace fossils

(Thalassinoides), the depositional environment of Ngrayong Formation is in inner shelf to

middle shelf environment Moreover, the lithologies and sedimentary textures of the Ngrayong Formation points to deposit on a transitional environment as indicated by presence of quartz sandstone and limestone beds, and sandy claystone or shale respectively, particularly in sand bar environment and in lagoonal environment The sea level change has also been ascertained during the depositional processes of Ngrayong Formation There were upward deepening up to the middle sequence section and shallowing again upward to the top of the sequence The age of Ngrayong Formation is Middle Miocene (N7-N9) The conclusive proofs were shown in this thesis

Key words: Depositional environment, paleontology, petrography, grain size

analysis, heavy mineral, provenance, reservoir quality of sandstone, Ngrayong Formation

INTISARI

Formasi Ngrayong telah dipelajari sejak lama Meskipun demikian, lingkungan pengendapan Formasi Ngrayong selah di determinasi secara berbeda dari satu peneliti ke peneliti lainnya Oleh karenanya, Formasi Ngrayong menjadi menarik untuk dipelajari lebih dalam sentang lingkungan pengendapannya

Pengulunan stratigrafi telah dikerjakan sepanjang sungai Braholo, dari desa Ngampel hingga desa Sendangharjo untuk mendapatkan kalom litologi penampang vertical Berdasarkan sekuen stratigrafi zona Rembang, Formasi Ngrayong terletak selaras distas Formasi Tawun, sedangkan Formasi Bulu memumpang tidak selaras diatas Formasi Ngrayong

Lingkungan pengendapan Formasi Ngrayong serta karakterisasi reservoar batu pasiunnya dipelajari dari segi sedimentologi Sedimentologi menekankan pada foraminifera, fosil jejak, analisis ukuran butir, analisi pertrografi (sayatan tipis), serta provenan Porositas dan permeabilitas batupasir Formasi Ngrayong juga diukur untuk evaluasi kualitas reservoar Porositas dan permeabilitas diukur dengan alat “consolidation permeameter”

Analisis ukuran butir menunjukan karakterisasi batupasir yaitu halus sampai menengah (2.03 - 3.14 phi), sortasi cukup baik hingga baik (dengan simpangan baku berhiasan dari 0.36 sampai 1.0 phi), membulat tanggung hingga membulat, merupakan sedimen super-mature dan batulemgpung pasiran mencirikan sortasi buruk (dengan simpangan baku berhiasan dari 1.09 sampai 1.75 phi), merupakan sedimen immature Lingkungan pengedapan batupasir menandakan lingkungan sand-bar, dan batulempung pasiran mencirikan lingkungan lagoon Dengan cara ploting diagram QFL, provenan batupasiran Formasi Ngrayong berasal dari blok kontinental Selain itu, komposisi batupasir dan mineral berat (apatit, rutil, kyamit, zirkon, hematit, magnetit, dan turmalin) mencerminkan endapan Formasi Ngrayong berasal dari batuan beku plutonik dan batuan metamorf Porositas berhiasan dari 31 persen sampai 44 persen Pemeabilitas berhiasan dari 61 milidarey sampai 372 milidarey Porositas dan permeabilitas ini sangat baik dan memungkinkan batuopasir Formasi Ngrayong menjadi batuan reservoar Akan tetapi, permeabilitas lebih tinggi dari 1500 milidarey berdasarkan hasil analisis ukuran butir Hasil studi ini menunjukkan bahwa perubahan muka air laut relatif telah mempengaruhi proses dan produk sedimentasi di Cekungan Jawa Timurlaut selama kala Miosen Tengah

Pengendapan Formasi Ngrayong terdiri dari enam siklus Setiap siklus terdiri dari suatu deret lapisan-lapisan batugamping pasiran – batupasir kuarsa – batulempung

pasiran atau serpih Fosil Foraminifena ( Amphistegina lessonii, Elphidium sp,

Quinqueculina sp, Asterorotalia gaimadii, Lepidocyclina sp, Cyclocypues sp) dan fosil

jejak (Thalassinoides) menunjukkan bahwa lingkungan pengendapan tepian tengah

(middle shelf) hingga tepian dalam (inner shelf) Umur Formasi Ngrayong yaitu Miosen Tengah Litologi, fosil dan tekstur pengendapan mencirikan bahwa Formasi Ngrayong terbentuk pada lingkungan transisi Khususnya pada lapisan batupasir kuarsa dan batugamping diendapkan pada lingkungan sand-bar, serta batulempung pasiran dan serpih diendapkan pada lingkungan lagoon Akan tetapi, maka air laut mengalami perubahan selama proses pengendapan Formasi Ngrayong Kenam memingirkan telah terjadi pendalaman ke arah atas hingga penampang sekuen tengah dan pendangkalan kembali ke arah atas hingga puncak dari sekuen Kesimpulan itu ditunjukkan dalam tesis ini

Kata kunci: Lingkungan pengendapan, paleontologi, petrografi, analisis

ukuran butir, provenan, kualitas reservoar batupasir, Formasi Ngrayong

ACKNOWLEDGEMENT

I would like to thank JICA and AUN/SEED-Net Secretariat at Bangkok which provided me the scholarship; to Gadjah Mada University at Yogyakarta, especially the Geological Engineering Department, that accepted me to study Master degree; and to the Ho Chi Minh City University of Technology that supported my study in Indonesia

I want to express my gratefulness and esteem to Dr Subagyo Pramumijoyo (my advisor at the Gadjah Mada University, Indonesia) and Prof

Dr Koichiro Watanabe (Co-advisor at Kyushu University, Japan), who guided and criticized me during my thesis preparation and writing

I would like to show my gratitude to lecturers who taught me in two first semesters and lecturers who did not teach me but nevertheless gave me their helpful advice during the completion of my thesis

Lecturers (Wartono Rahardjo, Budianto Toha, MSc, Bambang Budiyono

in Paleontology Laboratory; Widiasmoro, MT, Dr I Wayan Warmada in Mineral Resources Laboratory; Agus Hendratno, ST, MT, in Optical Laboratory; Jarot Setyowiyoto MSc., in Sedimentography Laboratory), assistants, and students (Akmaluddin, ST, Moch Indra Novian, ST, Didit Hadi Barianto, ST, Gayatri Indah Meilani, Fathoni Mukti Raharjo in Paleontology Laboratory; Wanni, ST, Fenny Thamba, ST, Prakasa Ardiyanto in Mineral Resources Laboratory; Yuslan, Sarju Winardi, ST, in Sedimentography Laboratory), are thanked for all their help

in laboratory works

Mr Mamat (Directorate Geology and Mineral Resources, Bandung) and Dipl.-Ing Andri S Mubandi (Geological Engineering Department, Institute of Technology Bandung), who prepared thin section and gave their experiences enthusiastically in thin section analysis, are also thanked

I would like to express heartfelt thanks to Ir H Moch Yohannes PK, M.Sc, (Departmen Energi dan Sumber Daya Mineral, Pusat Pendidikan dan

Pelatihan Minyak dan Gas Bumi, Cepu)who accompanied me on field works and supported significant references

I have the honor of giving my deep gratitude to my parents for their kindness and love They always give me words of encouragement in whatever I

do

Finally, thanks to all my Vietnamese friends and colleagues who helped

me out during difficulty times in my study in Indonesia

I am glad to receive your warm suggestions to complete a good research or enhance my knowledge Thank you for all your kind attention

Ms Nguyen Thi Bich Ngoc

TABLE OF CONTENTS

Title page i

Approval sheet ii

Abstract iv

Acknowledgement vi

Table of contents viii

List of figures x

List of tables xiii

Chapter I INTRODUCTION 1

I.1 Background 1

I.1.1 The study location 3

I.1.2 Scope of study 3

I.1.3 The purposes of study 4

I.2 Research methodology 4

I.2.1 Paleontology analysis 4

I.2.1.1 Outcrops selection 6

I.2.1.2 Sampling methods 6

I.2.1.3 Sample preparation 6

I.2.1.4 Standard criteria 6

I.2.2 Granulometry analysis 7

I.2.3 Petrography 9

I.2.4 Consolidation permeameter 11

I.2.5 Heavy minerals determination 12

Chapter II GEOLOGY 14

II.1 Regional geology of NE Java Basin 14

II.2 Tertiary stratigraphy of Cepu-Blora area 18

II.3 The Ngrayong Formation along Braholo River 24

Chapter III PALEONTOLOGICAL AND SEDIMENTOLOGICAL DATA ANALYSIS 31

III.1 PALEONTOLOGY 31

III.1.1 Foraminifera 33

III.1.1.1 Factors of living foraminiferal distributions 33

IV.1.1.2 Sedimentary environment 36

III.1.1.2.1 Benthonic foraminifera 36

III.1.1.2.2 Planktonic foraminifera 39

III.1.1.2.3 Large foraminifera 39

III.1.2 Trace fossil 42

III.2 SEDIMENTOLOGY 46

III.2.1 Grain size analysis 46

III.2.1.1 Grain size data analysis 46

III.2.1.1.1 Histograms 47

III.2.1.1.2 Cumulative curves 47

III.2.1.1.3 Graphical treatment of grain size data 47

III.2.1.2 Interpretation 49

III.2.1.2.1 Grain size 49

III.2.1.2.2 Sorting 50

III.2.1.2.3 Roundness 53

III.2.1.1.4.4 Textural maturity 54

III.2.2 Provenance of sandstone in Ngrayong Formation 56

III.2.2.1 Description of mineralogical composition 59

III.2.2.2 Sandstone Provenance 64

III.2.2.2.1 Petrographic Analysis 64

III.2.2.2.2 Heavy Minerals analysis 67

III.2.2.3 Interpretation and discussion 72

III.3 RESERVOIR QUALITY 76

III.3.1 Influence of depositional environment on reservoir quality prediction 76 III.3.1.1 Influences of Composition 78

III.3.1.2 Influences of Texture 79

III.3.2 Porosity and permeability value 80

Chapter IV DISCUSSION, CONCLUSION AND RECOMMENDATION 84

IV.1 Discussion 84

IV.1.1 Paleontology 84

IV.1.2 Sedimentology 86

IV.1.3 Reservoir quality 88

IV.2 Conclusion 90

V.3 Recommendation 91

REFERENCE 92

Appendix A Histogram curves plotted from grain size analysis of Ngrayong Formation 99

Appendix B Cumulative curves with an arithmetic ordinate scale plotted from grain size analysis of Ngrayong Formation 106

Appendix C Results of heavy mineral analysis 113

Appendix D Results of paleontological analysis 120

Appendix E Definition of used terms in thesis research 125

LIST OF FIGURES

Chapter I Introduction

Figure I 1: Location map of study area 3 Figure I 2: Research scheme of thesis study 5 Figure I 3: Terminology and class intervals for grade scales 8 Figure I 4: Relation between framework composition of sandstones and tectonic setting 10 Figure I 5: A constant head permeameter is a device for measuring the

permeability of a coarse grained soil 11

Chapter II Geology

Figure II 1: Regional plate tectonic setting of SE Asia The area of East Java is highlighted by a red box 15 Figure II 2: Geological cross section of the East Java, showing the Ngrayong

Formation – Depositional Model 17 Figure II 3: The stratigraphic column of North East Java Basin 19

Figure II 4: Location map of study area showing sampling points within

Ngrayong Formation 20 Figure II 5: The sharp contact between Ngrayong Formation and Bulu Formation, picture to the south, Braholo River, Sendangharjo Village 24 Figure II 6: The sharp contact between Ngrayong Formation and Bulu Formation, picture to the west Braholo River, Sendangharjo Village 24 Figure II 7: The contact between Ngrayong Formation and Tawun Formation,

Braholo River, Ngampel Village 25 Figure II 8: The contact between Ngrayong Formation and Tawun Formation,

Braholo River, Ngampel Village 25 Figure II 9: Limestone intercalation with sandstone, bedding direction

N1000E/300S, first cycle, Braholo River, Ngampel Village 27 Figure II 10: Cross bedding was found in sandstone (on lap), bedding direction N1000E/320S First cycle, Braholo River, Ngampel Village 27 Figure II 11: Sandstone and thin limestone facies , direction N1000E/320S third cycle, Ngampel Village 28 Figure II 12: The nodule grain of limestone, fourth cycle, Ngample Village 28 Figure II 13: Ripple marks were found in claystone at Ngampel Quarry 29 Figure II 14: Burrow traces were found in sandy claystone at Ngampel Quarry 29 Figure II 15: Laminated claystone, sixth cycle, Braholo River, Sendangharjo

Village 30

Chapter III Paleontological and sedimentological data analysis

Figure III 1: The benthonic foraminifera (1-15) and planktonic foraminifera 19) were found in Ngrayong Formation 35

(16-Figure III 2: Classification of marine environments and habitat environment of benthonic foraminifera 37 Figure III 3: The microphotographies were taken from some limestone thin

sections that show large foraminifera in Ngrayong Formation, along Braholo River 41 Figure III 4: The photo of sandstone thin section show large foraminifera in

Ngrayong Formation, along Braholo River 41 Figure III 5: Schematic representation of the common marine ichnofacies with examples of characteristic trace fossils 43 Figure III 6: This photo shows the Thalassinoides ichnofacies at Ngampel village, third cycle 44 Figure III 7: Thalassinoides burrows showing a dominantly two-dimensional

network or maze (left) and a three-dimensional network (right) 44 Figure III 8: Vertical section show the distribution of foraminifera in Ngrayong Formation and depositional cycle 45 Figure III 9: Grain size and sorting of Ngrayong Formation sediments Data base

on grain size analysis and petrographic description 51 Figure III 10: The frequency curve with determining well-sorted sediment and poorly sorted sediment 52 Figure III 11: Outlines of six roundness classes of sand-size particles having high and low sphericity 53 Figure III 12: Different stages of maturity of detrital sediment in relation to the energy available during transport 55 Figure III 13: Expected relationships between environments and textural

maturity 56 Figure III 14: Photomicrograph of a dominant quartz mineral (Qz) in sandstone (sample Md-1), at Medang village 60 Figure III 15: Photomicrograph of feldspar mineral (F) (sample NG-2) Feldspar has long shape and twin cleavage in the center, at Ngampel village 61 Figure III 16: Photomicrograph of rock fragment (sample SE-3) Rock fragment have high relief, located in the center, at Sendangharjo village 61 Figure III 17: Photomicrograph of biotite mineral (B) (sample NG-2) Biotite has reddish brown color, one direction of cleavage, at Ngampel village 62 Figure III 18: Photomicrograph of calcite cement (sample Md-1) Calcite cement filled in the pore between quartz grains and has high order color, at Medang village 63 Figure III 19: Photomicrograph of limonite (sample SE-11) Limonite also filled

in pore between quartz grains and has brown or black color, at Ngampel

village 64 Figure III 20: Ternary Plot showing provenance of the sandstone sediments of the Ngrayong Formation at study location based on the scheme of Dickinson and Suczek, 1983 66

Figure III 21: Diagram illustrating analysis of provenance based on properties of

plagioclase determined with the standard petrographic microscope 67 Figure III 22: The pie chart show percent of heavy mineral of sample BR2-5A, at Ngampel village 70 Figure III 23: The pie chart show percent of heavy mineral of sample NG-1B, at Ngampel village 71

Figure III 24: The pie chart show percent of heavy mineral of sample BR1-1C, at Sendangharjo village 71 Figure III 25: Early Tertiary paleogeography showing potential source for quartz-rich sediments 73 Figure III 26: Ternary Plot showing provenance of the sandstone sediments and their model 74 Figure III 27: Map showing three main structural configurations from the north to south of East Java Basin 75 Figure III 28: Porosity-permeability (maximum) cross-plot illustrating the five lithofacies (1) Conglomerate, (2) medium-grained sandstone, (3)

conglomeratic sandstone, (4) grained laminated sandstone, (5)

fine-grained “shaly” sandstone 77 Figure III 29: Diagram shows filling of cement in the open space, which

decreases porosity of rock 78 Figure III 30: Diagram show poorly sorted sediments and fine-grained sediments usually have lower porosity than well-sorted sediments because the fine-

grained fragments tend to fill in the open space 79 Figure III 31: Distribution of porosity and permeability, and stratigraphic

sequence of Ngrayong Formation, at along Braholo River 83

LIST OF TABLES

Chapter I Introduction

Table I 1: Categories of detrital grains were determined by models for visual

estimation of percentage composition of sandstone 10

Chapter III Paleontological and sedimentological data analysis Table III 1: Distribution of the major foraminifera of Ngrayong Formation in the study area 32

Table III 2: Explanation of figure IV.5 44

Table III 3: Phi (Φ) value of percentiles from cumulative frequency curves 48

Table III 4: Statistical parameters based on graphical method 48

Table III 5: The qualitative guideline of stages of textural maturity 54

Table III 6: The major provenance terranes, their tectonic setting, and typical sand compositions 57

Table III 7: Detrital mineral suites characteristic of source rock types 58

Table III 8: The calculated categories of grains from sandstone thin section 64

Table III 9: The calculated percentages of Q, F, L parameters 66

Table III 10: The table shows the relationship between heavy mineral and sandstone provenance 68

Table III 11: Results of heavy mineral analysis from Ngrayong sandstone sample 69

Table III 12: Summary of laboratory test results of porosity and permeability 81

Table III 13: Semiquantitative scale of prosity 81

Table III 14: Permeability values are calculated by Krumbein and Monk equation 82 Table III 15: Semiquantitative scale of permeability 82

Chapter I Introduction

Chapter I INTRODUCTION

I.1 Background

The Northeast Java Basin has been discovered and developed into the petroleum area of Indonesia for a century (1893 - 1994) However, geologists are still searching for oil and gas in this area now (Koesoemo, 2004) The prospective reservoir in Cepu area is the sandstone of Middle Miocene Ngrayong Formation Almost all anticlinal - structural traps of Ngrayong Formation as a reservoir target has been intensively explored and developed such as Kawengan, Ledok, Nglobo, Semanggi and Wonocolo oil fields in 1990s

The sandstone of the Ngrayong Formation was considered as the reservoir

in Cepu-Tuban Blocks but there are many different conclusions about their depositional environment According to Ardhana (1993), this sandstone is clean, quartzose, fine to medium grain, well-sorted, cross-bedded It was deposited in the shelf to upper slope at northern part of Tuban Block and in lower slope to deep marine at southern part of Tuban Block Musliki (2000) stated that the Ngrayong Formation was deposited in the littoral, coastal, tidal or dune facies in the northwestern part of the Rembang zone It was deposited in the submarine channel, submarine canyon and submarine fan facies at the boundary between the Rembang and Randublatung zone Additionally, Koesoemo et al (1995) showed the existence of shallow marine environments on the Middle Miocene periods in Rembang zone, but according to Lunt (1991), the Ngrayong Formation was deposited in significantly deeper marine settings

Soeparyono and Lennox (1989) described processes to form structural trap For example, deformation in the early Middle Miocene caused reactivation

of the basement faults in the Nglobo-Semanggi area with wrenching and the initial development of flower structures He described the local deltaic facies within the Ngrayong member in the north of Cepu

Chapter I Introduction

Rahardjo (1998), described the Ngrayong sequence as a member of the

Tawun Formation, consisting of Orbitoid limestone and shale in the lower part,

and sandstone with intercalations of limestone and lignite in the upper part The age of this unit is Middle Miocene, zones N9-N12 of Blow (1969) zonal scheme The depositional environment of this unit is fluvial or estuarine in outcrops in the north (Jatirogo, Tawun) and becomes more marine to the south It is the main reservoir unit in the Cepu oil fields, but seems to shale out immediately South and East of these fields

According to previous researches, Ngrayong Formation was deposited in many environments from non-marine (fluvial) to deep marine environment However, the Ngrayong Formation crops out completely along Braholo River, about 10 Km North of Blora town This river cut across Pakel anticlinal axis of the Rembang zone The outcrop is at the southern flank of Pakel anticline, which

is accessible and shows good quality exposure At this section, the Ngrayong Formation is overlain unconformably by the Bulu Formation and overlies conformably on the Tawun Formation Moreover, in the Ngrayong Formation can

be found large foraminifera, trace fossils, ripple marks, amber, and gypsum The sandstones are often well-sorted, often medium grained with abundance of quartz grain, cross – bedding, trace fossils, and frequent iron staining Its appearance could be compared to the characteristics of shallow marine deposits

The complete section of the formation, its accessibility and the controversy

of its depositional environment become attractive point to study deeper of the sedimentary facies, depositional environment of Ngrayong Formation, and its sandstone provenance The reservoir characteristics of the Ngrayong sandstone (its porosity and permeability) could also be measured

Chapter I Introduction

I.1.1 The study location

The location of study is along the Braholo River between Sendangharjo Village to the south and Ngampel village to the north, Blora District, Central Java Province, between coordinate 60 53’ 51’’ - 60 54’ 30’’ South and 1110 25’ 47’’ -

1110 26’ 49’’ East Ngrayong Formation at Medang village was also observed and samples were collected (Figure I.1)

I.1.2 Scope of study

This study emphasizes on interpretation of depositional environment, sedimentary characteristics of the Ngrayong Formation, and the provenance and reservoir characteristics of sandstone of Ngrayong Formation along Braholo River, Blora District, Central Java Province The study was carried out base on the outcrop data and laboratory analysis

Figure I 1: Location map of study area

8 Km Study Location

Chapter I Introduction

I.1.3 The purposes of study

The study tries to analyze facies of Ngrayong Formation in term of the depositional environment and sedimentary characteristics, and to identify the provenance of sandstone of Ngrayong Formation in accordance with its quality as

hydrocarbon reservoir, especially Ngrayong sandstone along Braholo River between Sendangharjo Village to the south and Ngampel village to the north, Blora District, Central Java Province

I.2 Research methodology

The research is mainly based on the measured section and laboratory analyses which include paleontologic, petrographic, granulometric, and consolidation permeameter analyses of the samples taken from the measured section (Figure I.2)

The paleontologic analysis is mostly for determining depositional environment and age of representative samples The petrographic analysis is focused on determining sedimentary characteristic and provenance of representative samples The granulometry is also for determining depositional environment, sedimentary characteristic, and the consolidation permeameter applies for porosity and permeability description in order to determine its reservoir quality

I.2.1 Paleontologic analysis

Paleontologic analysis was utilized to determine the depositional environment and the age of Ngrayong Formation The study concerned only on

foraminiferal fossils such as benthonic foraminifera, planktonic foraminifera, and

large foraminifera Moreover, trace fossils were also discussed to get more information about depositional environment

Chapter I Introduction

Figure I 2: Research scheme of thesis study

Consolidation permeameter

Laboratory work

Sampling Measured section

Source of sediments & Sedimentary

characteristics Depositional environment

Age & Bathymetry Rock texture

composition

Thesis Result Reservoir characteristics (K & Φ), and provenance of Ngrayong Sandstone, the depositional environment and the age of

Ngrayong Formation

Chapter I Introduction

I.2.1.1 Outcrops selection

Outcrops of claystone, sandstone and soft limestone along the section were

collected due to its possibility rich in fossil content The present of fossils on

sediments can be examined under a loupe and calcareous fossils can be checked

by dropping HCl 0.1 on the sediments

I.2.1.2 Sampling methods

Firstly, collecting tools and bags must be clean to prevent contamination

Secondly, the sample must be taken from fresh rock To avoid contamination,

rock sampling should not be undertaken close to sedimentary contacts, borings,

and burrows below unconformities Bags should be labeled on side and details

entered in a field notebook including the exact positions of the samples

I.2.1.3 Sample preparation

The rock samples were disaggregated to separate the microfossils from the

rock matrix The fine grade rock particles, smaller than the microfossils, that

passed through sieve-mesh of 200 were discarded Hornibrook et al (1989), Jones

(1956), and Toha (1993) described the preparation of sample specifically on how

to separate microfossil from the rock samples About 200 gram of each sample

was disaggregated The fragmented samples were then boiled for about 15

minutes using a boiling pot on a gas burner Then, about two teaspoons of sodium

hexametaphosphate were added to the solution The solution was kept for one

night and sieved using sieve-mesh of 40 to sieve-mesh of 200 The washed sample

usually contains a considerable proportion of remaining rock particles from which

the microfossils have to be picked under a low-power microscope

I.2.1.4 Standard criteria

In this study, the following criteria were used for the quantitative counting

The present term is used if species’ number is fewer than 5 of the same species in

Chapter I Introduction

the same sample Similarly, common term is used for 6 to 20, and abundant term

for more than 20 of the same species from the same sample

I.2.2 Granulometry analysis

The granulometry analysis was applied mostly for sandstone to sandy

claystone In laboratory, different grain sizes were separated by sieve method The

purpose of grain size analysis is to determine grain size, sorting, and textural

maturity One hundred grams of dry sand were sieved for 15 minutes using a

Retsch sieving machine Sieves used followed the U.S standard sieve mesh

system i.e 10, 18, 35, 60, 120, 230, 270 and remainder in pan (Figure I.3) The

sand that remains in each sieve was weighed and the resulting values were used

for the construction of cumulative curves and histograms of individual weight

percent

Statistical analysis were then carried out and the resulting data such as

grain size, sorting, roundness and textural maturity were used in determining the

depositional environment of study area

The method used to evaluate grain size of sediments by using sieve

technique can be found in many books or journals such as Haggar, et al (2002);

Anonym C (2004); Friedman and Sanders (1978); Lewis (1984); Anonym E

(2004); Boggs (1987) and Sperazza et al , (2002) This study applied the

following method : Calculate the following descriptive parameters using the

cumulative percent curve for each sample respectively In the formulas below, the

subscript in the phi terms (φx) refers to the grain size at which x % of the sample is

coarser than that size, or also, the size at which x % of the sample is retained on

that particular sieve size and any coarser screened sieves above that particular

sieve Geometric graphic mean and inclusive graphic standard deviation are

parameters to analyze the grain size distribution

Chapter I Introduction

Figure I 3: Terminology and class intervals for grade scales (Pettijohn and

Siever , 1987).

Chapter I Introduction

a Geometric graphic Mean (M 2 ):

Grain size is a sedimentary parameter that has been studied to identify the

behavior of rock The geometric graphic mean is the average grain size that can be

determined for most samples by graphing grain size distribution as a cumulative

curve The geometric graphic mean can be calculated using the formula below:

b Inclusive graphic standard deviation (σ I ):

Inclusive graphic standard deviation is a mathematical measure of the

distribution about a mean Inclusive graphic standard deviations are a reflection of

sorting For example, very well sorted sediments have a standard deviation of less

than 0.35 Phi These standard deviations were calculated from the following

formul

I.2.3 Petrographic analysis

Petrographic analysis involved examination of rock samples thin section

under polarized microscope In this study, the analysis was focused on

determining the composition of the rock, its porosity and percentage of minerals

(mostly quartz, feldspar, and lithics) in order to determine its provenance

Thin section analysis is very useful in determining the porosity of

sedimentary rocks All thin sections must be impregnated with blue epoxy first in

order to expose the intragranular voids and then point counting was utilized to

measure the porosity

Dickinson and Suczek (1970) demonstrated that standard QFL (quartz,

feldspar, lithics) diagram for plotting framework modes of sandstones could be

used for the provenance interpretation of detrital material in relation to its tectonic

6 6 4

5 95 16

φ φ φ

M

Chapter I Introduction

setting (Figure I.4) Folk (1968) showed genetic classification of quartz mineral

that can be used to determine its provenance

Figure I 4: Relation between framework composition of sandstones and tectonic

setting (After Dickinson et al, 1983 Pettijohn and Siever, 1987)

The grain components are described according to categories of grains

(Table I.1) Presence of other components, such as biotite and cement were noted,

however they were not counted

Table I 1: Categories of detrital grains were determined by models for visual

estimation of percentage composition of sandstone

Categories of detrital grains Modal constituents

Q = total quartz = Qm + Qp Qm = monocrystalline quartz,

Qp = polycrystalline quartz = microcrystalline +

cryptocrystalline

F = total feldspar = P + K P = plagioclase,

K = K-feldspar = orthoclase + perthite

L = unstable lithic fragments

= Ls + Lv + Lm

Ls = sedimentary = claystone + mudstone +

siltstone,

Lv = volcanic = mafic (basaltic) + intermediate

(andesitic) + silicic (rhyolitic),

Lm = metamorphic = quartz-mica tectonite +

slate-phyllite

Accessory constituents Mica = muscovite + biotite

Other: calcite cement, limonite

Chapter I Introduction

Consolidation permeameter is utilized for porosity and permeability

measurement The permeameter is a Perspex cylinder which has about 100 mm in

diameter and 300 mm long and fitted with a brass base and flanged brass head that

are clamped together by vertical tie rods (Figure I.5) A balancing cylinder is

connected to the base plate and another connection from the head is taken to a

measuring cylinder Two connections from the cylinder are connected to

manometer tubes

Figure I 5: A constant head permeameter is a device for measuring the

permeability of a coarse grained soil (Sutton, 1975)

The soil or rock sample is placed in the cylinder between two filters and

the test is performed by allowing water to flow through the soil or rock sample at

a head controlled and kept constant by the overflow in the balancing cylinder

Chapter I Introduction

When steady conditions are obtained, the time taken to collect same time the

difference in the level of the water in the two manometer tubes is recorded This

represents the loss of head as the water flows through a length of soil/rock equal

to the distance between the connections The internal diameter of the cylinder is

also measured

From Darcy’s law, if k = coefficient of permeability

seepage velocity v = k.i = k.h/l

i = the hydraulic gradient, mm/s

h = the loss of head

l = the length of the soil sample Porosity and permeability are the most important attributes of reservoir

rock because these determine the amount of fluid and rate of flow in a rock

Samples were cut into core sample with diameter 1 cm and 3 cm in length

Darcy’s law was applied to determine the porosity and permeability of the

sandstone samples from the Ngrayong Formation, which were collected for

petrographic analysis Porosity and permeability were also calculated from

petrographic and grain size analyses This study provides a summary assessment

of the relative porosity of the rock types

The depositional environment is essential factor in understanding porosity

and permeability characteristics Each environment has particular characteristics

as texture and mineral composition that control the porosity and permeability of

rocks There is a major connection between high porosity and quartzose sand

grains

After sediment sampling from the different areas along Braholo River at

Ngampel and Sendangharjo village, the sand samples were brought back and sent

to the sedimentography lab to be examined for heavy minerals content The sand

sample was sieved and the 0.125- to 0.5- mm sand fraction was chosen to extract

Chapter I Introduction

heavy mineral Each sample was cleaned, and placed in a dense liquid of 2.89

(bromoform solution) Funnel of above mixture were covered by filter-paper and

kept into an Erlenmeyer box and then it started to precipitate The separation of

the heavy minerals from the rest of the sand grains was observed and the heavy

minerals were later examined under binocular microscope in order to determine its

type and percentage The standard book of minerals description was used to

compare the minerals under the microscope with the features of the minerals in

the standard book (Mange and Heinz, 1992; staff of sedimentology assistance,

2001) The process continued until hundreds of grains were counted, and each

mineral had been recognized Each mineral type was then counted, and its percent

composition was determined The experiment was then repeated

Chapter II Geology

Chapter II GEOLOGY

Regional geology setting of the Ngrayong Formation, especially in the study are, will be expressed in this chapter and exposed firstly on sub chapter of regional geology of North-East Java Basin, followed by sub chapter of geology of Blora-Cepu area, and then the Ngrayong Formation along Braholo River

II.1 Regional geology of NE Java Basin

The Northeast Java Basin is Tertiary foreland basin in the western portion

of Indonesia This basin is bounded by the Java volcanic belt or geanticline in the south, the Karimunjawa and Bawean arc in the north, the Pati and Cirebon trough

in the west and the Madura ridge and Madura Basin in the east (Figure II.1)

Approximately three quarter of the basin is onshore and the rest lies

offshore below the shallow Java Sea (Ardhana et al, 1993; Koesoemo et al, 1995)

(Figure II.2)

Based on the tectonics, stratigraphy and paleogeography, the Northeast Java Basin can be divided into three tectonic physiographic zones, from south to the north: the Kendeng zone, the Randublatung zone and the Rembang zone (Koesoemo, 2004; 1997)

Kendeng Zone

It corresponds to the Kendeng hills (van Bemmelen, 1949) It is a hilly zone; build from the products of very intensive tectonism that resulted to uplift as the Kendeng anticlinorium (Koesoemo et al, 1995) Tectonic intensity in Kendeng zone decreased from west to east In contrast, the hydrocarbon prospects increased from west to east There are many oil seepages in the middle of the zone although

Chapter II Geology

no oil fields have been discovered there There is no indication of hydrocarbon in the west There are three developed oil fields near Surabaya in the eastern part of Kendeng zone (Musliki, 1991) The general anticline trend is east-west, with overthrust faults of the same strike orientation There are few indications of normal faulting The thrust and overthrust faults usually do not penetrate into the basement This is result in a detached structural style (Musliki, 1999)

Figure II 1: Regional plate tectonic setting of SE Asia The area of East Java is

highlighted by a red box (Smyth et al., 2003)

Dominant lithologies of this zone are volcaniclastic rocks and marls with few intercalations of shale, sands and carbonates The rocks are interpreted as having been deposited in a deep-sea basin environment Source capacity (fine sediment) is bigger than the reservoirs (coarse sediments) The rate of deposition

is faster and possibly greater than the rate of subsidence

Chapter II Geology

Randublatung zone

This zone corresponds to Blora and Cepu hills, Ngimbang and Dander hills (van Bemmelen, 1949) or the transition zone The zone is gently deformed

and is structurally lower than the Kendeng and Rembang zones

The general trends of anticline in Randublatung zone are E– W, SEE and NW-SE direction The reverse faults have the same direction of the above trends, while the normal faults usually have the NE-SW directions only The zone is dominated by normal faults, which usually extend into basement Thus, a basement was created that involved structural style (Koesoemo et al, 1995)

NWW-Dominant lithologies of the Randublatung zone are marls and clays with intercalations of sands, calcarenites and carbonates These rocks act as an important reservoir and could have been deposited on an undulating continental slope

It seems that the zone has the same capacity between the source and reservoir rocks Rate of deposition is moderate and is less or equal to the rate of subsidence More than 20 oil fields have been developed within the zone since

1893

Rembang Zone

The zone extends across the northern margin of Java and is separated from the Randublatung zone by Lusi trough in the west, Kening trough in the middle and Solo trough in the east It is hilly zone Its tectonic intensity is higher than that

of Randublatung zone but lower than that of the Kendeng zone

Chapter II Geology

Figure II 2: Geological cross section of the East Java, showing the Ngrayong Formation – Depositional Model (Ardhana, 1993)

KELADI-1 KEPODANG-1

MERPATI-1

JS 16-1

JS 3A-1 LODAN PRANTAKAN BANYUBANG-1

CEPU NGLOBO SEMANGGI&LEDOK GRIDIS BARAT-1

GONDANG-1 NGASIN-1 GOGOR-21 KE-11E KE-11C KE-11G BD-1 JOB PERTAMINA - TREND TUBAN

BA AN CH

KA RI

M UN

JA WA

A RC H

JAVA EAST

Cross section line

Chapter II Geology

General trend of anticlines in the Rembang zone varies from E-W to NNW- SEE direction The reverse faults have the same direction as above, whereas, the normal faults usually have the NE-SW direction The zone is dominated by normal faults These faults usually extend into basement suggesting that the zone has also a basement involved structural style as in the Randublatung zone

Dominant lithologies of the area are sandsstone and carbonates with intercalation of marls and clays These sediments were interpreted as having been deposited on a continental shelf

The zone has reservoir capacity bigger than the source rocks The rate of deposition is slow and slower than the rate of subsidence About five oil fields have been developed in the zone during the same decade as the other zones

II.2 Tertiary stratigraphy of Cepu-Blora area

The nomenclature of formations varies in Cepu area This research follows the stratigraphic description of Musliki (1991) and Soeparyono and Lennox (1989) The oil and gas bearing formations are in the upper part of the stratigraphic sequence ranging from Miocene to Pleistocene (Musliki, 1991, 1997, 1999) Koesoemo et al (1995) defined that the oil and gas area produced from the upper part of the Tawun Formation, the Ngrayong Formation, the lower part of Wonocolo Formation and the Selorejo Formation in Cepu area However, according to Musliki (1991), oil and gas were produced from the Ngrayong Sand Formation, calcarenites from the Ledok Formation, Wonocolo Formation, and Tawun Formation The main reservoir is the Ngrayong Formation in the Cepu area

The rocks in the Cepu-Blora area were deposited in the transgression – regression cycle The stratigraphic column of this area is shown in Figure II.3 Deposition of Ngrayong Formation occurred during early to mid Middle Miocene and was shown on geological map at study location (Figure II.4) The names and descriptions of the lithologies in this area are as follow:

Chapter II Geology

Figure II 3: The stratigraphic column of North East Java Basin (Musliki,,

1997)

EL A NTICLINE

Platungan Tmt

Tmpm

 ) ) Sendangharjo

Sandstone, shale, claystone and siltstone with interbeds of limestone, coal and lignite Claystone and limestone, with sandstone, siltstone and calcarrenite interbeds

Strike and dip of bedding Location of sample Observation location

Strike slipe fault

Normal fault D

Tmb

EXPLANATION

Tmn Tmt

Claystone with interbeds of limestone,and glauconitic sandstonein the lower part.

Massive marl, whitish grey, rich in planktonic foraminifera

Referrence: Geological map of the Rembang Quadrangle, Jawa By Darwin Kadar and Sudijono, 1993.

Geological research and development centre.

Scale 1:100.000

Wonocolo Formation Ledok Formation Mundu Formation Paciran Formation

45 45

57 52

6 54

10

1 3 7 9

18 17

13 15 16

12 5 4

6 11

Surabaya Semarang Bandung Jakarta

J

A VA

Chapter II Geology

Ngimbang Formation

The Ngimbang Formation consists of basal coarse sandstone, shale and minor coal seams which changes up section into predominantly micro-grained limestone with minor marls and sandstone According to Musliki (1991), this unit

is an effective source rocks This formation represents deposition in a fluviatile to shallow marine environment during transgression upon a pre-Tertiary basement

Kujung Formation

In general, the Kujung Formation consists of claystones, shales or marls intercalated with sandy calcarenites These sediments of the Kujung Formation were deposited in a deep-water environment During the formation of Kujung, a deeper basin was probably located in the eastern part of the Northeast Java Basin This unit is also an effective source rocks (Musliki, 1991)

Tuban Formation

The Tuban Formation consists of fine clastic sediments Its base has never been reached in the East Java – Madura region Therefore, its thickness is not known The Tuban Formation can be considered as source rocks

Tawun Formation

Tawun Formation consists of grey carbonaceous shales and calcarenites The calcarenite is rich in orbitoidal foraminifera suggestive of a shallow water environment (Soeparyono and Lennox, 1989) It is formerly called “Lower Orbitoiden-Kalk”, and included in the Rembang Beds by van Bemmelen (1949)

In Ngampel village, the exposed section is about 30 m thick The strata contain

planktonic foraminifera of zone N8, Early Miocene age (Globigerinoides diminutus, Preorbulina transtoria, and Globigerinoides sicanus) The benthonic

foraminiferal associations suggest that the strata were deposited in a shallow, open

a facies change within the lower, regressive part of the Ngrayong Formation; unit III (mudstone) represents the upper, trangressive part of the cycle and overlies the other two units (Figure II.2) In the JOB Pertamina – Trend Tuban classification, the Ngrayong sediments include the interbedded quartzose sandstones, mudstones, thin limestones, and the overlying sandy, bioclastic limestones (the Platen or Bulu limestones) This formation has been found only in the Cepu area, where it accounts for the main producing zone of East Java Its thickness, which is over

300 m in the North, decreases southward (Brouwer, 1957)

Bulu Formation

This unit was formerly called “Platen-Complex” by Trooster (1937) It consists of platy, sandy limestone with intercalations of sandy marl Extremely

large foraminifera (Cycloclypeus annulatus) are abundant locally Harsono

(1983) formally describes this unit as a formation by establishing a type locality near Bulu Village, Rembang District This formation is widely distributed especially in the northern Rembang anticlinorium The unit thickens to the west, and at the Larangan River it reaches a thickness of about 360 m To the east, at the Besek River it is about 80 m thick Its lithologies and fauna indicate a shallow-open marine depositional environment

Chapter II Geology

Wonocolo Formation

In general, Wonocolo Formation is composed of monotonous blue-green shales, usually unstratified It is rich in foraminifera, sometimes small molluscas and solitary corals (Brouwer, 1957) Lithologies and fauna indicate deposition in deeper marine, outer sublittoral environment The marls are cap rock of Ngrayong Formation and Wonocolo Formation (Musliki, 1991)

Ledok Formation

Brouwer (1957) described this unit as consisting of glauconitic sandstone, platy limy sandstone and sandy limestone, sometimes cross-bedded This unit is rich in microfossils, but macrofossils are rather scarce On the geological map of Rembang quadrangle, Ledok Formation consists of grey claystone, marl and bedded calcarenite, occasionally containing glauconitic sandstone interbeds

Mundu Formation

Mundu Formation is generally composed of monotonous Globigerina

marls This unit is poor in macrofossils, but extremely rich in microfossils,

accounting for local developments into Globigerina oozes (Brouwer, 1957) On

the geological map of Rembang quadrangle, Mundu Formation consists of massive limestone characterized by karren surface due to weathering

Selorejo Formation

Selorejo Formation consists of almost 100% foraminiferal sand, with a few quartz grains and some glauconitic pellets It contains practically no terrestrial material (Brouwer, 1957)

Lidah Formation

Lidah Formation consists of dark grey claystone which occasionally contains molluscan bearing horizons and coal interbeds

Chapter II Geology

II.3 The Ngrayong Formation along Braholo River

Along Braholo River, at Ngampel Village, the Ngrayong Formation is overlain by Bulu Formation that consists of platy, sandy limestone with intercalations of sandy marls (Figure II.5 & II.6) and underlain by Tawun Formation that consists of mainly greenish grey claystone with intercalations of limestone (Figure II.7 & II.8)

Figure II 5: The sharp contact between Ngrayong Formation and Bulu

Formation, picture to the south, Braholo River, Sendangharjo Village

Figure II 6: The sharp contact between Ngrayong Formation and Bulu

Formation, picture to the west Braholo River, Sendangharjo Village

Bulu Formation

Ngrayong Formation

Ngrayong Formation

Chapter II Geology

Figure II 7: The contact between Ngrayong Formation and Tawun Formation,

Braholo River, Ngampel Village

Figure II 8: The contact between Ngrayong Formation and Tawun Formation,

Braholo River, Ngampel Village

Ngrayong

Formation

Tawun Formation

Tawun Formation

Ngrayong Formation

The contact

contact

Chapter II Geology

Ngrayong Formation was discussed in term of depositional cycle Depositional cycles are of particular interest because the documentation of these cycles in the rock record provides an approach for regional correlation, changes in sea level and environment Individual cycles are separated by the stage changes in dropping water in the sedimentary rock record as recognized from repetition of lithofacies or, exposure surface The Ngrayong Formation exhibits six depositional cycles Each cycle is discussed in detail below

The lowest part of the Ngrayong Formation overlies unconformably the limestone of Tawun Formation This formation started by a first cycle of sandstone - limestone that can be considered as a depositional progradation cycle The first cycle consists of bedded sandy limestone and sandstone with bedding direction of N1000E/300S (Figure II.9), and thickness of about 36 m This unit is covered by sandy claystone, sandstone and thin limestone facies (cycle 2) Cross bedding can be observed clearly in sandstone (Figure II.10), thickness is 45 m, with bedding direction of N1000E/320S The upper part of this cycle is covered by third cycle that consists of nearly similar sequence

The third cycle has three types of sedimentary facies (Figure II.11) Sandy limestone, sandstone and sandy claystone covered the second cycle, with bedding direction of N1000E/320S The estimated total thickness of the third cycle is 30 m The upper part of the third cycle is covered by the fourth cycle

Chapter II Geology

Figure II 9: Limestone intercalation with sandstone, bedding direction

N100 0 E/30 0 S, first cycle, Braholo River, Ngampel Village

Figure II 10: Cross bedding was found in sandstone (on lap), bedding direction

N100 0 E/32 0 S First cycle, Braholo River, Ngampel Village