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

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).

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.

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.

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

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.

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.

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.

FIGURE1. Location Map of West Natuna Basin

Pontianak

N

Kilometers

0 250

West Natuna Basin

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

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