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STATUS OF THESIS
ASSESSMENT OF A SYNRIFT PLAY IN SOUTH DENT GRABEN
(OFFSHORE EAST SABAH)
I, HARMINZAR BINTI MANSOR hereby allow my thesis to be placed at the
Information Resource Center (IRC) of Universiti Teknologi Petronas (UTP) with the
following conditions:
I. The thesis becomes the property of UTP
2. The IRC ofUTP may make copies of the thesis for academic purposes only
3. The thesis is classified as
CJ 0
Confidential
Non confidential
Reason:
As instructed by Petroleum Management Unit (PMU), Petronas.
Endorsed by
PRAM,PMU PRAM,PMU
Level 22, Tower 2, Level 22, Tower 2,
Petronas Twin Towers, Petronas Twin Towers,
KLCC KLCC
Date: 22/1/08
11
UNIVERSITI TEKNOLOGI PETRONAS
Approval by Supervisor
The undersigned certify that they have read, and recommend to The Postgraduate Studies
Programme for acceptance, a thesis entitled "Assessment of a synrift play in South
Dent Graben (offshore East Sabah)" submitted by Harminzar Mansor for the
fulfillment of the requirements for the degree of Master of Science in Petroleum
Signature:
Main supervisor: Dr Arthur Van Vliet
Date: zz/•jo~
Geoscience.
Ill
UNIVERSITI TEKNOLOGI PETRONAS
ASSESSMENT OF A SYNRIFT PLAY IN SOUTH DENT GRABEN (OFFSHORE
EASTSABAH)
By
HARMINZAR MANSOR
A THESIS
SUBMITTED TO THE POSTGRADUATE STUDIES PROGRAMME
AS A REQUIREMENT FOR THE
DEGREE OF MASTER OF SCIENCE IN PETROLEUM GEOSCIENCE
BANDAR SERI ISKANDAR,
PERAK
JANUARY, 2008
IV
DECLARATION
I hereby declare that the thesis is based on my original work except for quotations and
citations which have been duly acknowledged. I also declare that it has not been
previously or concurrently submitted for any other degree at UTP or other institutions.
Signature :~ Name : Harrninzar Mansor
Date
v
ASSESSMENT OF A SYNRIFT PLAY IN SOUTH DENT GRABEN (OFFSHORE
EASTSABAH)
ABSTRACT
South Dent Graben is located offshore of East Sa bah at the south part of the Sandakan
Sub basin. This basin is a very lightly explored area. There is only one exploration well
drilled here. The stratigraphy of this basin mainly divided into two parts: synrift
sediments and post rift sediments. They are separated by the breakup unconformity that
represents the Middle Miocene Unconformity (MMU). Post rift sediment consists of the
Sebahat Formation, the Ganduman Formation and the Togopi Formation. The
interpretations and petroleum play analysis of the post rift sediments have been done in
2005. Basically this project is a continuation from the 2005 project but focused more on
the synrift sediments. Synrift sediments are represented by Segama Group (Libung and
Tungku Formation), deposited during Early- Middle Miocene. Five horizons have been
interpreted: Top Basement, Base Purple, Base Brown, Base Green and Middle Miocene
Unconformity. From the seismic interpretation and mapping that has been done, no
economic size structural trap or stratigraphic play appears in the synrift sediments. Synrift
sediments only present in a small part of the basin and later overlain by extensive postrift
sediments through out the whole basin. Even though the north part of the basin has
hydrocarbon occurrences in the synrift sediments (Tanjong Formation/Segama Group),
the reservoir appears more equivalent to the Tanjong Formation. High proportion of
volcaniclastic sediments and conglomerates is the reason that the Segama Group is not a
good reservoir. Besides that, a calculation from the Vrms also shows very high interval
velocity ranging from 3000ms'1 to more than 4000ms'1, suggesting that the sediment is
compacted, has a low porosity and may not be a good reservoir.
VI
TABLE OF CONTENTS
STATUS OF THESIS
APPROVAL PAGE
TITLE PAGE
DECLARATION
ABSTRACT
TABLE OF CONTENTS
LIST OF TABLE
LIST OF FIGURES
CHAPTER ONE: INTRODUCTION
I. I OBJECTIVE
1.2 LOCATION AREA
1.3 PROJECT DATABASE
l.3.I SEISMIC
1.3.2 WELL
I.4 PROBLEMS ENCOUNTERED
CHAPTER TWO: LITERATURE REVIEW
2.I TECTONIC EVOLUTION OF SABAH
2.2 STRATIGRAPHY OF SABAH
2.3 VOLCANIC ACTIVITY IN SABAH
2.4 SANDAKAN SUB BASIN
2.4.I DENT PENINSULA STRATIGRAPHY (ONSHORE)
2.4.2 STRATIGRAPHY
2.4.3 SOURCE ROCK POTENTIAL
2.4.4 RESERVOIR
2.4.5 MATURITY
CHAPTER THREE: METHODOLOGY
3.I DATA COLLECTING
3.2 SEISMIC INTERPRETATION
3.3 MAPPING
VII
II
Ill
IV
v
VI
VII
IX
X
2
2
3
4
5
9
IO
IO
I2
I3
I4
I5
I5
I6
I6
22
3.4 VELOCITY CALCULATION
CHAPTER FOUR: RESULT
4.1 SEISMIC INTERPRETATION
4.2 MAPPING
4.3 VELOCITY CALCULATION
CHAPTER FIVE: CONCLUSION AND RECOMMENDATION
REFERENCES
APPENDIX A: Velocity calculation results
VIII
22
25
42
48
51
LIST OF TABLE
Table
4.1 Color code that have been used to interpret horizon in this project 25
IX
Figure
1.1
1.2
1.3
2.1
2.2
LIST OF FIGURES
Location area of this project
Basemap of the seismic lines in the SB306 block
Summary of Sebahat-1 well
NW -SE cross section across Sa bah during Cretaceous-Eocene
NW-SE cross section across Sabah during Oligocene-Early Miocene
2.3 NW -SE cross section across Sa bah during Early Miocene
Middle Miocene
2.4
2.5
2.6
2.7
NW -SE cross section across Sa bah during Late Miocene-Pliocene
The structural elements in Sandakan Sub Basin
Lithostratigraphy map of South Dent
Lithostratigraphy ofSandakan Sub Basin
3.1 SB30604-04 seismic line show the erosional truncation as
the indicator to Base Green
3.2
3.3
3.4
Other erosional truncation as indicators of Base Green
Onlap end to basement
On lap seismic reflections as indicator of Base Purple
3.5 Graph of relative error versus interval thickness in ms (using the
Dix formula)
4.1 Interpreted SB30604-04 seismic line
4.2 Interpreted SB30604-06 seismic line
4.3 Interpreted SB30604-08 seismic line
4.4 Interpreted SB30604-1 0 seismic line
4.5 Interpreted SB30604-12 seismic line
4.6 Interpreted SB30604-14 seismic line
4.7 Interpreted SB30604-05 seismic line
4.8 Interpreted SB30604- I I seismic line
4.9 Interpreted SB30604-19 seismic line
4.10 Interpreted SB30604-23 seismic line
4.11 Interpreted SB30604-27 seismic line
4.12 Interpreted SB30604-31 seismic line
4.13 Interpreted SB30604-39 seismic line
X
2
3
4
6
7
8
9
II
12
14
18
19
20
21
23
26
27
28
29
30
31
32
33
34
35
36
37
38
4.14 Interpreted SB30604-43 seismic line 39
4.15 Interpreted SB30604-45 seismic line 40
4.16 Interpreted SB30604-53 seismic line 41
4.17 Middle Miocene Unconformity (MMU) time structure map 43
4.18 Base Green time structure map 44
4.19 Base Brown time structure map 45
4.20 Base Purple time structure map 46
4.21 Basement time structure map 47
4.22 Summary of the velocity results on SB30604-04 49
4.23 Summary of the velocity results on SB30604-06 50
XI
CHAPTER I
INTRODUCTION
1.1 OBJECTIVE
The main objective of this project is to assess synrift sediments and prospectivity in
the South Dent Graben, Sabah, Malaysia.
1.2 LOCATION AREA
The south Dent Graben is located offshore of the East part of Sabah. It is under
SB306 acreage. This area is virtually unexplored. Total area of this block is
8184km2 The block is currently open block area. The basin in this area is known as
Sandakan Sub Basin. This project only focuses on the south part of the Sandakan Sub
Basin.
Sulu Se.
S'OO' 58332
Figure 1.1 Location area of this project
1.3 PROJECT DATABASE
1.3.1 SEISMIC
Two seismic surveys have been done in SB306:
1. ESB99 Series - This survey has been shot by Petronas in 1999. It comprises
1500 line km
2. SB30604 Series- Shot by Petronas in 2004 and comprises 1471 line km.
For this project only SB30604 have been used in the interpretation to avoid misties
between the two vintages.
2
3
Figure 1.2 Basemap of the seismic lines in the SB306 block
1.3.2 WELL
There is only one exploration well in the SB306 acreage which is Sebahat-1. The well
was drilled in 1973 by Sabah Teiseki Oil. Total depth of this well is 3700m TVDSS.
Sebahat-1 is a dry well and only has minor gas shows from the cuttings.
4
Fo rm~tlon lithology Foss il Shows Seismic Ties
predoo'nlrl!y sen:tstooe no tom no sl1cHv Togopl
SB8.6Ma
Gandurnan rMrlf ITIUd!tone wth
seemsOIII{Tde calareous ,....,.,
no shaN
S811.6Ma meil'tf M$1one, coral frll9flCR$ go$ $hoi¥
Sebahat -ones end some (tronspaled?)
lmeUones
TO 3700m I Adapted from PETRONAS (2006) I
Figure 1.3 Summary of Sebahat-1 well
1.4 PROBLEMS ENCOUNTERED
There are some problems encountered during this project like:
1. Lack of data such as check shot or VSP data. The only information is velocity
data from the seismic (stacking velocities).
2. The interpretation doesn't have calibration from the Sebahat-1 well. This
because the well has been drilled to 3700m and only penetrated the post rift
sediments.
CHAPTER II
LITERATURE REVIEW
2.1 TECTONIC EVOLUTION OF SABAH
There are several stages of tectonic evolution of Sa bah based on Tongkul, 1991 a and
PETRONAS, 1999:
I. Pre Cretaceous
5
This refers to Sabah Crystalline Basement as 'older metamorphosed oceamc
basement with rare acid intrusive rocks'.
2. Early Cretaceous- Early Eocene
There were several unstable deep troughs in a block faulted domain that emerged
from the older basement in Early Cretaceous time. Shallow marine limestones were
deposited on the highs and the oceanic basins were filled with ophiolite complexes
and covered with pelagic sediment. The ophiolite complex is known as 'new oceanic
basement' and forms the basement to the thick sedimentary systems of Sabah
(Hutchinson, 1989). In the centre of deep marine throughs are thick successions of
predominantly argillaceous sediments, coeval with Chert Spilite Formation. These are
6
represented by the Sapulut, Trusmadi and East Crocker Formations. The ophiolite
complex and argillaceous sediments later have been deformed and uplifted in the
Eocene tectonic episode.
The Eocene deformation has been described as related to the southeastward
subduction of the proto South China Sea oceanic lithosphere under the NW margin of
Borneo. This resulted from NW -SE extension in the southern continental margin of
China and was enhanced by anticlockwise rotation of the continental basement of
Borneo, the West Borneo basement. The former subduction trench zone, probably
trending NE-SW, would have been located in central Sabah, west of Labuk Valley.
Closing of the deep through of the 'hypothetical' proto South China Sea basin could
also have been the result of opposing movements of two continental masses from the
southern continental margin of China and the Borneo micro continental plate. Xia and
Zhou (1993) proposed that the Nansha Block: a micro continental block of
transitional crust in the Dangerous Grounds-Reed Bank area collided with palaeo
Borneo in the Late Eocene, before the initial opening of South China Sea. This Late
Eocene collision folded the Palaeogene and older units in Sabah.
'R-.f4DI Soa' SapuluL, Eut. Crocker,
Tru•madi
Oceanic cruat
Adapted from Tongkul (1991 a). PETRONAS (1999)
Volc&Dic arc, ophiolite. Cbtrt. - Splllte
Figure 2.1 NW -SE cross section across Sabah during Cretaceous-Eocene
3. Oligocene- Early Miocene
Subsequent to the Late Eocene deformation, uplift and erosion of the Rajang Fold
Thrust Belt fed new depocentres to the west and east (Palaeogene Basins). The NW
Sabah continental plate then moved southward and linked to the opening of South
China Sea resulted in the initial uplift of the Temburong and west Crocker
Formations. To the south, shallow marine sediment (Meligan) was deposited. While
7
to the north, deep marine clastic deposition continued (Stage III and Kudat Fm). The
widespread carbonates were formed on uplifts of the Crocker Fold Thrust Belt. In the
eastern Sabah, two rift zones began to form which are NE trending Sandakan rift and
SE trending Tarakan rift, accompanied by chaotic deposits (olistostromes) and
explosive volcanism.
NW
Carbonnte layer
~· w .. t Crocker (Olcltr Croclc.er) Fold-TbrUel Belt
and imbrication or ophiolite! complex
I Adapted from Tongkul (1991a). PETRONAS (1999)
Figure 2.2 NW -SE cross section across Sabah during Oligocene-Early Miocene
4. Early Miocene-Middle Miocene
SE
Eastern Sabah rift zones widened and were infilled by synrift deposits, the shallow
marine Tanjong and K.apilit Formations. Intense compressive tectonism occurred with
the collision from southward directed movement of the NW Sabah continental plate
against the northwest margin of Borneo. According to Tongkul (1997), the
deformation was due to collision of micro continents against older fold belts in the
NW Borneo region. To the north, the Palawan and Reed Bank terrane collided with
the fold belt of NW Borneo as a result of N-S opening of South China Sea. To the
south, the Celebes Sea oceanic lithosphere was subducted northwestward as a result
of NW collision of the Banggai- Sula platform with older fold belt of eastern
Sulawesi. The collision formed bending of the Crocker Fold Thrust Belt. Uplift and
erosion of Crocker Fold Thrust Belt fed new depocentres to the north and northwest.
Thrust sheets were formed during this time. The eastern and central Sabah ophiolitic
complex was popped up and in eastern Sabah, volcanic activity continued.
The regional tectonic deformations in Sabah and neighboring areas,
compressional in west, northwest Sabah and Palawan areas and extensional in eastern
8
Sabah and Southeast Sulu Sea area have caused a major period of uplift and erosion,
which produced the Deep Regional Unconfonnity.
NW }Wang BDd Crocker Fold-Tlmaet Bolte,
'pop-up' alivere orub lb
CollJelon zone
Spreading ui1
r-M_a_p-ted_f_ro_m_T-on-gk-ul-(1_$_1-a)-,P-Cffi--0-~---(1-$-~~~ t
SE
Figure 2.3 NW-SE cross section across Sabah during Early Miocene-Middle Miocene
5. Late Miocene- Early Pliocene
During the Late Miocene in western offshore Sabah, there is defonnation with large
scale N-S strike slip movements. The Inboard Belt was uplifted and eroded to feed
new depocentres: East Baram delta and Outboard belt. The major erosional surface is
the Shallow Regional unconfonnity. In eastern Sabah, the Miocene synrift deposits
are not only uplifted and eroded but fonned into circular and sub circular shapes,
caused by both shale diapiric movement and wrench faulting. The erosion of the
synrift deposits fed new depocentres: north, east and southeast. The eastern Sandakan
sub basin has considerable sediment thicknesses. A new phase of volcanic activity
(Tawau-Semporna-Sulu volcanics) and igneous intrusions (Kinabalu) occurred from
Late Miocene to Quaternary. According to Tongkul (1997), the late Miocene
defonnation was probably related to northwestward shortening and uplift of the
Sabah fold belt as a result of the collision of the Banda Arc with northern Australia
9
and NW movement of the Phillipine Mobile Belt against the southeastern margin of
Asia.
NW SE Semporoa - Tawau - 811lu,.. N~ volta1Uee 1 ., Noo,ene i •edJment. /.;-•
Adapted from Tongkul (1991 a) . PETRONAS (1999)
Figure 2.4 NW -SE cross section across Sa bah during Late Miocene-Pliocene
6. Pliocene- Recent
Compressional events affected most of Sabah. The Pliocene compressive event was
probably a continuation of the Late Miocene event. The volcanic arc in eastern Sa bah
appeared to have migrated from the Dent Peninsula southward to the Tawau area and
Semporna Peninsula. Subduction of the Celebes Sea towards the north may have
produced the extensive volcanic activity.
In eastern Sabah, the Late Miocene and the younger Dent Group sediments were
deformed into a series of en echelon folds with ENE trending axis. These folds are
high relief on Sabah side and low relief on Philippine side. Recent offshore well
results indicate that the Late Miocene sections towards the northwest have almost
been eroded. In southern eastern Sabah, the Miocene sediments were also affected by
the Pliocene compression with uplift occurring in response to continued northeast
southwest compression.
2.2 STRATIGRAPHY OF SABAH
In Sabah, five distinct tectonostratigraphic provinces can be recognized (Lim, 1985):
l. An ophiolite complex, which is considered to form the basement to the
sedimentary succession of Sabah
10
2. The Rajang-Crocker accretionary prism, a belt consisting of deformed deep marine,
Eocene Oligocene sediments
3. Broken formations and melanges which show characteristics of tectonic,
sedimentary and diapiric origin and are thought to have formed in a series of related
events in Early Middle Miocene times
4. Neogene sedimentary rocks, which are mostly shallow marine to fluvio-deltaic
facies deformed into sub circular to elliptical shaped, fault bounded areas which are
known as the 'circular basins' of Sabah
5. The Sempoma-Sulu Arc, a region with andesitic to dacitic volcanic activity of
Miocene to Quaternary age in the Dent and Sempoma Peninsula.
2.3 VOLCANIC ACTIVITY IN SABAH
Regional geology of the accretionary wedge in Sabah indicates southeastward
subduction of Proto South China Sea and continental lithosphere during Middle
Eocene and Early Miocene. A volcanic arc would be expected in south Sabah. There
may be older volcanic rocks beneath young volcanic rocks of the Dent and Sempoma
Peninsulas. Younger volcanic rocks, 12 Ma in Sempoma Peninsula, are possibly
related to northward subduction of the Celebes Sea. There is a young phase of
basaltic activity in south Sabah (Pliocene and younger) which is different in character
to older Miocene-Pliocene subduction related volcanic activity (Balaguru, 2006).
2.4 SANDAKAN SUB-BASIN
Sandakan sub-basin is located along the southwest margin of the Sulu Sea. The sub
basin is bounded by Keenapusan Ridge and Cagayan Ridge to the northwest and by
the Sulu Ridge towards the east. The Offshore Sandakan sub-basin in Malaysia
acreage is divided into two blocks. The north part is the SB305 block and the south
part is the SB306 block.
11
There are some parts of the basin lying onshore Sabah in the Dent Peninsula known as
SB331 block. Below is the geology map ofthe Dent Peninsula (Figure 2.5).
a·N
114'(
NW SABAH /" PLATFORM/
, / !C' - ~ o.Ji
/' '/' ~--,/',;"
I
, /
;
/
[ZJ Ophool•t e Rock• ~ Pe•aeogen.-..eogene Se<llmerts - P!e .r.,uary 0 Neogel'e Sediments
(./ Fold-Tilruat Ball ,_A Normal Faullt /'/ Wronch Faultt D Neogana lgnaoua
,. ROCka
Adapted from Petronas (1999)
Figure 2.5 The structural elements in Sandakan Sub Basin
12
730000
680000
LITHOSTRATIGRAPHY OF SOUTH DENT
600000
118'30'
MUD VOLCANO DEPOSITS
TOGOPI FORMATION
[_ GANDUMAN FORMATION
• SEBAHAT FORMATION
• TUNGKUFORMATION
SILASUKAN VOLCANIC BRECCIA
• LIBONG TUFFITE FORMATION
• AVER FORMATION
LASANG FORMATION
CHERT SPILITE FORMATION
Adapted from Petronas (2006)
Figure 2.6 Lithostratigraphy map of South Dent
2.4.1 DENT PENINSULA STRATIGRAPHY (ONSHORE)
The oldest rocks that crop out in the southern dent Peninsula are the outer shelf to deep
water sediments of the Lower Miocene Labang Formation (Haile & Wong, 1965). Middle
Miocene Ayer and Libong Formations consist of sub littoral bathyal tuffs, slump
breccias, boulder beds, cherts and pebbly mudstones and highly deformed lie
unconformably over the Labang Formation. Libong Formation then unconformably is
overlain by Tungku formation. There is some igneous rock found in this area belonging
to the Baganak Member of the Tungku Formation outcrop on the southern plains of the
Dent Peninsula (Balaguru, 2006). Late Miocene to Recent sublittoral - neritic clastic
marine sediments of the Sebahat and Ganduman Formations are found in the eastern part
of Dent Peninsula and unconformably over lie Tungku formation (Haile & Wong., 1965).
The contact is poorly exposed. Pleistocene terrace sand and gravel and Quaternary river
and coastal alluvium are the youngest rocks mapped (Lim, 1985).
13
2.4.2 STRATIGRAPHY
The Sandakan sub-basin consists of post rift sediments and synrift sediments (Figure 2.7).
The post rift sediments are represented by the Sebahat Formation, the Ganduman
Formation and the Togopi Formation while synrift sediments are represented by the
Segama Group. The postrift sediments show strong progradation from west to east. The
age of the postrift sediment is Middle Miocene up to Pleistocene.
Major unconformities are present within the basin. Biostratigraphic age
determinations are often unreliable because of reworking and the lack of age diagnostic
biota (Murphy, 2000).
The Segama Group is represented by the Tungku Formation and the Libung
Formation. The Tungku Formation comprises sandstones, conglomerates, claystones,
volcanic breccias and tuffs. The age of Segama Group is Early Miocene to Middle
Miocene. Fluvial sandstones with good porosity have been observed at outcrop in Dent
Peninsula. On the Philippines side, well 409-1 indicates porosity values above 15% at
depths down to 4000m. However, within the offshore Sabah area the provenance may
have included significant amounts of volcanic material and burial related diagenesis is
expected to have reduced the reservoir potential more extensively (Murphy, 2000).
The Sebahat Formation and the Ganduman Formation are making up the Dent
Group. Murphy (2000) put sands and claystones/shales deposited in variety of marine
environments (shallow (paralic) to bathyal) under Sebahat Formation. Paleobathymetry
increased from northwest to southeast. Onshore the Dent Peninsula, the Sebahat
Formation contains 40%-60% sandstones, while offshore exploration wells indicate a
sand content varying between 25% and virtually zero.
The Ganduman Formation which overlies the Sebahat Formation consists of
claystones and sands. Depositional environments varied from shallow water to deep
marine. Likewise environments are becoming deeper in a southeasterly direction.
The Togopi Formation overlies unconformably the Dent Group and comprises
reefal limestones, marine shales and sands deposited in shallow marine environments.
There are numerous gas seeps reported in the onshore areas of the Dent Peninsula.
They are associated with mud volcanoes and salt springs (Haile & Wong, 1965).
14
ONIHOP.E
EPOCH ITHOITRUGIA PHV FORMATION
D EPOSITIOHAL EtMROPIMENT TECTONIC
w ~lBIOCEIE ~ SHPLLOYV M~RIIIE FINPL EnEt-SIOil
z Cl
PliOOEIE ~ TP.ANSPRESSION l:
.. ~.8Jt~ ~~ PliFT
::: 0 ("') m z m
c 'tJ 'tJ
~
::: 0 0 ,.. m
r 0 ~ m ;c
OLOOOEIE
)>
11'1 :I> z Q :I> ;,~;
:I> :z
;,~;
:I> 'tl
E -1
BA~GGAI SU--4 SLOCI' COWS/i!li
REGIO~AL FOLDING
~fJ~oc.<.s Ca.J..JS.ON
POST-RIFT NVERSION l: UPLIFT
G~O'v\TH FAULTING "'-'-"V""""'-'IiN'I'IAAIVV'VV•JV\.I'vl I' IIU PAI.A'Ntti SLOCY 1!.
SHALLOW MARINE
UAR NE
CAGAY/J/1 APC COWS/ON
SYI\"-RIFT ~, BLOCK 3 GROWTH '\~ FAULTING
IUBIIOE~CE
DEEP MI.RINE EACI\lRC :xraUOI ......,...~ ~"' ~ CFSUlUI "A
REGIO'lAL FOLDING ~" FAJL TING S UPLIFT
& ER0$1011 BATHY.\L
DEEP MARINE TURBIDITES
DEEP MI.RINE JV..A ~ ... RQ.~~es ~
FOREARC SETTING
EOCEilE IIIDOIN. I'OliUO
~ FAUlliiQL UPliFT ""'- LE~CI
DEEP MARINE TURB
Adapted from Balaguru (2006)
Figure 2. 7 Lithostratigraphy of Sandakan Sub Basin
2.4.3 SOURCE ROCK POTENTIAL
Poor to marginal hydrocarbon source rock potential are reported from the Tanjong
Formation /Segama Group and the Dent Group, namely Ganduman and Sebahat
Formation (Bates, 1995). Total organic carbon contents of the Tanjong Formation
equivalent are generally poor to good: 0.10-2.23 wt% and the Hydrocarbon Index
(HI) is between 86-243 (Petronas, 1999).
The Dent Group contains only poor to fair organic contents: 0.11-1.38 wt% TOC.
The generally low HI values ( 45-177) indicate these formations would generate
mainly gas.
15
Outcrop samples from the Dent Group (Ganduman and Sebahat Formation) and
the Segama Group/Tanjong equivalent all exhibited poor to negligible hydrocarbon
source rock potentials with Types IIVIY kerogen dominating.
2.4.4 RESERVOIR
Proven oil and gas reservoirs are in the Sebahat Formation and the Segama Group/
Tanjong Formation sandstones. Sebahat Formation has up to 25% porosity and a
permeability of I Darcy. However, from the drilling experience, Sebahat Formation is
absent in the north part of Sandakan Sub basin.
The underlying Tanjong Formation or equivalents to thin sand within
volcaniclastic Segama Group, have porosities of around 20-25% and permeabilities of
I 0-300md. However, the sand occurring in the basin is more equivalent to Tanjong
Formation (Petronas, 1999).
The best potential reservoirs are the Upper Miocene sandstones of the basal
Ganduman Formation (Murphy, 2000).
2.4.5 MATURITY
A geothermal gradient of 21 °C/km was observed in Sebahat-1 well and is thought to
be low a result of blanketing by the thick Sebahat Formation shale sequence. This
effectively moves the top of the hydrocarbon generation window in the southern Dent
Peninsula area to a relatively deeper level (Petronas, 1999).
16
CHAPTER III
METHODOLOGY
3.1 DATA COLLECTING
Data gathering started from the first week of August. The data included the reports from
Petronas UDRS, well data, paper review etc. All seismic lines were already loaded in the
workstation since this project is a continuation of the project that was done in 2005 by
PMU.
3.2 SEISMIC INTERPRETATION
The interpretations are divided into two stages. At the first, the hard copy interpretation
has been done to get a general idea about the area and then the full interpretation
continued on the workstation using Landmark software, Seisworks 20.
For seismic interpretation, only SB30604 vintage has been used to avoid
interpretation misties between the ESB99 vintage and SB30604 vintage. The
interpretation is based on the seismic facies because the Sebahat-1 well only penetrated
the post rift sediment, whereas the synrift doesn't have any control from the well.
Besides seismic facies, seismic reflection terminations also give good indication
to interpret the horizons. There are some seismic lines that show very clear erosional
truncation and on lap. Figure 3.1 and Figure 3.2 are some examples of erosional truncation
to interpret Base Green. Onlap seismic reflection terminations also make a boundary
between some seismic sequences. Figure 3.3 and 3.4 are some examples of onlap
17
terminations.
The horizons that have been in interpreted are Top Basement, Base Purple, Base
Brown, Base Green and Middle Miocene unconformity (MMU).
Figure3.1 SB30604-04 seismic line show the erosional truncation as the indicator to Base Green -00
19
20
21
(I)
tU (I)
c: 0 .... ti
11.)
c;:: 11.) M u
· ~
"' .... ~
"' ~
'"a 0 ~ M
Q.) M
So .... 1-Lf
22
3.3 MAPPING
All the horizons in Seisworks 20 have been transferred to ZMAP Plus to continue with
mapping process. Details of the mapping projection are listed below:
Projection: Universal Transverse Mercator
Spheroid: Everest 1830
Hemisphere: Northern
UTM Zone Code: 117
Five horizons have been mapped. There are Top Basement, Base Purple, Base Brown,
Base Green and Middle Miocene Unconformity (MMU).
3.4 VELOCITY CALCULATION
Based on the horizons interpreted, a calculation of the velocity from Vrms data of the
seismic has been done. This velocity calculation is important in this study because there is
no other calibration that been done and seismic velocities are the only data that we have.
The formula that been used to calculate interval velocity is Dix formula.
Where Vint: interval velocity
t1: traveltime of the first reflector
t2: traveltime of the second reflector
Vrms 1: root mean square velocity of the first reflector
Vrms2: root mean square velocity of the second reflector
The Dix formula should only be applied in thick intervals (more than 150ms). If the
interval is thinner than 150ms, the results error is higher. Figure 3.5 shows the relative
error of the Dix formula vs interval thickness in two way time.
AV.,fVj (%}
soi
I 401
I 30-l
20 -l
IOJ
500
. -·- ·------
DixFormu/a
V? =T2*V$122 -T1"V$1,2 I T2. T1 I
/
~
400 300 200 100
Interval Thickness In ms 7WT
Figure 3.5 Graph of relative error versus interval thickness in ms (using the Dix formula)
J J... 50
I J... 40
I ~30
~20
Lo
25
N w
24
Two seismic lines have been select in order to calculate the velocity. There are
SB30604-04 and SB30604-06. For seismic line SB30604-04, seven selected shot point
been choose to do the velocity calculation. The shot points are 120 I, 1361, 1561, 1841,
2041,2161 and 2241.
For seismic line SB30604-06, eight shot points have been selected. There are
1681, 1921,2081,2281,2441,2561,2761 and 2961.
Results of the velocity calculation are attached in the appendix.
25
CHAPTER IV
RESULT
4.1 SEISMIC INTERPRETATION
From the seismic facies and seismic stratigraphy, five horizons are interpreted. These
are Top Basement, Base Purple, Base Brown, Base Green and Middle Miocene
Unconformity (MMU).
HORIZON NAME STRATIGRAPHY
Blue Break up unconformity represent MMU
Blue to Green Tungku fm
Green to Brown Upper Libung fm
Brown to Purple Middle Libung fm
Purple to Dark Green Lower Libung fm
Dark Green Basement
Table 4. I Color code that have been used to mterpret honzon m thts proJect
26
Cl.)
~ ..... ........
27
(.)
28
0 Q ·....... u
29
30
UJ
.9 ..........
31
d.)
~ . ...... ........ u . ...... e IV) . ...... d.) IV)
32
d.)
~ ..... ....... u ...... 8 1;1) ...... d.) 1;1)
Figure 4 .8 Interpreted SB30604-11 se:smic line w w
34
35
36
37
38
cu ~ ..... .......
39
Figure 4 .15 Interpreted SB30604-45 seismic line ~ 0
.,J:>.. -Figure 4 . 16 Interpreted SB30604-53 seismic line
42
Based from the seismic interpretation, the synrift sediments fill up the small part of
South Dent Graben. The synrift sediments deposit from I OOOms up to 3500ms. The
synrift sediments (Base Green, Base Brown and Base Purple) were eroded by Middle
Miocene Unconformity (MMU). And from the interpreted seismic lines, the synrift
sediments were thinning to the east. Besides that, the synrift sediments also end at
seismic line SB30604-12 and towards the south it is already basement high.
4.2 MAPPING
All five interpreted horizons have been mapped using the ZMap Plus. Figure 4.17
shows Middle Miocene Unconformity time structure map. Basically, this map shows
Middle Miocene Unconformity spread through out the whole basin. Towards the
south the MMU stopped to the fault or basement high. The MMU is deepening to the
east part of the basin.
Figure 4.18, 4.19 and 4.20 represent the time structure map of Base Green,
Base Brown and Base Purple. From this entire map, we can conclude that the synrift
sediments are deposited in the syncline which represents the depocenter of the basin.
No structural play is evident from these three maps. For Base Purple, the anticline
arising in the west is only a ZMap extrapolated result because there is no seismic line
in that part.
Figure 4.21 shows the basement time structure map in the South Dent Graben.
The main faults in this area are trending from west to east and the minor faults are
trending from northwest to southeast.
0 1:500000
10 20 KILOMETERS
Figure 4.17 Middle Miocene Unconformity (M:MU) time structure map ~ w
0 lt500000
10 20 ~~~I L:OME T'ERS
Figure 4 .18 Base Gree:1 t:me structure map ~ ~
45
(()
Ct:: w t-UJ z 0 ~
a _J Q)
~ ~
...... tJ ::3
~ i:l "" w
('S) .§ ......
N ~ ~ 0
~ Q)
gj ~
(S) 0\ -
IS) -.;:t
Q) ~
CSJ ::3 bO ·-
CSJ CS) 1..1-4
cs:> ~ Lf) 9'9'
.__.
46
(f)
0::: w 1-w > 0 ~ _J Q.)
~ B tJ
~ £ <;/)
Q.)
cs:> .§ .....
N Q.) -~ ~
Q.) CA ro ~ 0
(S) N
v (SJ Q.)
~
<SJ & ~
(S) (S)
CSJ~
Ln <04-
,.......
SABP,H
Figure 4.21 Basement time structure map
20 KILOMETERS
.;:.. -....]
48
4.3 VELOCITY CALCULATION
Velocity calculations have been done on two of the selected seismic lines: SB30604-
04 and SB30604-06.
Figure 4.22 show the velocities on seismic lines SB30604-04. Velocity from
Middle Miocene Unconformity (MMU) to Base Green horizon is 2685ms·1-3810ms·1•
For the Base Green horizon to Base Brown horizon, the velocity is 3400ms"1to
3990ms·1• Base Brown horizon to Top Basement shows really high velocity. The
velocities are more than 4000ms·1• Different sediment thickness lying on the top of
the synrift sediments maybe the reason of the differences velocity in one horizon for
example Middle Miocene Unconformity (MMU) to Base Green horizon that have
range of velocity from 2685 ms·1-3810ms·1•
Seismic line SB30604-06 also gives almost the same velocity result as
SB30604-06. Middle Miocene Unconformity (MMU) to Base Green horizon velocity
is 2849 ms·1-3404 ms·1• The Base Green horizon to Base Brown horizon is 3201 ms·1
-
4116 ms·1• And for Base Brown horizon to Top Basement, the velocity is more than
4000 ms· 1• The velocity result for S30604-06 is summarized in figure 4.23.
Both of the seismic lines show that the synrift sediments in South Dent area
have very high velocities and it suggests that the sediments are highly compacted and
have low porosity. In terms of reservoir, the synrift sediments are not likely to be a
good reservoir.
Figure 4.22 Summary of the velocity resdts on SB30604-04 ~ \0
50
51
CHAPTERV
CONCLUSION AND RECOMMENDATION
A serious factor negatively affecting the prospectivity of the South Dent Graben is its
very small size. Effectively this size is further reduced for exploration because more
than half of the basin rises to the onshore outcrop area.
From the seismic interpretation and mapping that has been done, no economic
s1ze structural trap or stratigraphic play appears in the synrift sediments. Synrift
sediments only present in a small part of the basin and later overlain by extensive
postrift sediments through out the whole basin. Even though the north part of the
basin has hydrocarbon occurrences m the synrift sediments {Tanjong
Formation/Segama Group), the reservoir appears more equivalent to the Tanjong
Formation. High proportion of volcaniclastic sediments and conglomerates is the
reason that the Segama Group is not a good reservoir.
Besides that, velocities calculation results also show a very high velocity for
each interpreted layer. It suggests that the sediment is compacted and has low porosity
and therefore is unlikely to be a good reservoir.
Nevertheless, exploration opportunities might still exist in postrift sediments
in hypothetical carbonate and turbidite plays in this South Dent Graben area.
REFERENCES
Balaguru, A., 2006. Tectonic evolution and sedimentation of the NE Sa bah basin and Sulu Sea (CCOP-Danida Field study in eastern Sabah).
Bates, C.R., 1995.Geologic well evaluation reports: Kuda Terbang-1 ;Mutiara Hitam-1. WMC Petroleum (Malaysia) Sdn. Bhd.
Haile, N. S. and Wong, N. P. Y., 1965. The geology and mineral resources of the Dent Peninsula, Sabah. Geological Survey, Borneo Region, Malaysia, Memoir 16.
Hutchinson, C. S., 1989. Geological evolution of South-East Asia. Clarendon Press, Oxford, 368.
Lim, P.S., 1985. Geological map of Sabah (3'd Edition), Geological Survey of Malaysia.
Murphy Oil Corporation, 2000. Prelimanary assessment of hydrocarbon prospectivity in blocks SB305 & SB306, Offshore Sabah, Malaysia.
Petronas, 1999. The petroleum geology and resources of Malaysia. PETRONAS.
Petronas, 2006. Seismic Sequence Stratigraphy & Petroleum Play Analysis of Southern Dent Peninsular, Block SB306 Offshore Sabah, Malaysia.
Tongkul, F., 1991a. Tectonic evolution of Sabah, Malaysia. In: Nichols, G. and Hall, R. (eds.) Proceeding of Orogenesis in Action Conference London, 1990. Journal of Southeast Asian Earth Sciences, 6, 395-405.
Tongkul, F., 1997. Polyphase development in the Telupid area, Sabah, Malaysia. In: Geosea 1995 Proceedings, Journal of Asian Earth Sciences, 15, 175-184.
Xia, K. Y. and Zhou, D., 1993. The geophysical characteristics and evolution of northern and southern margins of the South China Sea. Bulletin of the Geological Society of Malaysia, 33,223-240.
APPENDIX A: Velocity calculation results
:)J:SJUt>U4-U4 veiOCit1 catclllanon resun t2Vrms'2- t2Vrms'2-t 1 Vrms'2 /!2-
SP time Vrms Vrms'2 T lsecl t x Vrms'2 t1Vrms'2 !2-!1 !1 SORT
1201 1080 2040 4161600 1.08 4494528 1380 2280 5198400 1.38 7173792 2679264 0.3 8930880 2988.458 1550 2480 6150400 1.55 9533120 2359328 0.17 13878400 3725.372 1760 2640 6969600 1.76 12266496 2733376 0.21 13016076.19 3607.78
1361 1100 2100 4410000 1.1 4851000 1350 2220 4928400 1.35 6653340 1802340 0.25 7209360 2685.025 1620 2560 6553600 1.62 10616832 3963492 0.27 14679600 3831.397 2120 2940 8643600 2.12 18324432 7707600 0.5 15415200 3926.22 2200 2980 8880400 2.2 19536880 1212448 0.08 15155600 3893.019
1561 1380 2280 5198400 1.38 7173792 1660 2580 6656400 1.66 11049624 3875832 0.28 13842257.14 3720.518 2050 2860 8179600 2.05 16768180 5718556 0.39 14662964.1 3829.225 2600 3190 10176100 2.6 26457860 9689680 0.55 17617600 4197.332 2700 3260 10627600 2.7 28694520 2236660 0.1 22366600 4729.334
1841 1550 2480 6150400 1.55 9533120 1850 2740 7507600 1.85 13889060 4355940 0.3 14519800 3810.486 2250 3000 9000000 2.25 20250000 6360940 0.4 15902350 3987.775 2750 3300 10890000 2.75 29947500 9697500 0.5 19395000 4403.975 3400 3520 12390400 3.4 42127360 12179860 0.65 18738246.15 4328.77
2041 1820 2240 5017600 1.82 9132032 1950 2350 5522500 1.95 10768875 1636843 0.13 12591100 3548.394 2300 2540 6451600 2.3 14838680 4069805 0.35 11628014.29 3409.987 2750 2860 8179600 2.75 22493900 7655220 0.45 17011600 4124.512 3700 3520 12390400 3.7 45844480 23350580 0.95 24579557.89 4957.778
2161 2150 2460 6051600 2.15 13010940 2300 2540 6451600 2.3 14838680 1827740 0.15 12184933.33 3490.692 24~0 21'>40 ? 4~ 17075520 2236840 0.15 14Q122f\f\.f\7 ~Rf\1 F\4
2650 2780 7728400 2.65 20480260 3404740 0.2 17023700 4125.979 3700 3520 12390400 3.7 45844480 25364220 1.05 24156400 4914.916
2241 2150 2460 6051600 2.15 13010940 2250 2500 6250000 2.25 14062500 1051560 0.1 10515600 3242.777 2400 2600 6760000 2.4 16224000 2161500 0.15 14410000 3796.051 2520 2720 7398400 2.52 18643968 2419968 0.12 20166400 4490.702 3480 3380 11424400 3.48 39756912 21112944 0.96 21992650 4689.632
-
SB30604-06 veiOCI y calculatiOn result I X
SP time Vrms Vrms•2 T (sec) Vrms•2 t2Vrms•2-t1Vrms•2 12-11 12Vrms•2-t1Vrms•2/l2-11 SORT 1681 1100 2050 4202500 1.1 4622750
1350 2220 4928400 1.35 6653340 2030590 0.25 8122360 2849.97544 1600 2400 5760000 1.6 9216000 2562660 0.25 10250640 3201.66207 1950 2680 7182400 1.95 14005680 4789680 0.35 13684800 3699.29723
1921 1500 2350 5522500 1.5 8283750 1820 2560 6553600 1.82 11927552 3643802 0.32 11386881.25 3374.44532 2180 2800 7840000 2.18 17091200 5163648 0.36 14343466.67 3787.27695 2560 3040 9241600 2.56 23658496 6567296 0.38 17282357.89 4157.20554 2620 3120 9734400 2.62 25504128 1845632 0.06 30760533.33 5546.21793
2081 1680 2480 6150400 1.68 10332672 1940 2600 6760000 1.94 13114400 2781728 0.26 10698953.85 3270.92553 2380 2940 8643600 2.38 20571768 7457368 0.44 16948563.64 4116.86332 2820 3220 10368400 2.82 29238888 8667120 0.44 19698000 4438.2429 3100 3380 11424400 3.1 35415640 6176752 0.28 22059828.57 4696.78918
2281 1550 2380 5664400 1.55 8779820 1800 2500 6250000 1.8 11250000 2470180 0.25 9880720 3143.36126 2300 2880 8294400 2.3 19077120 7827120 0.5 15654240 3956.54395 2900 3300 10890000 2.9 31581000 12503880 0.6 20839800 4565.06298 3550 3580 12816400 3.55 45498220 13917220 0.65 21411107.69 4627.21382
I 2441 I 2ooo I 236o I 55696oo I 2 l111392oo I -1 I I
2100 2420 5856400 2.1 12298440 1159240 0.1 11592400 3404.76137 2500 2660 7075600 2.5 17689000 5390560 0.4 13476400 3671.02166 2900 2900 8410000 2.9 24389000 6700000 0.4 16750000 4092.67639 3650 3300 10890000 3.65 39748500 15359500 0.75 20479333.33 4525.40974
2561 2100 2420 5856400 2.1 12298440 2200 2480 6150400 2.2 13530880 1232440 0.1 12324400 3510.61248 2500 2660 7075600 2.5 17689000 4158120 0.3 13860400 3722.95581 2800 3020 9120400 2.8 25537120 7848120 0.3 26160400 5114.72384 3300 3100 9610000 3.3 31713000 6175880 0.5 12351760 3514.50708
2761 2420 2600 6760000 2.42 16359200 2500 2660 7075600 2.5 17689000 1329800 0.08 16622500 4077.07003 2620 2720 7398400 2.62 19383808 1694808 0.12 14123400 3758.11123 2900 2900 8410000 2.9 24389000 5005192 0.28 17875685.71 4227.96472
2961 2700 2750 7562500 2.7 20418750 3000 2950 8702500 3 26107500 5688750 0.3 18962500 4354.59527 3300 3100 9610000 3.3 31713000 5605500 0.3 18685000 4322.61495
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