Relation of Syn-Depositional Faulting with Carbonate Growth using Miocene Carbonate Platforms of Central Luconia Province, Malaysia

Abstract

Interpretation of two Miocene carbonate platforms in Southeastern part of Central Luconia Province has provided an insight of the structural history of the platforms, relating syn-depositional faulting with the platform evolution and it’s internal architecture. Six horizons have been interpreted on both platforms. These horizons represent four different stages of carbonate growth, starting with build-out phase, followed by build-up and build-in phase. Internal architecture of these platforms ranges from standard layer-cake of carbonate platform with flat top (Platform FY) to sub-circular, steep sided margins that appear tilted and ends-up in pinnacle look-alike before carbonate production stopped (Platform EX).
Siliciclastic influxes from Borneo deltas have favoured platform growth by suppling nutrients to the reef at the time of carbonate deposition. It also affected the demise of the platforms by providing extra clastic input that created unfavourable water environment for carbonate growth, and leds to subsidence and collapse of the platforms due to increase of gravity loading from rapid accumulation of sedimentary deposits above the platform. Faults are widespread throughout both platforms. All faults are normal, and seem to govern the overall north-south elongation of platforms in Southeastern part this province. However, individual faults are preferably trending NE-SW with dipping angle of approximately 15°-50°. The platforms were tilted, due to major faulting and increase of overburden from growing carbonate layers. As the platform tilted, lateral environmental facies on the platform changed from reef build-up to quiet lagoonal facies environment. The changes are observed laterally over an approximately 2.5- 3.5km distance.

Full text

Relationship between syn-depositional faulting and carbonate
growth in Central Luconia Province, Malaysia
S.N. Fathiyah JamaludiN*, maNuel Pubellier & david meNier
Geoscience Department, Faculty of Geosciences & Petroleum Engineering,
University Technology Petronas, 31750 Tronoh,Perak, Malaysia
*Email address: fathiyah.jamaludin@yahoo.com
Abstract: Using 3D seismic and well data, detailed seismic interpretation has been conducted on two carbonate Platforms
EX and FY located in the Central Luconia Province, Malaysia. The results provide an insight to understand the relationship
of faulting with syn-depositional carbonate growth. Five geo-seismic units were interpreted from the Late Oligocene
to present day sedimentation in the basin. Structural interpretation of both platforms shows that almost all the faults
in the deeper part of the platforms are normal listric faults that resulted from the nal rifting stage of the South China
Sea. Small-scale, steep normal faults within the carbonate units behave relatively as syn-depositional faults that became
the base and template for the reefs to grow as these faults created conjugate and branching faults systems. Apart from
becoming templates for the reefs to grow, these syn-depositional faulting events had also interrupted the growth of reefs
especially those that were located at the platform margin. Slight movement of the branching faults induced sub-marine
landslide to create reefs collapse. The establishment of relationship between faulting and carbonate growth in Central
Luconia Province might be useful for revisiting mature hydrocarbon reservoirs.
Keywords: Central Luconia Province, South China Sea, seismic interpretation, fault, carbonate
INTRODUCTION
Successful carbonate deposition requires a combination
of several environmental factors such as suitable structural
template, sufcient water depth, salinity and enough nutrient
supply. This was the case in the Central Luconia Province
in offshore Malaysia during the Miocene where extensive
carbonate reefs deposited on tilted horst blocks at the time
when the sea oor remained at a shallow depths for a long
time (period of tectonic stability) (Franke et al., 2014).
Research focusing on the sedimentology of the carbonate
platforms in the Central Luconia Province had begun with
the rst publication by Epting (1980) and Doust (1981).
They had suggested that the development of the carbonate
platforms in the Central Luconia were essentially unaffected
by tectonic deformations, and only governed by eustatic sea
level changes. However, improved seismic imaging in the
Central Luconia’s subsurface shows that faulting and folding
were widespread, throughout Miocene to Pliocene. Thus,
apart from eustatic sea level and nutrient supply, signicant
tectonic activity also played a role in the development of
the carbonate platforms in this area.
To investigate the relationship between the growth of
carbonate platforms and structural effects, two reference
platforms, EX and FY located in the southeastern part of
this province, were studied using 3D seismic volume and
well data. Recognition of the relationship between syn-
depositional faulting and growth history of the carbonate
platforms is the key to increase hydrocarbon production
in this basin. This study aims to establish the relationship
between structural framework within carbonate platforms
and its growth in response to syn-depositional tectonics.
GEOLOGICAL SETTING
The Central Luconia Province (Figure 1a) is a relatively
broad and stable structural block at present (Zampetti et al.,
2004a) covering an area of 240km x 240km (Vahrenkamp,
2004) in offshore Sarawak and has become the target for
many prolic oil and gas exploration since 1968. It has
undergone Eocene extensional tectonics and subsidence in
the north and compressional tectonics in the south. To date,
more than 200 drowned carbonate platforms ranging in size
from a few km2 to more than 200km2 across have been
seismically mapped (Ali and Abolins, 1999) with 55 elds
proven to contain commercial quantities of hydrocarbon,
particularly gas (Ali, 1995; Pierson et al., 2010).
In the Late Early Miocene, when the tectonic activities
were slowing down, reefs started to flourish where
favourable conditions prevailed. Reefs had become more
prolic throughout the Early Middle Miocene to Late
Middle Miocene. The carbonate reefs were interpreted to
be deposited in the inner neritic environment. Inuence
of prograding clastic materials from the nearby Baram
deltas and increase of sediment load from the NW Borneo
Wedge and major sea level falls had subsequently stopped
the carbonate deposition. Clastic deposits continued to
prograde into the Central Luconia Platform since the Late
Miocene until today, and promoted gravitational tectonics
within this province.
Compared to the other basins in the South China Sea,
there was a delay in carbonate deposition at Central Luconia
(Franke et al., 2014) because the Luconia Block was still
colliding and subsiding beneath Borneo during the Late
Oligocene, whereas the other parts of South China Sea had
Bulletin of the Geological Society of Malaysia, Volume 60, December 2014, pp. 77 – 83

S.N. Fathiyah JamaludiN, maNuel Pubellier & david meNier
Bulletin of the Geological Society of Malaysia, Volume 60, December 2014
78
settled in their rifting processes. Carbonate deposition in
the Central Luconia took place in the Late Early Miocene
but become more prolic in the Middle to Late Miocene
(Zampetti et al., 2004a,b).
DATA AND METHODOLOGY
The data which include 3D seismic and well data of
Platforms EX and FY (Figure 1b) were made available
by the Petroleum Management Unit (PMU), Petronas and
Shell Sarawak Berhad. The seismic volume used for this
research is a ltered, scaled and migrated seismic volume.
The inline interval was 25m with crossline interval of
12.5m (grid size = 25m x 12.5m), with 245 inlines and
1124 crosslines for Platform FY, and 272 inlines and 492
crosslines for Platform EX.
Interpretation of seismic data began with creation of
a synthetic seismogram. Moderate to good correlations
between synthetic and seismic data were achieved
through seismic to well tie analysis. Horizons for seismic
interpretations were selected according to the obvious
reectors correlated from the seismic to well tie process.
Since the available wells only penetrate up to the depth
of 2280 m (to the base of carbonate layers), interpretation
on the deeper horizons were only based on the continuity
of the seismic reectors, as observed on several seismic
attributes. Maps for each of the horizons were created and
overlaid with fault surfaces.
The internal geometries of the seismic reections within
Platforms EX and FY were also used to understand and
interpret the environment of deposition for each carbonate
unit. In this study, the geometries of seismic reections were
identied and interpreted for the purpose of understanding
the features seen on each interpreted sequence. Sequence
stratigraphy (i.e.: on-lap, top-lap and down-lap) was not
applied in this study. Three seismic geometries were
identied in Platforms EX and FY (Figure 3).
RESULTS AND DISCUSSIONS
Horizons and Geo-Seismic Units
Eleven horizons (H1-H11) have been interpreted in this
study, with ve geo-seismic units, labelled as Geo-Seismic
Unit 1 to 5 (Figure 2). These units were interpreted based
on a combination of continuity of amplitude displayed on
seismic data, well data (to the extent of its last penetrated
depth) and gathering of information from the available
literatures. The horizons were interpreted within a geo-
seismic framework and tied to the biostratigraphic markers
that correspond to time intervals from recent to 15.97 Ma
(Early Middle Miocene). Biostratigraphic markers for the
deeper horizons were not available for Platform FY and EX.
Well F6 from nearby platform within the Central Luconia
Province, together with the other deeper wells from Balingan
Province and North Luconia Province were incorporated to
interpret the lithology of the deeper horizons.
Horizons were interpreted from the sea bottom, labelled
as Sea Bottom (SB), followed by Horizon 1 (H1) to Horizon
11 (H11) in the deepest part of the seismic section. Horizon
11 to Horizon 7 have the same seismic pattern, and are
grouped as Geo-Seismic Unit 1 (GSU 1). The top of Horizon
7 to Horizon 5 shows the same seismic characteristics and
are grouped as Geo-Seismic Unit 2 (GSU 2). Geo-Seismic
Unit 3 (GSU 3) consists of the carbonate units interpreted
from Horizon 5 to Horizon 3. Geo-Seismic Unit 4 (GSU
4) is grouped from the top of Horizon 3 to Horizon 2. The
nal interpreted Geo-Seismic Unit 5 (GSU 5) is bounded
between Horizon 2 to the sea bottom (SB). The main internal
geometry within each geo-seismic units interpreted in this
study is shown in Figure 3.
Well Logs Interpretations
Gamma Ray Logs for Well EX-2, EX-3, FY-2 and FY-6
have been used in interpreting the lithology for Platform
EX and FY (Figure 4). Regrettably, Gamma Ray Logs are
Figure 1: (A) Location of Central Luconia
Province in the bigger South China Sea,
highlighting the carbonate Platform EX and
FY used in this research. (B) Top carbonate
map of Platform EX and FY with “No data
zone” labelled in between the two platforms.
“No data zone” represents the area without
seismic data (not provided for this research).

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79
Bulletin of the Geological Society of Malaysia, Volume 60, December 2014
not available from top to bottom of the wells. Most of the
logs and cores are tight within a depth of 1700m to 3300m.
All available wells are exploration wells, thus they only
penetrate to either the top of carbonate or inter-carbonate
section within Geo-Seismic Unit 3 of our interpreted data.
None of these wells penetrate into the deeper Geo-Seismic
Units 2 and 1. The lithological interpretation of Platform
EX based on Well EX-2 and EX-3 was made methods by
Gartner et al. (2004) and Zampetti et al. (2004b). In Platform
EX, some limestone breccias have been observed especially
towards the edge of the platform (Figure 5). Three limestone
breccias intervals with a minimum thickness of 1m were
recognized at the top of core in Well EX-3 (Gartner et al.,
2004). Based on gamma ray logs plotted in Figure 4, gamma
ray values ranging from 50 to 80API are common within
limestone intervals, indicating the existence of shales within
the carbonate intervals (contaminated carbonate). Clean
limestones have gamma ray values of 10-30 API. Lithological
interpretation of Platform FY remains condential within
industrial companies.
Structural Interpretation
Faulting affects the whole area covered by the entire
3D seismic survey for both Platforms EX and FY. Structural
interpretation was mainly based on the observable offset
of seismic reectors in the reectivity data or lineaments
observed in the variance (coherence) volume. A total of
50 faults has been interpreted within Platform EX and FY
(Table 1). These faults are mainly planar in nature within
Geo-Seismic Unit 3 and turn into listric shape in the deeper
seismic units. Most faults died out in the Geo-Seismic
Unit 3 but some continues to displace Geo-Seismic Unit
4. No major faults were observed within Geo-Seismic Unit
5, except for small-scale localised fractures. Observation
of seismic data between Platform EX and FY shows that,
there is potential for normal faulting in the saddle area that
separated these two platforms (no data zone in Figure 2).
All faults are normal faults, generally oriented N-S and
NE-SW which seems to govern the north-south elongation
of platforms in the south eastern part this province.
Fault Classes
Faults have been classied into few classes, based on
the units they displaced. The fault classes are summarized
in Table 1.
Platform EX
Platform EX grew on a few small scale domino system
fault blocks. These faults are interpreted to be related to
the last rifting stage of the South China Sea. Throughout
carbonate growth in Platform EX from Middle to Late
Miocene, small scale normal faults are observed. Some of
them were active during carbonate deposition, promoting
platform collapse especially in the Eastern and Western
Table 1: Faults classes in Platforms EX and FY.
Fault Class Criteria Platform EX Platform FY
ADisplaced all geo-seismic units except Geo-Seismic Unit 5.
Displacement in Geo-Seismic Unit 4 is small compare to the older
units. Associated with conjugate faults in the NE of Platform FY.
Not available for
interpretation 10 faults
BFault(s) that displaced Geo-Seismic Units 1 to 3. 14 faults 15 faults
CFault(s) that do not displace Geo-Seismic Units 3, 4 and 5. The
displacement(s) are only visible in Geo-Seismic Unit1 and 2.
Not available for
interpretation 6 faults
DEarlier fault(s) that only displaced Geo-Seismic Unit 1. Difcult to
image in seismic but might be related with rifting fault. Not available for
interpretation 5 faults
Figure 2: Seismic cross-section of Line
AA’ showing ve geo-seismic units (Geo-
Seismic Units (GSU) 1-5) and saddle area
(no seismic data zone) between Platform EX
and Platform FY. Both platforms have been
directly affected by normal faults.

S.N. Fathiyah JamaludiN, maNuel Pubellier & david meNier
Bulletin of the Geological Society of Malaysia, Volume 60, December 2014
80
Figure 3: Seismic geometry based on seismic reection pattern on
Platform EX and Platform FY.
Figure 4: Lithological interpretation based on core interpretation and gamma ray for Well EX2, EX3, FY2 and FY6.
ank of Platform EX (Gartner et al., 2004; Zampetti et
al., 2004a,b). The majority of the faults are closely spaced,
steeply dipping with dipping angles ranging from 40°-70°
with dominant fault system strikes from 015°-040° and
040°-070° (Figure 6).
Fourteen faults have been interpreted within Platform
EX, and they are concentrated in the eastern and western
margins of the platform. Structure interpretation in Platform
EX is only limited to the carbonate section of Geo-Seismic
Unit 3 reasonably due to the small size of the provided
seismic data, hence all the faults in this platform falls in
Class B Fault. Velocity pull-up effects were taken into
account when doing interpretation as extremely high velocity
had passed through the carbonate build-ups of Platform
EX and resulted in fold-look-a-like structure underneath
the carbonate build-up.
Faulting and fractures within this carbonate platform
had somehow created an outline for the platform as
they showed straight segments where the platform edge
coincides with the faults or are guided by the fracture
systems. Multiple sliding events had been interpreted to
be responsible for submarine landslide in Platform EX.
Zampetti et al. (2004a,b) had classied patchy, chaotic and
hummocky seismic geometry to describe slide deposits on
the eastern and western anks of Platform EX. Slide scars
were represented as sharp discontinuities forming crescent-
shaped embayments associated with seismic chaotic bodies
of slide deposits.

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Platform FY
Faulting in Platform FY is relatively more pervasive
compared to faulting in Platform EX. Most of the faults cut
through the carbonate platform. Some of the fault(s) were
initiated earlier before the carbonate started to deposit on this
platform and continued to move slowly during deposition of
carbonate, making a syn-depositional faulting event. Sets of
conjugate faults can be seen at the edge of Platform FY (in
NE of the platform). In the northern part of Platform FY,
the faults are oriented in ENE-SW direction while in the
southern part, the faults are oriented about N-S direction
(Figure 7a). The dip angles of these faults are ranging from
40°-80°. Different fault directions within Platform FY might
be indicative of a localised strike-slip fault in this area with
an approximately N-S oriented basement fault, promoting
E-W extension.
Conjugate faults in Platform FY have similar dips of
approximately 30°-40°, oriented NE-SW and NW-SE. A
detailed inspection within Geo-Seismic Unit 3 carbonate
shown that Fault Class B and C develop various numbers of
conjugate fault systems, crossing each other and developed
X-patterns within the carbonate unit(s). Some of the faults
Figure 5: Wells EX2 and EX3 in Platform EX. (A) The wells location on the surface timeslice. (B) Crossline 1, showing Well EX3
penetrates to the depth of approximately 1960m. (C) Inline 2, showing Well EX2 penetrates to the depth of approximately 2090m. (D) Core
images of limestone breccia in the upper part of Well EX3, consist of grainstone, packstone and wackestone (modied after Gartner et al.,
2004). (E) Core images of argilaceous limestone at the base of Well EX2. Exact depth is unknown. (modied after Warrlich et al., 2010).
(F) Core sample showing dolo-mudstone in core of EX3. Exact depth is unknown (modied after Warrlich et al., 2010). (G) Thin section
from Well EX2 showing foraminifera with platy corals in lime-mudstone. Exact depth is unknown (modied after Gartner et al., 2004).
failed to create conjugate systems and later changed into
distributed branching faults, creating positions of mini
horsts and graben patterns within Geo-Seismic Unit 3 that
become the templates for the reefs to grow. These branching
conjugate normal faults within the carbonate units were
tilted and rotated and provided changes in the environment
of deposition from reef build-up to lagoonal facies. This
is proven by fossil assemblages such as corals, corraline
algae, echinoderms and operculina found in Well FY-6. The
fossil assemblages represent shallow open marine close
to reef slope as the possible environment of deposition in
Platform FY (Figure 8).
CONCLUSION
Detailed seismic interpretation of Platforms EX and
FY in the Central Luconia Province has shown well
dened carbonate build-ups that had been affected by syn-
depositional faulting during their growth. Both Platforms
EX and FY have ve geo-seismic units (1-5). The older
geo-seismic units 1 and 2 are dominated with older listric
normal faults. Their displacements in each of the geo-seismic
units are small (to seismic scale), suggesting that these faults

S.N. Fathiyah JamaludiN, maNuel Pubellier & david meNier
Bulletin of the Geological Society of Malaysia, Volume 60, December 2014
82
Figure 8: Environmental changes from
lagoon (back reef) to reef at/front, observed
in seismic data of Platform FY. The changes
in environment of deposition were due
to template provided by the branching
conjugate faults system in carbonate units
of Platform FY.
Figure 6: Faulting in Platform EX.
Interpreted faults on the section (cross-section
K-K’) and horizontal (coherence (variance))
slice at time 1515ms. The faults are dipping
40°-70°. Faulting stopped during the nal
stage of carbonate deposition and hardly
affected the clastic layers of Geo-Seismic Unit
4 above the platform. Coherence (variance)
slice shows that the top carbonate (green
contour) is bounded by faults on both anks.
Figure 7: Fault trends in Platform FY. (A)
Faults are dominated by two sets. Set 1:
Oriented in N-S direction (red colour) and Set
2: Oriented in ENE-WSW direction (green
colour). (B) Fault trends as observed in the
cross-sections UU’ and YY’.

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Bulletin of the Geological Society of Malaysia, Volume 60, December 2014
formed during or just after the nal rifting stage of the
South China Sea. Some of the older listric faults continue
to move during deposition of Geo-Seismic Unit 3 that is
the main carbonate depositional unit in Platforms EX and
FY. Slight movement within the carbonate depositional unit
had either encouraged the reefs to grow on the topographic
highs such as at the tip of a faulted block, or promoted
sub-marine landslides as what we observed in the anks of
Platform EX. In general, syn-depositional faulting within
carbonate platforms in the Central Luconia Province had
provided templates for the reefs to nd suitable bases for
them to grow during Miocene time. However, at the same
time, syn-depositional faulting during carbonate deposition
had led to reef collapse together with other environmental
factors such as storm and wind directions. In conclusion, the
branching conjugate normal faults and older listric faults in
the carbonate deposits might serve as potential hydrocarbon
traps and migration paths within the carbonate reservoirs
in the Central Luconia Province, which can be useful for
exploration revisits.
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