PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Fifteenth Annual Convention, October 1986

PROBLEMS OF SOURCE ROCK IDENTIFICATION IN THE SALAWATI BASIN, IRIAN JAYA

Rudy S.K. Phoa* Luki Samuel**

ABSTRACT

The Salawati Basin is the most prolific oil basin in Eastern Indonesia. More than 500 exploratory and developrrle~~t wells have been drilled and around 300 tnilliorl barrels of oil produced from 15 fields.

Oil typing indicates that tlte oils of the Salawati basin are similar and were generated from the same source rock type. Recent geochemical analyses suggest that the oils were sourced from kerogen rich in fresh-brackish water algae and higher plants with a significant input of marine type 11, sulphur rich kerogen.

The Miocene marine Klasafet and Klamogun shales and carbonates have been regarded as the source rocks for the oils now trapped in reefs of the Miocene Kais Forr~iation. However it is possible that the oils have more than orle source. Potential soul-ce rocks in the Salawati Basin 21-e the Klasafet and Klamogun Formations (Miocene), Sirga Formation (Oligocene), Ken? be larig an (CI-et aceous-.I urassic), Tipuma (Triassic) and Aifa~n (Per~nian) Forrnations.

The oil of the Salawati Basin may corlsist of a 11iixt~11-e of oils generated from these formation and migrated into the Kais Formation during the Plio Pleistocene.

I. INTRODUCTION

Some oil was also produced by NNGPM from the same Miocene carbonates in Wasian (1939) and Mogoi (1941) fields (Bintuni Basin).

NNGPM recognized that the oil accumulations were present in reefal build-ups but the exploration techniques available at that time were unable to locate them (Visser and Hermes, 1962). In 1960 the Consortium withdrew from Irian Jaya.

In 1970, Petrorner Trend Corporation signed a Producti- on Sharing Contract with Pertamina. By using modern geo- logical and geophysical techniques, a concerted effort of exploring for carbonate build-ups paid off with the dis- covery of the Kasirn field in 1972. Subsequent exploration using digital seismic resulted in the discovery of Jaya, Walio (Rednlond 61 Koesoernadinata, 1976) and Kasim Utara in 1973, Kasirn Barat in 1974, Cendrawasih in 1976, Arar in 1977, Moi in 1979 and Klagagi and Klalin recently (Fig. 2).

Pertamina Unit EP-V, who retained the Klamono and Linda-Sele Blocks, added oil production from subsequent discoveries in South Sele (1975), Linda-A (1977), Linda-C (1 985) and Linda-G (1 985).

A major discovery from the same carbonate was also made by -PHILLIPS on Salawati Island.

The Miocene marine Klasafet shales and Klamogun - The Salawati Basin (Fig. 1) is a prolific Indonesian oil carbonates have been regarded as the main source rocks

basin, which has been intensively explored. More than which have generated the oils in the Salawati Basin. How- 500 exploratory and development wells have been drilled. ever, recent geochemical analyses indicate that the oils

The current oil production from the eastern part of may have more than one source. Indonesia comes mainly from this basin (35,000 BOPD). This paper intends to discusses the problems of source It has produced in excess of 300 MMBO and peak produc- rock identification for the oils found in the Salawati Basin. tion was reached in 1978 with an average flow of 121,000 BOPD. Oil in the Salawati Basin is mainly produced from the Miocene reefal carbonates of the Kais Formation.

Exploration for hydrocarbons in Irian Jaya (Salawati and Bintuni Basins included) was initiated in 1935 by NNGPM, a consortium of BPM, STANVAC and Far East Pacific Investments. Twenty-five exploratory wells were drilled resulting in five discoveries of which the Klamono Field (1936), in the northern part of the Salawati Basin, was considered commercial. This field has produced more than 30 MMBBLS of low API gravity oil (19O API) from highly porous Miocene limestone at an average depth of only 100 meters.

* Petromer Trend, Jakarta, Indonesia ** PERTAMINA EP, Jakarta, Indonesia

11. GEOLOGY

The Salawati Basin is a late Tertiary local depression, located in the westernmost part of the Kepala Burung (Bird's Head), Irian Jaya (Fig. 3).

The basin is presently bounded to the north by the Sorong Fault Zone which separates the Australian Con- tinental 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 carbo- nates 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.

In the Salawati Basin, exploration for hydrocarbons has been chiefly restricted to the Late Tertiary sequence.

© IPA, 2006 - 15th Annual Convention Proceedings, 1986

Seismic surveys and most of the wells in the basin were designed to evaluate the Miocene Kais Formation. Only a few wells have penetrated the entire sedimentary sequen- ce present in the basin.

The Salawati Basin initially took shape duri~lg Mio- Pliocene time. Prior to that time, early Tertiary seas trans- gressed over the entire area above a major unconformity which exposed the Late Paleozoic sedimentary rocks (Fig. 4).

The Late Pdeozoic rocks in this basin are represented by the Aifam Group which consist of conglomeratic sand- stone and shale, grading upward into calcareous mudstone and minor; argillaceous limestone. Paralic sedimentation, in the form of calcareous mudstone and sandstone with coal seams, ends this Carboniferous-Permian sequence.

In part of the Salawati Basin, it appears that during Triassic and Cretacesous time, there was a period of non- deposition or erosion. The Paleocene through Lower Oligocene sediments (Waripi and Faumai Formations) were deposited directly on an eroded surface of the Aifam Group.

The Faumai Formation consists of shallow marine carbonate. with associated evaporites while the Waripi Formation represents deeper marine, pelagic limestone deposition.

The Late Oligocene Sirga Formation consists of sands and shales and represents a regressive sedimentary cycle which succeeds the shallow marine deposition of the Faumai Formation.

Beginning in the Early Miocene, downwarping of the basin or deepening of the seas took place. Dark grey to brown deeper marine limestone with shale interbeds were deposited in the basin depocentre. These sediments are assigned to the Klamogun Formation. This formation grad- es laterally toward the basin margin into high energy carbonates which constitute the early phase of the Kais Formation.

The Kais Formation in general consists of two major types of carbonates, i.e. reefal and shelf or platform carbo- nates. The latter grades laterally into deeper waterlbasinal carbonate, toward the centre part of the basin. The reefal facies of the Kais Formation appears to have a diachronous stratigraphic relationship with the overlying Klasafet Formation, which generally consists of off-reef and fine clastic se dirnents.

Uplift during Mio-Pliocene time along the Sorong Fault Zone to the north and the Ayamaru Plateau to the east, separated the basin (sensu lato) into the Salawati Basin to the west and the Bintuni Basin to the east. This uplift led to the deposition of the thick clastic sequence of the Klasaman Formation and terminated the flourishing reef development in the Salawati Basin.

Finally, during Plio-Pleistocene time, after a regional uplift of the basinal area, the fluvial Sele sands and conglomerates were unconformably deposited over the older formations.

III. HYDROCARBON SOURCE ROCKS

Since the first oil was discovered in the Miocene reefal limestone (Kais), exploration was concentrated almost

entirely on this prolific objective and resulting in the discovery of numerous fields. The Miocene marine Klasafet and Klarnogun shales and carbonates have been regarded as the source which generated the oils. Seismic surveys were designed to evaluate only the Miocene carbonates and drilling was usually terminated within these carbonates.

However, as geochemistry started to be more popular and used as an additional tool to support exploration, the likelihood of the Klasafet and Klamogun shales and carbo- nates being the principal oil source for the basin began to be questioned.

Present data suggest that in the Salawati Basin, the Klasafet shales are only mature in the deeper part of the basin and contain below average to average amounts of organic matter consisting of inertinite and vitrinite which have low oil source potential. Similarly, where penetrated, the Klamogun shales and carbonates generally contain low to average amounts of organic matter though alginite is much more cornmon. The Klarnogun shales and carbonates are only mature in the deeper parts of the basin.

The assumption that the Klasafet and Klarnogun shales and carbonates are the primary source for the Salawati Basin is open to question based on recent geochemical analysis. The oils may have more than one source.

Robertson Research have analyzed 5 wells and 2 oils (Fig. 10) for their study of the Petroleum Geochemistry of Indonesian Basins. Core Laboratories have carried out for TREND, gas chromatography analysis on 13 oil samples from various fields in the Salawati Basin, and recently conducted an oil characterization and correlation study on oils from five fields.

The geochemical analyses by Robertson Research in- dicate 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 by Corelab suggest that the crude is generated from a mixture of terrestrially derived organic matter and bacterial bodies (algae), deposit- ed under rather acidic, low oxygen conditions. Generation of oil is at thermally mature levels.

However, correlation study of five oil samples by Core- lab indicate that the oil has the same source (Fig. I 1-1 3) and . the material is marine, consisting mainly of type 11 (algae) sulfur rich kerogen with a secondary input of type 111 kerogen from terrestrial influx. The oil has been generated at normal maturity levels.

In the Salawati Basin several formations, which were deposited in shallow marine or paralic environments could be considered as potential hydrocarbon source rocks (Fig. 4).

Klasaman Shales

The Plio-Pleistocene Klasaman shales contain high levels of organic matter, but they are immature in most parts of the basin. These shales are unlikely to produce any signifi- cant hydrocarbons.

Klasafet Shales

In the deeper parts 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 250°F (llO°C) or 10,000' depth. How- ever, during Plio/Pleistocene time, before the uplift, the paleo temperature of the Klasafet shales was in excess of 350°F (150° C) or higher than the cracking temperature. If' the Klasafet shales generated liquid hydrocarbons, they would have been cracked immediately into gas. Consequent- ly, only gas and high gravity oil (condensate) could be expected from' fields in which the hydrocarbons are gene- rated from a Klasafet source only.

By contrast, the majority of the oil fields in the Sala- wati Basin have a low GOR. The associated gas is negligable, which wohld preclude the Klasafet as being the primary source for hydrocarbons. The gas-condensate fields discover- ed by Phillips on Salawati Island and in the northern part of the Salawati Basin by Petromer Trend may suggest some contribution from the Klasafet. However recent GC/MS studies have indicated a common source for these gas- condensates and the oils in the southern part of the basin.

Klamogun Carbonates or Calcareous Shales

The Type I1 sulphur rich kerogen of the Klamogun Formation may have been mature to generate hydro- carbons. Data on the Klamogun source rock potential is very limited and n o definite source rocks have been identi- fied. The Klamogun Formation may have contributed to the oils generated in the Salawati Basin.

Sirga Shales

The Sirga shales have been penetrated in only a few wells. They contain Type I and I1 kerogens in one well and Type IV in others. They are partly matllre in the basin. Part of the oil in the Salawati Basin may Le sourced from this formation.

Pre-Tertiary Formations

Recent geochemical studies in the Bintuni Basin (Che- vallier & Bordenave, In press) indicate that extracts from surface samples of the Permian (Aifam Formation) and the Juro-Cretaceous (Kembelangan Formation) show good correlation with the oils pooled in Mogoi, Wasian, Wiriagar fields and Kalitami-IX well. The Triassic Tipurna shales are also of source potential.

To the southwest, in the Bula Basin of Seram Island, CSR (AAR) produce oil (cum. prod. 13.5 MMBO, Dec., 1984) from the Pleistocene Fufa sands, Pleistocene reefal carbonates and recently from the Triassic Kanikeh sands (Fig. 5).

O'Sullivan et al, 1985 suggest that the Bula oils were generated from the Juro-Triassic organic rich Manusela calcareous beds. Other Mesozoic and Early Tertiary sequen- ces contain kerogen of terrestrial origin and are gas prone or too lean in organic matter to produce the oils. The Late Tertiary sequence is gas prone and immature.

Distribution of maturity levels of various horizons have been reconstructed and are shown in figures 6 to 8. Since Vitrinite Reflectance data are limited, Lopatin subsidence profiles were added to reconstruct the maturity levels. It appears that the mature areas follow the present

crescent shape of the basin. The sediments are mature in the centre of the basin and immature along the margins. This suggests that, whatever the source, oil generation is related to the Late Miocene to Pliocene subsidence of the Salawati Basin.

Figure 9 depicts possible generation and migration of hydrocarbon in the Salawati Basin. If oil was generated in the pre-Kais sequence then migration would have taken place via faults into the Kais reservoirs. The pre-Tertiary sequence may be connected with the Kais reservoirs by the normal faulting present in the Salawati Basin.

IV. CONCLUSIONS

The Klasafet and Klamogun shales and carbonates have up to now, been regarded as the principle source rocks for the Salawati Basin.

However, it is now apparent that the Klasafet Formation is unlikely to be the main source.

Oil source rocks are likely to occur in the Klamogun and Sirga Formations. Recent evidence suggests that the pre- Tertiary shallow marine or paralic sediments of the Juro- Cretaceous Kembelangan, the Triassic Tipuma and the Permo-Carboniferous Aifam Formations could also be potential source rocks.

TTI data and the distribution of oil fields in the Sala- wati Basin, indicate that mature source rocks are located in the present day centre of the basin.

ACKNOWLEDGMENTS

The authors wish to thank the managements of PER- TAMINA and PETROMER TREND CORPORATION for their approval to publish this paper. Special thanks are also expressed to our colleagues for their valuable advice, discussion and assistance.

V. SELECTED REFERENCES

Chevallier, B., and Bordenave, M.L., In press (1986) "Contribution of the Geochemistry to the Exploration in the Bintuni Basin, Irian Jaya".

Collins, J.L. and Qureshi, M.K., 1977, "Reef Exploration in Bintuni Basin and Bomeberai Trough - Irian Jaya", IPA Proceedings, 6th Annual Convention, Jakarta.

Core Laboratories, 1984, "Gas Chromatography Analysis of 13 oil samples in the Salawati Basin, Irian Jaya", Trend's In-House Report.

Core Laboratories, 1986, "Oil Characterization and Cor- relation Study, Salawati Basin, Irian Jaya", Trend's In-

House Report. Froidevaux, C.M., 1977, "Tertiary Tectonic History of the

Salawati Area, Irian Jaya, Indonesia", IPA Proceedings, 6th Annual Convention, Jakarta.

Harper, J.D., Redmond, J.L. and Whitaker, J.T., 1978, "Stratigraphic Seismic Analysis and Hydrocarbon History", Petromer Trend's In-House Report.

Koesoemadinata, R.P., et al., 1970, "Petroleum Possibilities

in West Vogelkop Area, Irian Jaya (a re-evaluation of Salawati Basin)", Pertamina Pusat In-House Report.

O'Sullivan T., Pegum, D., and Tarigan, J. 1985, "Seram Oil Search, Past Discoveries and Future Oil Potential; IPA Proceedings, 14th Annual Convention, Jakarta.

PERTAMINA-BEICIP, 1982, "Petroleum Potential of East Indonesia".

Redmond, J.L., and Koesoemadinata, R.P., 1976, 'WaIio Oil Field and the Miocene Carbonates of Salawati Basin, Irian Jaya, Indonesia", IPA Proceedings, 5th Annual Convention, Jakarta.

Robertson Research Int ., 1983, "Petroleum Geochemistry of Indonesian Basins".

Vincellette, R.R., 1973, "Oil Bearing Reefs in Salawati Basin of Irian Jaya, Indonesia", AAPG Bulletin, V. 60,

Vincellette, R.R., and Suparyadi, R.A., 1976, "Reef Exploration in Irian Jaya, Indonesia", IPA Proceedings 2nd Annual Convention, Jakarta.

Visser, W.A., and Hermes, J.J. 1962, "The Exploration for Oil in the Netherlands New Guinea", Verhandelingen van het Koningklijk Nederlands Geologisch Mijnbouw Kundig Genootschap. Geol. Serie, V.XX. PP. 265.

RE

GIO

NA

L I

ND

EX

MA

P

AU

ST

RA

LI

A

n - C

)

50

0

1 0

(1

0

- - -- -

- -

I M

ILE

S

PE

RT

AM

lNA

CO

RP

OR

A7

'lO

N

-Z> i

SA

LA

WA

Tl

BA

SIN

LO

CA

TIO

N O

F O

IL F

IEL

DS

>

AN

D

EX

PL

OR

AT

ION

WE

LL

S

PE

RT

AM

INA

P

etr

om

er

-re

nd

6%

C

OR

PO

RA

TIO

N

. S

AL

AW

AT

I B

AS

IN

ST

RU

CT

UR

AL

EL

EM

EN

TS

HA

Y IL

TW

, IM

412

FIGURE 4

M A T U R E IN DEEPEST P A R T O F THE B A S I N

P A R T L Y FAATURE

E R O S l O N

J i J i i A S S I C N O N - D E P O S I T I O N

- - - - -

PENETRATED

E X P L O R A T O R Y

CAi?ECNl FEnWJS

SALAWATI BASIN, IRlAN JP.Y A

P35S1siE n r D R O C A f f B O k SOURCE ROCX

PE

RT

AM

lNA

CO

l~O

RA

rlO

N -.

SA

LA

WA

TI

BA

SIN

THER

MA

L M

ATU

RA

TIO

N

PR

OF

ILE

RE

GIO

NA

L S

TR

UC

TU

RE

M

AP

ON

TO

P

OF

K

AlS

FO

RM

AT

ION

( C

.1

1000

FE

ET

)

EA

ST

-W

ES

T

CR

OS

S -

SE

CT

ION

S

HO

WIN

G

TH

ER

MA

L

MA

TU

RA

TIO

N

BY

F

OR

MA

TIO

N.

4 16 FIGURE 8

SA

LA

WA

TI

B

AS

IN

E

'

- )

C14

7 A

MIN

,\

/,(i~.;b'

i'e

/io

rne

r T

ren

d

/. U

b/

-

Co

rPO

RA

TIO

N

'~

;3

.

SC

HE

MA

TIC

SE

CT

ION

SH

OW

ING

PO

SS

IBLE

HY

DR

OC

AR

BO

N

GE

NE

RA

TIO

N A

ND

MG

RA

TIO

N

PERTAMlNA Petromer Trend

CORPORA~ION

C 10+ SATURATED HYDROCARBON

DISTRIBLITIONS- SALAWATI O t S

41 9

FIGURE 1 1

hl s MOI OIL 1.1)

KLAMONO OIL

!

-

KASlM U T A R A OIL WALlO (Kais) OIL

CENDRAWASIH OIL

A .

>

I

KLAMONO OIL

PERTAMINA

STERANE DISTRIBUTIONS-SALAWATI OQS

(m/z 217 ) i