seismic stratigraphy of the tertiary sediments, offshore sarawak deepwater area, malaysia · ·...

AAPG Ill/eflla/lima! CII'!fmIlCl d E1:hilllium 'g4 AUjIM/2!-24, /994, KI/Ilia LUlllpur. Malay.,ul

Seismic sequen~e stratigraphy of the Tertiary sediments, offshore Sarawak deepwater area, Malaysia

ABDUL MANAF MOHAMMAD AND ROBERT H.F. WONG

PETRONAS P.O. Box 12444, Kuala Lumpur

Abstract: A seismic sequence stratigraphic interpretation was carried out in the Sarawak deepwater area with the main objective of developing a workable chronostratigraphic chart that defines stratigraphic boundaries within which depositional systems and lithofacies can be identified, mapped and interpreted. The dataset includes 8,000 km of seismic lines and well data from four drilled locations.

. The study has resulted in the identification and correlation of eight seismic horizons representing the tops of eight depositional sequences which are grouped into four mega/supersequences (A, B, C and D) based on regional tectonic events of the South China Sea. Six of the seismic horizons have been tied to the four wells and dated based on paleontologic data. Two other older horizons are dated based on correlation to the global sea-level chart. Higher order sequences are also interpreted from paleontologic, lithologic, paleofacies data and GR-Iogs from the four wells.

Seismic facies analysis have also been carried out in the study area where four main seismic facies (Facies I-IV) lunging from non-marine to deepmarine facies are interpreted. Seismic facies maps constructed for lower and upper portions of Oligocene-Lower Miocene Supersequence C indicate that it contains all four main facies. This supersequence is overall transgressive and its paleoshoreline runs in a NW-SE direction. A seismic facies map generated for Middle Miocene-Recent Supersequence D suggests that it contains mainly outer shelf to deepmarine facies (Facies III-IV) and its paleoshoreline runs in an East-West direction.

A workable chronostratigraphic chart has been developed where second to fourth-order sequences can be correlated within the study area. The chart is able to correlate the episodic rifting ofRu and Pigott (1986), the local structural history and Shell's sedimentary Cycles I-VIII (Ho, 1978) to the global sealevel curve (Haq et al., 1988).

This study also assisted in identifying potential play-types. Structural traps of non-marine to shallow marine facies are mainly confined to Supersequence C while stratigraphic traps of basin floor fans are located mainly in Supersequence D.

INTRODUCTION

The study area is located within the Sarawak Basin in the South China Sea. It lies approximately 200 to 400 km from the present coastline ofSarawak and covers an area of approximately 40,000 sq km (Fig. 1).

Water depths in the study area ranges from about 100 m to more than 2,000 m (Fig. 2). In general, the bathymetric depth increases towards the northeast but occasionally is interrupted by seamounts that reach up to less than 1,000 m of water depths. At the western part of the study area, a very smooth seafloor topography with gentle northeast gradient is observed, due to clastic sedimentation during Late Miocene to Pliocene. However, the shelf edge in central area forms a steep slope and is associated with oblique shelf progradation during the Upper Miocene to Recent period. Meanwhile, a protrusion at the eastern shelf edge is related to a carbonate growth which is Middle Miocene.

Geol. Soc. Malay"ia, Bulletin 37, July 1995,' pp. 345-361

The seismic sequence stratigraphic study was performed with the following objectives:-• To develop a workable chronostratigraphic

correlation chart that defines main stratigraphic boundaries within which deposition systems and lithofacies can be defined, mapped and interpreted. The correlation takes into consideration the regional tectonics, local structural history and the global sea-level curve;

• To determine the different types of paleoenvironment under which the sequences were deposited; and;

• To assist in identifying potential play-types which include both structural and stratigraphic traps.

DATA AVAILABILITY

The seismic sequence stratigraphic study was conducted based on the available geophysical and geological data.

The geophysical database comprises

346

LEGEND :

SUMATERA

200 WATER DEPTH 11'1 METRES


SOUTH EAST ASIA LOCATION MAP OF

STUDY AREA

SOUTH CHINA

SEA

Figure 1. Location map ofthe study area.

.... "---------

Figure 2. Bathymetry map.

o

20 Km

o 'WELL B

PHILIPPINES

Ceo!. Soc. l11a laYd ia, BulLetin 37

SEISMIC SEQUENCE STRATIGRAPHY OF THE TERTIARY SEDIMENTS, OFFSHORE SARAWAK DEEPWATER AREA, MALAYSIA 347

approximately 8,000 line-km of high-quality seismic data of 1989 vintage and reprocessed seismic data of 1969-1973 vintages. The seismic coverage provides a grid spacing of 5 x 10 km2 in the east and 15 x 15 km2 in the west. The lines are orientated mainly in NE-SW and NW -SE directions.

The well database for this study includes four wells - A, B, C and D that were drilled in the shelfal part of the study area.

The seismic and well location map is s40wn in Figure 3.

GEOLOGICAL SETTING

Based on structural trends and styles observed, the study area can be subdivided into three provinces: West Luconia, North Luconia and Central Luconia platform (Fig. 4).

The West Luconia Province is characterized by thrust imbricates and asymmetrical anticlines associated with zones of large expansion towards the landward side. The province contains very thick Miocene-Pliocene sections.

The North Luconia Province represents a deep Cenozoic rift basin. It is characterised by mainly N-S trending fault-bounded structures and listric faulted graben complexes.

The Central Luconia Platform is a stable province where very little clastics influx reached the area at least up to Mid-Miocene, allowing deposition of a carbonate bank which locally developed into shelfal buildups.

SEISMIC INTERPRETATION

Altogether, eight seismic horizons were correlated using regional lines in the study area. They represent tops of depositional sequences which are grouped into four MegaiSupersequences based on correlation of regional tectonic events i.e. the rifting episodes of the South China Sea (Ru and Pigott, 1986) to the global sea-level chart (Haq et al., 1988) (Fig. 5). They range from Megasequence A (Late Cretaceous-Middle Eocene), Supersequence B (Late Eocene-Late Oligocene), Supersequence C (late Oligocene-Early Miocene) and Supersequence D (Early Miocene-Recent).

The ages of the Blue, Purple, Dark Green, Orange, Brown and Light Green horizons have been dated based on seismic ties to the four wells located on the shelfal area which penetrated down to Lower Oligocene sediments. The Red and Pink horizons have been tentatively dated based on the ages of the regional tectonic events in the South China Sea and their comparison with the eustatic chart (Fig. 5).

An example of interpreted megalsupersequences

July 1995

within a listric-faulted graben complex is shown Figure 6. The boundaries are clearly distinguished by their angular relationship and/or regionally mappable horizons, usually associated with high amplitudes. Figure 7 is another example where the four megalsupersequences are identified and the onlapping of the overlying sediments of Supersequence B against the Red Horizon is clearly evident. The Red Horizon also truncates the upper part of Megasequence A.

In these examples, other simple sequence boundaries related to local structural history are also interpreted. One of them is the Purple Horizon which is interpreted to separate two major seismic facies - a lower continuous high amplitude parallel events and an upper weak reflectors to reflection free zone (Fig. 6).

WELL-LOG INTERPRETATION

Second order megalsupersequence and thirdorder simple sequence boundaries (Vail et al., 1990) can be readily identified on seismic data. Well logs are able to resolve up to fifth or sixth order of cyclicity (Van Wagoner et at., 1990). Hence, in this study, megalsupersequence, simple sequence and parasequence boundaries are also interpreted by utilizing well-logs combined with paleontologic, lithologic and paleofacies data.

The stratigraphy was previously interpreted by Shell using the sedimentary cycle concept (Ho, 1978). Altogether there are eight cycles with ages ranging from Early Oligocene to Recent. Cycle I

. (early Oligocene-Early Miocene is the oldest cycle whilst Cycle VIII (Pleistocene) is the youngest cycle (Fig. 5). Systems tract interpretation was conducted on the well sections and the intervals crossing major stratigraphic boundaries are depicted (Fig. 8 to Fig. 15).

The 30 Ma Supersequence Boundary (Blue Horizon) separating Supersequences Band C is recognised within Cycle I which was penetrated by Well A. Although there is no variation in the interpreted paleofacies, the coarsening upward sandy section is easily differentiated from the overlying thick, shaly interval, suggesting a drastic change from low to high accommodation space (Fig. 8). The coarsening upward section below the Blue Horizon is interpreted to be highstand systems tract and the shaly interval above it is interpreted to be a transgressive systems tract.

The third-order 22 Ma sequence boundary (Purple Horizon) within Supersequence C was interpreted to be the boundary dividing Cycle I and Cycle II (Fig. 9). The boundary is marked by a sudden change in lithology (sand prone below to shales and carbonate above) and depositional

SOUTH

CHI N A

SEA

50 Km


WELLD 20Km

Figure 3. Seismic and well location map.

SARAWAK DEEPWATER AREA

STRUCTURAL ELEMENTS MAP

Figure 4. Structural elements map.

LEGEND:

'------J NORMAL FAULT

~ REVERSE FAULT ASSOCIATED WITH TOE THRUSTING


CORRELATION OF SARAWAK CYCLES TO GLOBAL SEA LEVEL CHART AND REGIONAL TECTONICS

AGE SEQUENCE REGIONAL REGIONAL i. SARAWAK HELL' TECTONIC

ERA SYSTEM SERIES KEY S CYCLE STRATIGRA- RIFTING EVENTS OFFSHORE LOCAL

SBIMF HIC HORIZON EPISODE .U":'.'!':.!""":" STRUCTURAL HISTORY

aUATE R PLEIST VIII NARY OCENE 1.6

LIGHT Rift GREEN -

oW U VII onset

Oz 3.0 BROWN -~~ C')

Local minor L m VI D tensional faulting

I- 5.5 ORANGE - ift onset

W U W.Luconia Delta

V gravitational

z thrusting started

W W 11.6 Cl Z W.Luconia Delta I

W M IV listric faulting

0 () "" 15.0 starts Carbonate Reefin,

W m III on high blocks

0 I-

z 17.5 DARK - ·~ift ---::E GREEN onset Thermal uplift an

II faulting with detachment on

L Blue Horizon - - - - 22.0 PURPLE - ---

Local basement C detached faulting ...

m I-

W

Z U W

() - - - 30 -BLUE ~reakup c - -.-- E

>- 0 unc. ::J .. Cl

....

.:ot

II: L I u

-I '<t N-S tansform 0

0 ci: :l5 c:( B faulting in Normr faulting i I- S.China sea

()

U .,

I- W _L_ ~ 0 II: z 39.5 -RED - - ift

Rifting and uplift l! onset N W W

of high .2

W blocks with erosiorE

0 I- Cl Z C') E

W M ci: .,

Z ii

0 () I- u to

W W 0 Seafloor spreading .,

W ..

Continues with .. U -I ..J

49.5 slow basin

c:( ------ subsidence and

L PRE-I subduction

Q" ( S.W subbasin I

W U A

z W - - - - - 58.5 -PINK - reak up () unc. 0 W -I c:( L Q"

If)

U :::l ------ 68 a 0 Subduction of W II: Oceanic crust N U W

0 Q" ". along Lupar Line If)

c:( Q" W Iii :::l Counter clockwise ::E II: rotation of Borneo

Rw/corsawL

Figure 5. Correlation of Sarawak cycles to global sea-level chart and regional tectonics.

July 1995

Figure 6. Interpreted Line 1 crossing a listric-faulted graben complex.

Figure 7. Interpreted Line 2 showing major truncation by red horizon.

WELL -A WELL - B SEQUENCE STRATIGRAPHIC INTERPRETATION WITHIN CYCLE I SEQUENCE STRATIGRAPHIC INTERPRETATION OF

AGE MEGA SYSTEMS SHELL'S PALEO-UPPER CYCLE I AND LOWER CYCLE II

MA. SUPER SEQ. TRACT CYCLE FACIES GR DEPTH AGE EGA/ en

0 m MA UPER SYSTEMS SHELL'S PALEO- x

DEPTH en SEQ. TRACT FACIES

GR l- s:: CYCLE :::I 0 en m

a: 7 0 C TRANSGRESSIVE

c m

SYSTEMS w ~ z

HIGHSTAND ()

u w m TRACT a.. SYSTEMS Z 0 en

I-m -I

a.. TRACT a: ~ :D

« « 7 ~ ~

a: :s G5 w SB :D

30 z » ~ ""0

BLUE HORIZON :J: II 0 7 -<

..... 0 ..... w -10000 FT "Tl ..... « ~ -I

X :J:

a: TRANSGRESSIVE II) m w II)

all -I Z SYSTEMS w m

:D Z

TRACT i -I » 0 :D m -< a: en

7 « m U 0

~ w 22 C PURPLE m Z z

-I .... HIGHSTAND a: PB HORIZON Sf>

B SYSTEMS « .... 0

:s "Tl "Tl

TRACT 0 en

a: Z :J: ..... 0

w 0 « PB :D

-10500 FT m x ..... en ~

0 a.. ...

» I- :D

..... ..... » 0 HIGHSTAND « :i: « I-

» SYSTEMS I- '" ..... II)

II) 0 TRACT « m « 0

m 0 ""0

U :i: PB u »

a: -I

w PB m :D

~ » 0

:D m

..... :J> s:: »

Rw_AI. > -< -11000 FT en »

Figure 8. Well-A: sequence stratigraphic interpretation within Cycle I. Rw/ovlWBl

Figure 9. Well-B: sequence stratigraphic interpretation of Upper Cycle I and W Lower Cycle II. 01 .......

AGE EGA/

MA. UPER

SEQ.

D

17.5

C

Rw/DlllWB1L

WELL - B SEQUENCE STRATIGRAPHIC INTERPRETATION OF

UPPER CYCLE II AND LOWER CYCLE III

SYSTEMS SHELL'S PALEO- GR TRACT CYCLE FACIES

TST III

DARK GREEN

HORIZON

u -l-ll: w Z

lI: w Z Z

w Z -

HIGHSTAND II lI:

'" SYSTEMS ::iE 0

TRACT ....I

0 :I:

DEPTH

Figure 10. Well-B: sequence stratigraphic interpretation of Upper Cycle II and Lower Cycle III.


UPPER CYCLE III AND LOWER CYCLE IV

MA. UPER

. ..... -: .... _ .... -...... .

SYSTEMS

i 1(1) W I~ -I I ..... U I W >

SEQ. TRACT ! ~ U ------------ -------------- -----------------------------------------}------------------

I

PALEO

FACIES GR

o J: ~ DEPTH :::::i

--------.:==_ .. --------_ ... -.----_ ..

7 -6700 FT i

Figure 11. Well-B: sequence stratigraphic interpretation of Upper Cycle III and Lower Cycle IV.

W 01 I\)

~ o C r

s:: » z » ..."

s:: o I

~ s:: » o » z o :c o CD m ~ I

:-n ~ z G>


UPPER CYCLE IV AND LOWER CYCLE V

MA.

11.6 D

Rw'-'W8Z

SYSTEMS TRACT

HIGHSTAND SYSTEMS

TRACT

TRANSGRESSIVE SYSTEMS

TRACT

en • w ..J..J irlo :::c> enO

V

IV

PALEO· FACIES

w z a: <C :E ~ 0 .... ~ :c CI)

w Z

~ :E

~ w Q

GR o ::J: DEPTH I-::::i

·6000 FT

Figure 12. Well-B: sequence stratigraphic interpretation of Upper Cycle IV and Lower Cycle V.

AGE

MA.

5.5

WELL - C

SEQUENCE STRATIGRAPHIC INTERPRETATION OF

UPPER CYCLE V AND LOWER CYCLE VI

jv!EGAj ~w ~UPER

SYSTEMS -'-' PALEO· 0 -'U ::J: w> GR I- DEPTH

SEQ. TRACT ::J:U FACIES ::::i en

-4800 FT

TRANSGRESSIVE z SYSTEMS

0 :c ,

TRACT VI z :E :c

ORANGE S8 5000 FT

D HORIZON lI/i: -} ~

HIGHSTAND w

SYSTEMS !(

V z 0

TRACT CIQ a: (j

Rw/RIWCL

Figure 13. Well-B: sequence stratigraphic interpretation of Upper Cycle V and Lower Cycle VI.

(J) m en ;:: C'5 en m o c m z C"> m

~ ~ G5 ~ "'0 ::I: -< o "T1

~ m -I m ~ > ~ en m C 3: m

~ o ~ ::I: o :Il m

g?

I c m m

~ m :Il » :Il

~ s:: S en >

W 01 W

Ul 01 ~

WELL - D SEQUENCE STRATIGRAPHIC INTERPRETATION OF

UPPER CYCLE VI AND LOWER CYCLE VII

WEll- D SEQUENCE STRATIGRAPHIC INTERPRETATION OF

UPPER CYCLE VII AND lOWER CYCLE VIII

AGE iMEGAJ SYSTEMS SHELL'S PALEO- 0 MA ~UPER TRACT CYCLE FACIES

GR j: DEPTH

SEQ. :::J

-6600 FT

en w MA. SYSTEMS :.... ..... PALEO- GR

0 ..... u :c DEPTH TRACT w FACIES :c > 5 en u

TRANSGRESSIVE VII z

SYSTEMS :::E u TRACT J:

VIII ~ » w OJ

0

TS

LST l"i ... BROWN IsB

3.0 HORIZON

0 HIGHSTAND

SYSTEMS u TRACT 1= a: 7000 FT w z

~ c c 5i

IMFS VI

, II: w Z ~ w z a:

Z c II:

.-1.6 LIGHT GREEN w SB -3000 FT s::

z » HORIZON i!:

z » w

'T1

Z s::

I 0 ::J: » 3i: 3i:

0 » ...... 0

D ...... » 0 z ::c 0

PB :II 0 OJ

VII m ~

Z ::J: :i ~ ::c z ~ s: -3500 FT z

G>

« TRANSGRESSIVE :::E

SYSTEMS 9 0

PB

TRACT J:

I'" Z s:

7600 FT RwlaalWD1L

RwiaIWD

Figure 14. Well-D: sequence stratigraphic interpretation of Upper Cycle VI and Figure 15. Well-D: sequence stratigraphic interpretation of Upper Cycle VII and Lower Cycle VII. Lower cycle VIII.


environment (coastal to lower coastal plain below to shallow marine above). A transgressive systems tract and a highstand systems tract are interpreted to lie immediately above and below the Purple Horizon respectively.

On the well-log, the second-order 17.5 Ma Supersequence Boundary (Dark Green Horizon) is interpreted to separate Cycle II from Cycle III (Fig. 10) and coincides with the end of an overall coarsening upward clastics belonging to the highstand systems tract. Although no change of paleofacies is detected across this boundary the overlying sediments are more argillaceous with intercalations of thin carbonates, which are referred to the transgressive systems tract.

Well-B encountered carbonate sediments during Cycles III-IV (Middle-Upper Miocene). The Cycle IIIIIV and Cycle IVN boundaries are interpreted to be maximum flooding surfaces as shown in Figure 11 and Figure 12 respectively. The boundaries demarcate the change from transgressive systems tract to highstand systems tract as suggested by a change from finer grain, deeper marine facies to coarser grain shallow marine facies. The transgressive period signifies a time of build-in phase due to high accommodation space while the highstand period implies a build-out phase due to low accommodation phase (Epting, 1980).

The interface between Cycle V carbonates and Cycle VI clastics encountered by Well-C is shown in Figure 13. The change in lithology and paleofacies indicates a highstand deposition of carbonates followed by a fall in sea-level and erosion at 5.5 Ma. Subsequently, outer shelf shaly sediments were deposited during the ensuing rise in sea-level.

Well-D encountered a thick section of Cycles VI, VII and VIII clastic sediments. The lower part of the well portrays a shaly to silty Upper Cycle VI and Lower Cycle VII. The change in stacking patterns and paleofacies enables the sequence boundary to be recognized (Fig. 14). The underlying highstand systems tract exhibits a coarsening upward pattern whereas the overlying thin, lowstand systems tract reveals a fining upward pattern.

The upper part ofWell-D shows a more sandy, shallower marine environment. The boundary between Cycles VII and VIII is suggested by a change from an overall coarsening upward highstand systems tract to thick, shaly sediments of the transgressive systems tract of the next sequence (Fig. 15).

Hence, by interpreting the systems tracts at the four drilled locations, second order supersequence boundaries, third-order simple boundaries and fourth-order maximum flooding surfaces are distinguished. One supersequence

July 1995

boundary falls within Cycle I and another is located at the top of Cycle II. Four third-order simple boundaries differentiated the Cycles I1II, VNI, VII VII and VIINIII boundaries and two maximum flooding surfaces define the Cycles IIIIIV and IV N boundaries.

SEISMIC FACIES ANALYSIS

Based on the seismic facies analysis (Mitchum et al., 1977) carried out in the study area incorporating well control in the shelf area, the paleoenvironments were interpreted for Supersequence C and D. As no well has been drilled in the deeper waters, the predicted facies distribution is based mainly on the observation and interpretation of seismic data.

Four main seismic facies are interpreted in the study area (Fig. 16). Facies I includes subparallel to wedging, discontinuous to continuous, irregular to wavy reflections representing upper to lower coastal plain sediments. Fluvial fans and lacustrine shales are confined to this facies. Facies II comprises parallel to subparallel to wedging, continuous high amplitude reflections. They are interpreted as mainly coastal to inner neritic sediments comprising sand/shale interbeds. Facies III contains transitional parallel events overlying transparent to chaotic data and some mounded facies indicating shale-prone marine facies with some carbonates. Facies IV encompasses transparent to chaotic reflectors mimicking outer shelf to bathyal shales with some slightly mounded facies suggesting deposition of some turbidite fans.

Based on this analysis, seismic facies maps were generated for various zones within Supersequences C and D. Figure 17 displays the seismic map for the lower part of Supersequence C (between Purple and Blue Horizons) illustrating the deposition environments from non-marine in the southwest (Facies I) to marine in the northeast (Facies III). The variation of the various seismic facies is exhibited by Line 3 and Line 4 which portrays Facies I and Facies II respectively (Fig. 18).

A seismic facies map constructed for the Upper Supersequence C (between Dark Green and Purple Horizons) shows that the paleo shoreline continues to trend in a NW-SE direction (Fig. 19). This map also indicates a backstepping in the paleoenvironments as compared with the Figure 17 and implies an overall rise in sea-level within Supersequence C, which agrees with the Haq's curve (Fig. 5). Line 5 and Line 6 depict Facies II and Facies IIIIIV respectively (Fig. 20) of the Upper Supersequence C.

Another seismic f~ies map was produced for

356

2 o

~ w

= o

RW/SFAl

TRANSPARENT

TO

CHAOTIC.

SOME SLIGHTLY

MOUNDED FACIES

FMN I HMN - FON I HON

-BATHYAL

(MOSTLY SHALE.

SOME TURBIDITES)

FACIES IV

f;!i~;i::::::::~::i\;i Facies I ~ Facies II

Facies III


SEISMIC FACIES ANALYSIS TRANSITION PARALLEL

OVERLYING

TRANSPARENT TO

CHAOTIC.

SOME MOUNDED

FIN I HIN - FMN I HMN

(SHALE PRONE.

SOME CARBONATES)

FACIES III

pARALLEL TO

SUBPARALLEL TO

WEDGING. CONTINUOUS

HIGH AMPLITUDES

REFLECTIONS

LCP I COF I COL - FIN I HIN

(SAND - SHALE INTERBEDS)

FACIES II

Figure 16. Seismic facies analysis.

SUBPARALLEL TO

WEDGING. DISCONTINUOUS

- SOME CONTINUITY.

IREGULAR TO WAVY

REFLECTIONS

MOSTLY NON MARINE

UCP I LCP

(FLUVIAL FANS.

LACUSTRINE)

FACIES I

Figure 17. Seismic facies map, lower megalsupersequence C (between purple and blue).

Geol. Soc. MalaYJia, Bulletin 37


the Supersequence D. A major uplift at about 17.5 Ma (Dark Green horizon) shifted the paleo shoreline to a position running roughly parallel to the present day coastline (Fig. 21). Within the study area, only Facies III and N are recognised, representing outer shelf to bathyal environments. The seismic facies map also shows locations of ponded turbidites and turbiditic fans located within Facies N. Ponded turbidites are interpreted as high amplitude events encased by low amplitude data located within a depressed areas (Fig. 22).

POTENTIAL PLAY-TYPES

Once the seismic facies of the two important supersequences are mapped and the chronostratigraphic boundaries correlated, the study can then assist in identifying potential playtypes. Structural traps as well as stratigraphic traps containing the various seismic facies can be mapped, and their seal, reservoir and source rock

characteristics assessed. Figure 23 displays a structural trap of Upper

Supersequence C containing reservoir rock of Seismics Facies II sealed by Seismics Facies IV of Supersequence D. Source rocks are believed to be derived from underlying Seismics Facies IIII of Lower Supersequence C. Two stratigraphic traps of interpreted basin floor fans within Seismics Facies N of Supersequence D are demonstrated in Figure 24. Overlying deepwater shales provide the seal. Source rocks for this play are thought to be derived from deeper Seismics Facies IIII of Upper Supersequence C.

CONCLUSIONS

Four second-order megalsupersequences have been identified and correlated with the regional tectonic events of the South China Sea. Within the second order supersequences, third-order sequences are also recognised and they are related to the local

Figure 18. Line 3 (Facies I) and Line 4 (Facies II).

July 1.995

358 ABDUL MANAF MOHAMMAD AND ROBERT H.F. WONG

Figure 19. Seismic facies map, upper mega/super sequence C (between dark green and purple),

Figure 20. Line 5 (Facies II) and Line 6 (Facies III!IV).

SEISMIC SEQUENCE STRATIGRAPHY OF THE TERTIARY SEDIMENTS, OFFSHORE SARAWAK DEEPWATER AREA, MALAYSIA 359 11 2'OO'E

5'OO'N~mml

!llmtl Facies III

F==-=~=j Facies IV

6'OO'N ~ Turbidites

Figure 21. Seismic facies map, mega/super sequence D (post dark green).

Figure 22. Line 7 and Line 8 showing ponded turbidites .

1. Motle: Point. Activo hOrt201l: uummybasc

Figure 23. Structural play in supersequence C.

Figure 24. Interpreted basin floor fan.


structural history. Fourth-order sequences linked to the sea-level fluctuations are also distinguished within the third-order sequences.

A workable chronostratigraphic chart has been developed where the second to fourth-order sequences can be correlated. The chart is able to correlate the episodic rifting of the South China Sea (Ru and Pigott., 1986), local structural history and Shell's sedimentary Cycles I-VIII (Ho, 1978) to the global sea-level chart (Haq et al., 1988).

Four seismic facies (Facies I, II, III and IV) have been identified in the study area. The interpreted facies range from non-marine to deep marine facies. Seismic maps generated for the supersequence C' indicates its overall transgressive deposition which agrees with the global sea-level curve of Hag et al. (1988).

Potential play-types within Supersequence C and D are also derived from this study. The main play type in Supersequence C is a structural trap of Seismic Facies I1II, whereas stratigraphic traps of basin floor fans are found mainly in Supersequence D.

REFERENCES EPTING, M., 1980. Sedimentology of Miocene Carbonate

buildup, Central Luconia, Offshore Sarawak. Geol. Soc. Malaysia Bull. 12, 17-30.

HAQ, B.U., HARDENBOL, J. AND V AIL, P.R, 1988. Sea-level changes: An integrated approach. Society of Economic Paleon. and Min. Special Publication No. 42.

Ho, K.F., 1978. Stratigraphic framework for oil exploration in Sarawak. Geol. Soc. Malaysia Bull. 10, 1-13.

MITCHUM, RM., JR., VAIL, P.R AND SANGREE, J.B., 1977. Seismic stratigraphy and global changes of sea-level - part 6: stratigraphic interpretation of seismic reflection patterns in depositional sequences. In: Payton, C.E. (Ed.), seismic stratigraphy - application to hydrocarbon exploration. AAPG Memoir 20,117-134

Ru, KE AND PIGOT[, J.D., 1986. Episodic rifting and subsidence in the South China Sea. AAPG Bulletin 70, 1136-1155.

VAIL, P.R, AUDEMARD, F., BOWMAN, S.A., EISNER, P.N. AND PEREZ-CRUZ, G., 1990. The stratigraphic signatures of tectonics, eustasy and sedimentation. Department of geology and geophysics, Rice University, Houston, Texas.

V AN WAGONER, J .c., MITCHUM, RM., CAMPION, K.M. AND RAHMANI AN, V.D., 1990. Siliclastic sequence stratigraphy in well logs, cores and outcrops. AAPG Methods in Exploration Series No.7.

---------+ ••• -~-+.-.-------

Manuscript received 19 October 1994

July 1995

seismic stratigraphy of the tertiary sediments, offshore sarawak deepwater area, malaysia · ·...

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