XIII-1

XIII. SALAWATI BASIN

XIII.1 Introduction

The Salawati Basin is a late Tertiary local depression, located in the

westernmost part of the Kepala Burung (Bird's Head), Irian Jaya (Figure 1). The

basin is presently bounded to the north by the Sorong Fault Zone which

separates the Australian Continental Plate to the south from the Pacific Oceanic

Plate to the north. It is separated from the Bintuni Basin by the Mio-Pliocene

Ayamaru High, where Miocene shelf carbonates crop out. Southward, the basin

is limited by the Misool-Onin Geanticline. The continuation of the Sorong Fault

Zone bounds the basin to the west.

XIII.2 Regional Geology

XIII.2.1 Tectonic Setting

The Salawati Basin is located in the tectonically complex Eastern Indonesia

region, where three major crustal plates impinge. The basin is a structural and

stratigraphic feature that began to develop on the northern margin of the

Australian Plate during Miocene time. Structural development of the basin is the

result of the complex interplay of these three major crustal plates. Left lateral

strike-slip movement along the Sorong fault is primarily responsible for the

present day structural configuration of the basin. These structural elements are

well defined at the surface, and seismically in the sub-surface (Figure 2).

East-west folds and complex faulting dominate the local tectonic pattern. Most

of the faults are northeast-southwest trending normal faults, which predominate

over the basinal areas. These faults are generally down-stepping to the

northwest across the basin into the depocenter on northern Salawati Island, as

a result of the transtensional pull-apart regime induced by movement along the

Sorong fault initiated during Late Miocene time. The most prominent of these

faults is the "Line 6" Fault (or series of faults) which run through the Sele Straits

and across the Salawati island. Although this fault has significant strike-slip

movement, the major effects are down to the basin which allowed rapid

deposition of the Pliocene Klasaman Formation, which is best illustrated by the

rapid westward increase in the thickness of the Pliocene Klasaman Formation

XIII-2

toward the depocenter. Movement appears to be diverted to a more neutral left-

lateral displacement as the fault's orientation shifts to a more east-west direction

and crosses the southern part of Salawati Island, which suggests movement

began in post-Miocene, Early Pliocene times. With the exception of some areas

near the Sorong Fault, such as just north of Trend's Arar Block, faults with

evidence of strike-slip movement are presumably conjugate shears related to

the left-lateral Sorong Fault.

The final movement of the Sorong Fault during Plio-Pleistocene time created a

series of northeast-southwest trending folds. These are located south of the

Sorong fault in the northern part of Salawati Island.

XIII.2.2 Stratigraphy

In general, the Salawati Basin can be grouped into fourth sedimentary regimes,

these are: 1) Pre-Carboniferous Basement, 2) Permo-Carboniferous Sediments,

3) Jurassic-Cretaceous Sediments, and 4) Tertiary Stratigraphy (Figure 3).

Pre-Carboniferous Basement

Kemum Formation

The Kemum Formation (Visser & Hennes, 1962) forms a basement block in the

central part of the Birds Head where it is bounded by the Sorong Fault Zone to

the north and the Ransiki Fault Zone to the east. To the south and southwest,

rocks of Late Paleozoic, Mesozoic and Cainozoic age, overlie the basement

block with angular unconformity.

The lower contact of the Kemum Formation is not exposed and the unit has a

minimum thickness of a few thousand metres. The age of the unit is based on

sparsely distributed Silurian graptolites and Devonian ostracods. A K-Ar age of

about 1250 my for a granodiorite pebble in a meta-conglomerate indicates a

Precambrian provenance. The Kemum Formation is intruded by Late

Carboniferous and Perm-Triassic plutons of the Anggi Granite, and by dykes of

basaltic or andesitic composition yielding Pliocene K-Ar ages.

The unit consists dominantly of low-grade metamorphic rocks comprising thinly

interbedded pelitic and psammitic layers with sedimentary textures and

structures typical of distal turbidites. The main rock types are slate, slaty shale,

XIII-3

argillite and metawacke; meta-arenite and meta-conglomerate are less

common. Thin intercalations of recrystallised limestone and dykes or sills of

metavolcanics are rare. A much less widespread sandy facies consists of locally

calcareous quartz-rich metawacke and meta-arenite and siliceous slate or

argillite.

Permo-Carboniferous Sediments

Aifam Group

The Aifam Group was defined by Pigram and Sukanta (1982) who upgraded the

original definition of the Aifam Formation of Visser and Hermes (1962). The type

area for the group is the Aifam River, a tributary of the Aifat (Kamundan) River,

in the central Birds Head.

The Aifam Group crops out in the Birds Head, southern Birds Neck, along the

southern margin of the Central Range and is known from a few petroleum

exploration wells. In the Birds Head region the Aifam Group crops out along the

south side of the Warsamson Valley, and as a belt extending eastwards from

the Aifat River to the Mios River. In the Birds Neck the Aifam Group is restricted

to thin metamorphosed slivers along the west flank of the Wondiwoi Mountains.

In the Warsamson Valley the Aifam Group is undifferentiated and consists of a

basal arkose overlain by well-bedded quartz sandstone, calcareous shale and

shaley limestone in turn overlain by black shale. The group appears to rest on

the Early Carboniferous Melaiurna Granite. However, a sample of limestone in

float yielded thelodont fish scales of Devonian aspect (Young and Nicoll, 1979).

In the central Birds Head the Aifam Group is divided into three formations. The

lowest is the Aimau Formation and consists of basal thin red conglomerate,

sandstone and shale with silicified wood, overlain by a sequence of well-bedded

siliceous sandstone and greywacke interbedded with shale, siltstone and grey

limestone. The overlying Aifat Mudstone consists of black calcareous mudstone

with abundant concretions, minor dirty limestone and rare thin quartz sandstone

beds. The uppermost Ainim Formation consists of interbedded carbonaceous

silty mudstone, quartz sandstone, greywacke and siltstone, and contains coal

seams up to 1 m thick. The Aifam Group rests unconformably on the Siluro-

Devonian Kemum Formation.

XIII-4

The Aifam Group ranges in age from Middle Carboniferous to Late Permian at

the type locality. Numerous fossils throughout the group include silicified wood,

plant fossils, conodonts, corals, bryozoa, brachiopods, ammonoids, fusulinids,

crinoids and a single trilobite.

Jurassic-Cretaceous Sediments

Kembelangan Group

The Kembelangan Formation was originally defined Visser and Hermes (1982)

and raised to group status Pigram and Sukanta (1982). The Kembelangan

Group crops out throughout eastern Birds Head, Birds Neck and Central Range.

In the Birds Head the Kembelangan Group contains of the Jass Formation

(Pigram & Sukanta, 1982) where consists of black to brown partly calcareous

and mudstone, lithic sandstone, muddy sandstone and limestone with a little

quartz sandstone, and quartz or polymictic conglomerate. The maximum

thickness is approximately 400 m.

In the Birds Neck the Kembelangan Group is exposed in the cores of tight

anticlines of the Lengguru Fold Belt. In the west and centre the group consists

of alternating sandstone and mudstone which are progressively meta-

morphosed in an eastward direction. Along the eastern coast of the Birds Neck

and in the islands offshore in the Transition Zone between Continental and

Oceanic Provinces, the Kembelangan Group is dominated by mudstone which

has also been metamorphosed to slate.

In the Central Range around the Wissel Lakes, the Kembelangan Group

consists of alternating sand and shale in the south and a sequence dominated

by mudstone and partly metamorphosed in the north, largely in the Transition

Zone between the Oceanic and Continental Provinces. The same nomenclature

that was applied to the formations in the Birds Neck has been used in the

southern region. The Middle to Upper Jurassic Kopai Formation consists of light

grey quartz sandstone which is argillaceous, glauconitic and calcareous,

interbedded with black to grey silty mudstone, minor conglomerate, calcarenite,

calcilutite and greensand.

XIII-5

Tertiary Stratigraphy

Waripi Formation

The Waripi Formation (Visser & Hermes, 1962) out crop in the western

mountains of the Central Range from where it extends westwards into the

southern extremis of the Birds Neck.

The formation consists of well-bedded, sandy oolitic calcarenite and

biocalcarenite, calcareous quartz sandstone and red-brown oolitic

biocalcarenite. The limestone commonly dolomitic and in many places contains

foraminifera.

The maximum estimated thickness of the Waripi Formation is 700 m in the

upper Baupo River; Visser and Hem (1962) estimate a thickness of 380 m at

the west end its distribution range but state that the formation thick and

disappears in eastern Irian Jaya.

The Waripi Formation contains no age-diagnostic fossil. The Waripi Formation

is probably of Paleocene age. The clastic detritus in the formation was probably

derived from the south; the oolites suggest a shallow carbonate bank and the

formation was probably deposit on a very shallow shelf.

Faumai Limestone

The Faumai Limestone (Faumai Formation of Visser & Hermes, 1962) can be

recognized in outcrop only in the eastern part of the Birds Head, where it is

overlain by the clastic Sirga Formation and is separated by it from the later,

Miocene part of the New Guinea Limestone Group. The outcrop of the Faumai

Limestone extends from the eastern side of the Ayamaru Plateau eastwards to

the coast of Cenderawasih Bay.

The Faumai Limestone is a well-bedded arenacous limestone consisting of

calcarenite which is commonly muddy. It is about 250 m thick. The limestone

represents carbonate bank and shoal deposits. It contains abundant larger

foraminifera which date it as Ta to Tb or middle Eocene to Oligocene. Lateral

equivalents of the Faumai Limestone are present in the New Guinea Limestone

Group throughout western Irian Jaya, e.g. in the Yawee Limestone, but the

XIII-6

limestone is recognized as a lithostratigraphic.unit only in the Birds Head where

it is capped by the clastic Sirga Formation.

Sirga Formation

Oligocene of Sirga Formation found subsurface in the Salawati Basin west of

the ayamaru Plateau. The predominant rock types in the Sirga Formation range

from siltstone and mudstone in the west and south to quartz sandstone and

conglomerate in the north and east. It appears to have been derived from a

landmass occupying the present-day outcrop of the Kemum Formation, and to

form a lens-like sheet thinning both north and south from a maximum thickness

of 200 m in the Aifat River. Large and small foraminifera in the Sirga Formation

yield an early Miocene age. The formation is probably transgressive and

deposited in shallow water as sea-level rose after the world-wide drop recorded

by Vail and Mitchem (1979) late Oligocene times.

The Sirga Formation lies conformably on the Faumai Limestone and

disconformably on the Aifam Group near the Ayamaru Plateau. It is conformably

overlain by Kais limestone or, in some exploration wells in the Salawati Basin,

by Klamogun Limestone.

Kais Limestone

The outcrop of the Eocene of Kais Limestone (Kais Formation of Visser &

Hermes, 1962) forms a broad belt crossing the Birds Head from west to east. It

consists of calcarenite and muddy calcarenite; the patch reefs of the Salawati

Basin and the southern margin of the Ayamaru Plateau are formed largely by

boundstone or reef material in the position of growth. The thickness of the

limestone changes considerably, over short distances; the maximum reported

thickness is 557 m.

The Kais Limestone represents a reef complex comprising platform and patch

reef facies. The patch reefs are largely confined to the Salawati Basin. The age

of the Kais Limestone is most probably early to middle Miocene. The Kais

Limestone rests conformably on the Sirga Formation and unconformably on the

Aifam Group. It is laterally equivalent to the Klamogun Limestone, Sekau

Formation, and Klasafet Formation.

XIII-7

Klasafet Formation

The Klasafet Formation (Visser & Hermes, 1962) crops out discontinuously

across the Birds Head from west to east, though it appears to be almost

continuous subsurface in the Salawati Basin at least. The formation consists of

massive to well-bedded marl, micaceous and calcareous siltstone and a little

limestone.

Visser and Hermes estimate the thickness of the Klasafet Formation to be

approximately 1900 m. The formation is 500 m thick in the Klamono oil field.

The Klasafet Formation is contemperaneous with the Kais Limestone and is a

facies deposited in deeper water below wave-base in the same basin in which

abundant reefs grew and merged in shallow water to form the patch reefs and

platforms of the Kais Limestone. The marly sediment eventually built up to the

level of the reefs and smothered them. Visser and Hermes (1962) note that the

youngest sediments; shallow-water deposits and that a southward decrease

clastic material in the Klasafet Formation indicate northern source for the

material. The age of the Klasafet Formation is early to middle Miocene; it may

range into the late Miocene. The Klasafet Formation overlies and is probably

also partly equivalent the Klamogun Limestone. The Klasafet Formation seals

the oil-bearing patch reef of the Salawati Basin.

Klasaman Formation

The Klasaman Formation was defined by Visser and Hermes (1962). It crops

out over a large area of Salawati Island in the western Birds Head and along the

southern side of the Ayamaru Plateau as far east as the Kais River. The

Klasaman Formation has been penetrated in many wells drilled in the Salawati

Basin.

The late Miocene to Pliocene Klasaman formation consists of interbedded

sandy, partly calcareous mudstone and muddy, partly calcareous sandstone. In

the upper part conglomerates and lignite seams occur. Minor molluscan

coquina beds are also present. Conglomerates are more common to the north.

The maximum thickness is about 4500 m. Benthonic and pelagic foraminifera,

molluscs and bryozoa are the most common fossils.

XIII-8

The Klasaman Formation rests conformably on the Klasafet Formation to the

south and disconformably on it in the north. The Kalasaman Formation is

overlain unconformably by the Quaternary Sele Conglomerate.The Klasaman

Formation is an immature source rock. Some of the coarse clastic beds near

the northern parts of the Salawati Basin may have reservoir potential.

Sele Conglomerate

The Sele Conglomerate was defined by Visser and Hermes (1962). It crops out

on Salawati Island and in the western Birds Head, east of Sorong. It consists of

polymictic conglomerate with thin claystone and sandstone intercalations. Plant

remains are common. Maximum thickness is 120 m. No diagnostic fossils have

been found the formation and is therefore younger than Pliocene.

XIII.3 Petroleum System

XIII.3.1 Source Rock

The potential source rock base on geochemical analyses indicated that the

source rock is rich in fresh brackish water algae and higher plants and the oil

was generated at about middle maturity level. The gas chromatography

analyses suggest that the source of the crude is generated from a mixture of

terrestrially derived organic matter and bacterial bodies (algae), deposited

under rather acidic, low oxygen conditions. Generation of oil is at thermally

mature levels. In the Salawati Basin several formation, which were deposited in

shallow marine or paralic environments could be considered as potentially

hydrocarbon source rock.

Klasaman Shales

The Plio-Pleistocene Klasaman shale contains high levels of organic matter, but

they are immature in most part of the basin. These shales are unlucky to

produce any significant hydrocarbon.

Klasafet Shales

In the deeper part of the basin, where the Klasafet is mature based on the

Lopatin subsidence profile, the peak of oil generation (TTI 75 eq. Ro = 1 %) at

the present time, is at around 250o F (100oC) or 10,000’ depth.

XIII-9

Sirga Shale

The Sirga shale has been penetrated in only a few wells. They contain type I

and II kerogens in one well and type IV in others. They are partly mature in the

basin. Part of the oil in the Salawati Basin may be sourced from this formation.

XIII.3.2 Reservoir Rock

The Miocene Kais Formation, where porous reefal carbonate facies developed,

is the primary reservoir target in the Salawati Basin. According to Robinson and

Soedirdja (1986), the reefs grew on a widespread carbonate platform during

transgressive episodes in the Miocene and in the southern part of the Salawati

basin; three reef stages can be recognized. The reefal carbonates consist of

bioclastic packstones and wackestones with numereous biohermal and

biostromal build-ups.

XIII.3.3 Seal Rock

Intraformational shale of the Kais Formation suggestively form seal for

hydrocarbon accumulation in the Salawati Basin.

XIII.3.4 Migration and Trapping Mechanism

In the Salawati basin, the Neogene section may act as potential source where

time and depth of burial have slowed maturity to be reached. Updip lateral

migration is provided in a radial from away the “kitchen area” covering the Sele

strait and northern Salawati Island. In case of oil generated in the Aifam Group,

upward migration could be taken places vertically through fault in to the

overlying Kais reef traps. The structures of the oil fields in the Salawati basin

are mostly associated with normal fault which have connected the Permian

sequence with the Kais reservoir traps.

XIII.4 Hydrocarbon Play

The Miocene Klasafat calcareous fine clastics were regarded as the best

potential source rocks to generate hydrocarbons in the Salawati Basin. Most of

the produced oils in the basin are from the slightly anoxic calcareous marine

facies, which have a significant terrestrial kerogen component and were

generated at moderate thermal maturity levels. These hydrocarbons are

XIII-10

believed to have migrated through and been trapped in the Miocene carbonates

of the Kais reefs very recently, with hydrocarbon generation and expulsion

occuring only in the last few million years. This simple concept of hydrocarbon

migration assumed the possibility of normal faults down-stepping to the basin,

being conduits for vertical hydrocarbon migration from the Kais carrier beds into

the younger reservoirs (Figure 4). Conceptually the Pliocene carbonate build-up

play type was considered a good potential reservoir to trap such vertically

migrated hydrocarbons.

XIII-11

References

Pieters P.E., Piagam C.J., Trail D.S., Dow D.B., Ratman N., dan Sukamto R.,

1983, The Stratigraphy of Western Irian Jaya, Proceed. Indon. Petrol.

Assoc.12th Ann. Conv. pp 229-261.

Phoa R.S.K., Samuel L., 1986, Problem of Source Rock Identification In The

Salawati Basin, Irian Jaya, Proceed. Indon. Petrol. Assoc.15th Ann.

Conv. pp 406-421.

Djumhana N., Syarief A.M., 1990, Pliocene Carbonate Build-Ups A New Play in

the Salawati Basin, Proceed. PIT XIX IAGI, Bandung, pp 119-135.

FIGURE 1. Location Map of Salawati Basin

Jayapura

130Eo 1 3 5 Eo 1 4 0 Eo

130Eo 1 3 5 Eo 1 4 0 Eo

13030’E 131 0 0 ’ E 13130’E

Kemun High

Kilometer

0 10 20

N

FIGURE 2. Regional Tectonic of Salawati Basin

FIGURE 3. Regional Stratigraphy of Salawati Basin

AGEPLEISTOCENE

PLIOCENE

MIOCENE

OLIGOCENE

EOCENE

PALEOCENE

CRETA-CEOUS

JURASSIC

SIRGA

FAUMAI

WARIPI

UPPERKEMBELANGAN

MIDDLEKEMBELANGAN

LOWERKEMBELANGAN

U

L

U

ML

TRIASSIC

PERMIAN

CARBONIFEROUS

DEVONIAN KEMUM

AIMAU

AIFAT

AINIM

SELE

KLASAMAN

KLASAFET

KAIS

FORMATION LITHOLOGY NSDEPOSITIONAL

SYSTEMREGIONAL

EVENTHYDROCARBON

OCCURRENCE

CLASTICINFLUXFROMINVERTEDHIGHS

CARBONATPLATFORMWITHREEFS

CLASTICINFLUXFROMLOCALHIGHS

CARBONATEPLATFORMWITHLOCALLYREEFS

SHALLOW MARINE CLASTICINFLUXOPENMARINEFACIES(DEEPMARINE)

MIDDLETOOUTERNERITICFACIES

NONMARINETOTRANSITIONFACIES

COASTALPLAINSEDIMENTS

SHALLOWMARINESEDIMENTS

FINALTECTONIC(SORONGFAULTMOVEMENT)

OLIGOCENEUPLIFTCHANGEOFPACIFIXPLATEMOVEMENT

ENDOFCRETACEOUSUPLIFT(INITIALCOLLISIOANOFTHENORTHERNMARGINOFAUSTRALIANANDTHEPACIFIXPLATES)

TERUMBU-1

SALAWATIDEVONENERGYFIELDS

KLARI-1

MERAKEMAS-1

WIRIAGARDEEPPLAYFIELD

WIRIAGARFIELD

WIRIAGAR,VORWATA,UBADARI,ROABIBAFIELD

WIRIAGARFIELD

SR

SS

S

RR

R

R

R

S

Legend

S Source R ReservoarS Seal

FIGURE 4. Hydrocarbon Paly Model of Salawati Basin

PRETERTIARYGRANITE

KAIS KLASAFET

KLASAMAN

LINE 6FAULT

SALAWATIISLAND ARAR BLOCK

“TERUMBU”

PLAY CONCEPTWEST EAST

KLASAMAN

S

RS

R

R

RS

S

R

S

Source

Reservoar

Seal

Legend