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VI-1
VI. WEST NATUNA BASIN
VI.1 Introduction
The Location of West Natuna Basin is in the northern tip of the
Indonesia Island
Arc System, which was developed as an intra-continental rift
basin within the
Sunda Platform. The Eocene to Oligocene extensional phase and
Miocene to
Present day contraction and inversion motivated the basin
formation.
The boundary of the West Natuna Basin to the north is Khorat
Swell, which is a
south-dipping monocline basement high, however, to the south is
Sunda Shelf
and to the east is Natuna Arch. This Natuna Arch is a northern
protrusion of
Sunda Shelf separates the West and East Natuna Basins (Figure
1). The
periodical emergence of this ridge along with Khorat Swell
occurred during
Tertiary (White & Wing, 1978), therefore supplied the source
of sediments for
the two adjacent basins. At the northwest it opens to the NW-SE
elongated
Malay Basin, and at the southwestern tip it opens to a smaller
E-W elongated
Penyu Basin. The Malay Basin is separated from the Penyu Basin
by Tenggol
Arch, a ridge extruding east to the western part of the West
Natuna Basin.
VI.2 Regional Geology
VI.2.1 Tectonic Setting
There are several main structural elements can be distinguished
in the West
Natuna Basin such as Anoa Graben, South Kakap Graben, Northern
Central
High, Southern Central High and the Anambas Graben. The West
Natuna
structurally comprises of two main grabens, these are the North
Kakap Graben
and the South Kakap Graben (Figure 2).
Eocene-Oligocene Extension
During Eocene to Oligocene age was characterized by SW-NE
half-grabens
rifting phase and influenced by a right lateral stress regime,
which closely
related to the opening of the South China Sea in relation to the
formation of the
NNW-SSE orientation and led to development of pull-apart basins.
These
grabens are classified as an intercontinental rift system that
formed in the
Sunda Shelf. The basement high of Khorat Swell within the graben
systems and
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VI-2
the metamorphic/plutonic complex of the Natuna Arc were the
provenance of
sediments for both East and Natuna Basins.
Middle Oligocene to Early Miocene Tectonic Quiescence
Middle Oligocene to Early Miocene Age represented period of
tectonic
quiescence of the post-rifting phase (Gingger et al., 1993). The
deposition of
The Keras and Upper Gabus Formations took place during this
period.
However, the following deposition of Barat Formation was
categorized as post
rift as well as syn-rift sediment.
Miocene and Pliocene Compression
In the Early to Middle Miocene, the east block of China
relatively moved to the
southeast approaching the Sundaland, resulted in the opening of
the South
China Sea. The episode activated the right lateral faults in the
Thai Basin,
caused of changing of an extension regime to a compression
regime, which led
to folding in the Malay basin and activated NW-SE right lateral
faults and SW-
NE normal faults in the East Natuna Basin. In the West Natuna
Basin, the
normal faults were inverted into thrust faults along with a
local uplifting in the
northern part. During the Middle Miocene, the Indochina-Sunda
block rotated
resulted in a regime changing from compression to extension and
activation of
right lateral faults. This episode expressed in the form of a
braided river system
flowed into a lacustrine environment in the West Natuna
Basin.
The Middle Miocene right lateral motion along the NW-SE fault
system was
more intense; it resulted in the uplifting of the hanging wall
close to the fault.
The grabens were inverted into faulted fold structure (Sunda
Fold Type) and
subsequently were followed by an erosion of the Barat, Pasir
Formation and
Arang Formations.
VI.2.2 Stratigraphy
Pupilli (1973) was the first one who compiled the stratigraphic
framework of
both West Natuna Basin and East Natuna Basin based on
lithostratigraphy. The
Tertiary succession of West Natuna can be subdivided into four
(4) major
tectonostratigraphic megasequences such as syn-rift, post-rift,
syn-inversion,
and post-inversion (e.g. Ginger et al., 1993, Phillips et al.,
1997).
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VI-3
Underlying the Tertiary succession of the West Natuna Basin is
the Cretaceous
basement, which is comprised of amphibolites (Pollock et al.
1984) as well as
intrusive acidic-type igneous rocks such as quartz-diorite,
granite, and
metamorphic rocks that comprise of chlorite-schist and
gneiss.
The earliest sediment deposited in the West Natuna Basin is
thought to be older
than Early Oligocene. Based on the dating of the diabase
intrusion that was
found in the basal sediment, Lama Formation is indicated to be
of Late Eocene
(?) to Early Oligocene age. The Lama Formation mainly comprises
fluvio-
deltaic, fluvial and alluvial fan sandstones deposits. The Lama
Formation
stratigraphically is conformably overlain by the Benua
Formation. This formation
consists of lacustrine shales deposit. Overlying Benua shales
are interbedded
sandstones and shales of fluvio-deltaic and fluvial sediment of
the Oligocene
Lower Gabus Formation. The sandstones are fine to medium grade,
abundant
plant debris, typically form thick, blocky or fining upward
units, and usually
massive.
The Middle to Late Oligocene of the Keras Shale was deposited
above the
Lower Gabus Formation in lacustrine environment. This Keras
Shale was then
gradually replaced by the interbedded sandstone and shale of
Upper Gabus
Formation. The sandstone of Upper Gabus Formation is fine to
very fine
grained, in blocky or fining upward units similar to the Lower
Gabus. This
formation was deposited in braided delta and lacustrine plain
environment in
Late Oligocene to Early Miocene time. Palynological data shows
that the
Oligocene/Miocene boundary is within the topmost part of Gabus
interval.
Above Upper Gabus, subsequently the Early Miocene Barat Shale
was then
deposited in lacustrine environment with the influences of
marine condition at
some places. This assigned age is based on the non-existence of
Oligocene
and older markers, and the possible occurrence of Florschuetzia
levipoli.
Following the deposition of Barat Shale, the entire Arang
Formation was
deposited in a shallow marine environment with fluctuations to
coal-swamp
dominated coastal plain related to basin inversion and relative
sea level
changes. Palynological evidence gives an Early to Middle Miocene
age for this
formation.
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VI-4
Muda Formation was deposited unconformably above Arang Formation
in
shallow marine environment. The Base Muda unconformity is widely
recognised
at West Natuna basin. The Muda Formation consists of mudstone,
shales and
sands. This formation is formed since Late Miocene until the
present time
(Figure 3).
VI.2.3 Depositional Setting
Syn-rift
The syn-inversion sediments consist of Lama and Benua Formation
with the
age of Late Eocene to Early Oligocene. The Lama Formation is
composed of
lacustrine, deltaic and alluvial fan deposits. The sediments
filled the grabens or
half-grabens. The sandstone layers within the sediments are
potential to be
hydrocarbon-producing layers (reservoir). The Benua Formation,
which
conformably overlies the Lama Formation, is dominated by
shale.
Post-rift
The post-rift sediment is formed by lacustrine and river
deposits. Sandstones of
the river deposit thicken toward the depocentre, and they were
developed as
meander and braided stream. The post-rift sediments are composed
of fluvial
and fluvio-deltaic deposits of the Lower Gabus Formation in the
lower part that
consists of sandstone and shale intercalations. The overlaying
sediment is the
lacustrine shale deposit of the Middle to Late Oligocene of the
Keras Formation.
The youngest post-rift sediment is composed of sandstones and
shale
intercalations of Upper Gabus Formation that was deposited in
lacustrine,
braided stream and deltaic environments during the Late
Oligocene to Early
Miocene age. It was deposited conformably above the Keras
Formation.
Syn-inversion
The Late Oligocene to Late Miocene compressional tectonic
inversion led to the
uplifting of the grabens and half-grabens of the North and South
Kakap. During
this inversion phase, subsequent younger sediments were
deposited. These
syn-inversion sediments are: Early Miocene lacustrine shale of
the Barat
Formation, shallow marine sandstones of the Pasir Formation, and
the shore,
swamp and shallow marine deposits of the Arang Formation.
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VI-5
Post-inversion
The late Miocene to Recent deposit of mudstones, shales and
sandstones of
Muda Formation unconformably overlies the Arang Formation. The
sediment
was deposited in a shallow marine environment.
VI.3 Petroleum System
VI.3.1 Source Rocks
Based on the result of pyrolysis analysis, which specify the
abundance of
vitrinite, the occurrence of amorphous type kerogen, as well as
concentration of
liptinite type material that sufficient for oil prone kerogen,
it was believed that
lacustrine shale of Barat Formation acted as hydrocarbon source
for Kakap
Block (Mc Williams, 1983 & Pallock, 1984).
The geochemical studies of two wells in North Kakap Blok
identify gas and oil
shows present in thousand feet below the Barat Shale Formation.
In more
detail, the studies were performed after 1988 (i.e. Core Lab,
1988, Dembicki,
1989, and Spagnuolo, 1991) suggested that Eocene to Oligocene
shale of
Benua/Lama, Keras and Barat Shales were more potential to act as
source rock
for hydrocarbon in this area. Kakap oil was generated from
type-I kerogen of the
Eocene to Oligocene lacustrine shale (Benua/Lama and Keras
Shale;
Spagnuolo, 1991). Top of oil window occurred around 277o F at
the depth of
9000 ft, while Hodgson and Chalik (1993) study shows that oil
window level was
occurred at the depth of 7000 ft.
The Lower Gabus source rock, which is composed of low-medium TOC
value
occurs locally within the mudstones, thin carbonaceous
sandstones, and coal.
The higher vitrinite and inertinite composition as can be
identified in a certain
well indicates that the organic material was derived from a
humid land-spatial
and humid environment.
VI.3.2 Reservoir and Seal
The existing reservoir rocks are sandstones of Lama/Benua
Formation that
porosity ranges from 7 % with permeability of 0.1-2.3 md. The
other
sandstones, which belong to Lower Gabus Formation has an average
porosity
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VI-6
of 22% and produced oil and gas in Anoa field. Sandstones of
Keras Formation
have porosity of 16-23% and show gas at well AQ-1X and
Anoa-1.
Barat and Arang Formations predominantly comprise of shales,
therefore, they
act as suitable regional seals/cap rocks, as well as
intraformational shales. The
intensive thrusting of the graben during the inversion phase
resulted in faults
seal formation. The compression makes the fault gap became
tight.
VI.3.3 Trap and Migration
Since the depocentre of graben in West Natuna Basin was inverted
into Sunda
type fold, the favourable and easy to trap is anticline.
Meanwhile, sandstone
layers of the syn-rift sediments in the depth can be taken as
stratigraphic traps.
The other possibility is the combination of those two. The time
of hydrocarbon
migration was parallel or might be slightly earlier than the
first inversion time,
which are Oligocene time. The hydrocarbon migration could be in
two
directions. First, is up dip/lateral migration, which is from
source rocks to the
reservoir rocks. The second is vertical migration, which is from
the source rocks
to reservoir rocks through faults pathway, vertically.
VI.4 Hydrocarbon Plays
In West Natuna Basin is dominated by three plays (Figure 4):
A series of N-S trending normal splay faults developed along the
northern and
southern sides of the South Kakap Wrench Fault. Structural
closures are on the
footwall sides of these splays. The wrench zone is a focus for
hydrocarbon
migration from syn-rift and post-rift sequences. Sometimes,
hydrocarbons are
trapped in multiple, stacked and independent pay zones. Trapping
is dependent
on fault seals, wirh top seal provided by both regional shale
units as well as the
abundant intra-formational shales. The closures are small at
each level but
stacking provided the potential for volumes. The potential the
prospects is small
but low risks and commercial viability of prospects with
reserves > 2.0 MMBBLS
has established. The Play is mature for some procpects.
The syn-rift play has been identified in the west-central part
of the South Kakap.
Oil and gas was discovered in fluvio-deltaic sandstones of the
U. Lama
Formation.
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VI-7
The Sunda Fold Play in North Kakap has many structures. There
are prospects
whilst others are given lead satus. The structural inversion was
more intense in
North kakap resulting in uplift and erosion of regional sealing.
The potential
kitchen area in the North Kakap has shown that the main phase of
hydrocarbon
generation and migration pre-dates formation of most of the
recognized traps.
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VI-8
References
Daines, S.R., 1985 Structural History of The W Natuna Basin and
The Tectonic
Evolution of The Sunda Region, Proc. 14th Ann. Conv.,
Indonesian
Pet. Assoc., p. 39-61.
Ginger, D.C., Ardjakusumah, W.O., Hedley, R.J. & Pothecary,
J., 1993,
Inversion History of the West Natuna Basin: Examples from the
Cumi-
Cumi PSC, Proc., 22nd Ann. Conv., Ind. Pet. Assoc., p.
635-658.
White, J.M. & Wing, R.S., 1978, Structural Development of
the South China Sea
with Particular Reference to Indonesia, Proc., 7th Ann.
Conv.,
Indonesian Pet. Assoc., p. 159-177v. 87, p. 253-277.
Wongsosantiko, A. & Wirojudo, G.K., 1984, Tertiary Tectonic
Evolution and
Related Hydrocarbon Potential in the Natuna Area, Proc., 13th
Ann.
Conv., Indonesian Pet. Assoc., p. 161-183.
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FIGURE1. Location Map of West Natuna Basin
Pontianak
N
Kilometers
0 250
West Natuna Basin
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NATUNAISLAND
KHORAT SWELL
PENYUBASIN
ANABASGRABENSOUTHERN
CENTRAHIGH
SOUTHKAKAPGRABEN
?
?
ANOAGRABEN
NORTHERNCENTRALHIGH
MALAYBASIN
SUNDASHELF
106 Eo 107 Eo 108 Eo
5 No
6 No
7 No
105 Eo104 Eo
4 No
0 80KM
N
106 Eo 107 Eo 108 Eo105 Eo104 Eo
5 No
6 No
7 No
4 No
STRUCTURAL/BASINALLOW
EASTMALAYAMICROPLATE
STRUCTURALHIGH
LEGEND:
NORMALFAULT
THRUSTFAULT
FIGURE 2. Tectonic Elementof West Natuna Basin
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FIGURE 3. Stratigraphy of West Natuna Basin
AGE/SERIES
0
1
5
10
15
20
25
30
35
40
PLEISTOCENE
45
50
55
P3
P4
P5
P6
P7
P8
P9
P10
P11
P12
P13
P14
P15P16P17
P18
P19
P20/N1
P21/N2
P22/N3
N4
N5
N6
N7
N8
N9N10N11N12N13N14N15
N16
N17
N18N19
N21
N22N23
N20
(0.8)(1.65)
(3.0)
(4.2)
(5.5)
(10.5)
(12.5)
(13.8)
(15.5)
(16.5)
(21.0)
(22.0)
(25.5)
(26.5)
(28.4)
(30.0)
(33.0)
(38.0)
(39.5)
(42.5)
(44.0)
(48.5)
(51.5)
(52.3)
(54.5)
(58.5)
GLOBALRELATIVECHANGEOFCOASTALONLAPVAILETAL(1977)
LANDWARD1.0
BASINWARD00.5
(36.0)
(37.0)
NATUNARIDGE
ITHOSTRATIGRAPHYL
CHRONOSTRATIGRAPHYOFTHEWESTNATUNABASIN
MUDA
U.ARANG
M.ARANG
L.ARANG
U.GABUS
M.GABUS
L.GABUS
SAMBAS/BENUASH
BARATSH
GAJAHSH
BASEMENT
R
S
S
S R
S
R
S Source R ReservoarS Seal
-
MUDA
ARANG
LOWER GABUS
ARANG
MUDA
LOWER GABUS
ARANG
BENUA/LAMA
BASEMENT
SUBTHRUSTTYPE
STRATIGRAPHICRELATEDTYPE
NORMALFAULTRELATEDTYPE
WRENCHRELATEDFOLDTYPE
SUNDAFOLDSTYPE
BENUA/LAMA
BASEMENT
000
5000
10000
15000
20000
HYDROCARBON PLAY CONCEPT OF WEST NATUNA
FIGURE 4. Hydrocarbon Play of West Natuna Basin
S
R
S
S SS
S
S
S
SS
S
SS
R
R
R
R
R
R
R
R
R
R
R
R
R
R R
R
S
S S
S
:PossibleGas
:PossibleOil
:Migration
LEGEND:S
R
S
Source
Reservoar
Seal