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X. KUTAI BASIN

X.1 Introduction

The Kutai Basin is the largest (165,000 sq km.) and the deepest (12,000 -

14,000 meters) Tertiary sedimentary basin in Indonesia. The basin is bounded

to the north by the Mangkalihat High; to the south the basin hinges on the

Adang-Flexure (Adang-Paternoster Fault); to the west it is terminated by the

Kuching High-part of the Kalimantan Central Ranges; and to the east the opens

into the Strait of Makassar (Figure 1). The accretion of subduction related rafted

terrains onto the Sunda Shield continued through the Jurassic and Cretaceous.

In the west and northwest Kalimantan, basement is a complex accretionary

prism of metasediment, metavolcanic, magmatic arc martial and abducted

amphibolites complexes.

X.2 Regional Geology

X.2.1 Tectonic Setting

The basement under the Lower Kutai Basin is interpreted to be continental in

character and is typed as rafted transitional. The lower Kutai basement docked

with earlier rafted basement segments in the Late Cretaceous to Palaeocene

(70-60 Ma). The suture zone between this latest of rafted segments is the

boundary between the Upper and Lower Kutai Basin. The suture zone trends

NNE in the northern portion of the basin, parallel to the Meratus Ophiolite

Complex to the south. NW-SE trending Adang-Paternoster and Mangkalihat

Fault Zones controlled the north and south of the Kutai Basin boundaries are.

This zone separates the relatively shallow area of the Barito and Makalihat

Platforms in the south and north to the deeper part of the Kutai Basin. Further

northward the Kutai Basin is separated to the Tarakan Basin by the existing of

the Mangkalihat Ridge/Platform. Structurally the Kutai Basin can be divided into

Upper Kutai Basin in west and Lower Kutai Basin in the east (Figure 2).

The onshore portion of the Mahakam Delta overlies a series tightly folded

anticlines and broad synclines known collectively as the Samarinda

Anticlinorium, which resulted from inversion of the Palaeogene basin

(Chambers and Daley, 1995). Offshore Mahakam Delta areas show at least two

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phases of deformation. Middle Miocene and older rocks exhibit compressional

folding and thrusting, while the overlying Upper Miocene-Pliocene strata are

only affected by extensional faulting (Malecek et al., 1993).

Toward offshore area, the Lower Kutai Basin passes laterally into the North

Makassar Basin. The North Makassar Basin composes of three main sub-

basins: Mahakam Depocentre, Santan Gravity Low, and Lariang-Karama Sub-

basin. The offshore Kutai Basin is estimated to have over 9,000 m sediments

fill. However, only the upper 6,000 m, of Neogene sediments are the main

interest for petroleum exploration activities.

In the southeastern part of the section the West Sulawesi Deformation Front

Zone limits the development of the Lower Kutai Basin to the east. The western

Lariang Basin margin is marked by a dramatic structural change from flat-lying,

undisturbed events in the Makassar Straits to a very tightly folded and faulted

region, which is interpreted to contain the same sedimentary section as found in

the Kutai Basin. The high amplitude folds that exhibit a random, episodic style

of movement are an offshore extension of a well-known wrench fault mapped

onshore in Sulawesi (CGG, 1994).

X.2.2 Stratigraphy

The Paleocene to Eocene age of the Kutai Basin inception was initially as

extensional rift-graben. The Lower Eocene-Lower Oligocene Kuaro and Telakai

Formations (equivalent to the Ujoh Balang Formation in the Upper Kutai Basin)

unconformably overlie Cretaceous metamorphic basement and comprise a fine

upward sequence with terrestrial sandstones at the base and marine shales at

the top. On the northern flank of the basin near the Mangkalihat Ridge, a Middle

Eocene-Lower Oligocene sequence composed of volcanoclastics and marine

bathyal shales (Sembulu Formation) has been recognized. Following Upper

Oligocene (N-4) regressive clastic sequence deposition has been started since

the Upper Oligocene (N-4) continuous to the present day (Figure 3).

The occurrence of the Early Eocene syn-rift sediments can be detected in the

North Makassar Basin (CGG, 1994). The Middle Eocene syn-rift deposits can

also be detected in the deep North Makassar Basin, which is equivalent to the

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Toraja/Malawa formation in the Sulawesi Region and Tanjung Formation in

Barito Platform of the South Kalimantan Area.

Following the Palaeogene rift phase, the Kutai Basin developed as a sag basin

on the continental margin prograding progressively eastward throughout the

Neogene in a series of deltaic sedimentary piles (BEICEP, 1982).

The middle of Early Oligocene sediments in the Lower Kutai Basin/North

Makassar Basin was deposited on a marine shelf that regionally transgressed

the top of the Middle Eocene sediment. Generally, the area of the North

Makassar Basin and its adjacent area were quite during this period, limestones

were developed in the shallower part, and deep marine shales were deposited

in the depocentre.

The Upper Oligocene-Lower Miocene sediment (equivalent to the Pamaluan

Formation) marks the base of the Neogene section and the oldest sediments

penetrated in the offshore Kutai Basin. The Pamaluan Formation is comprised

of black-black, carbonaceous shales with rare, thin beds of fine sandstones

interpreted as bathyal marine deposit in much of the onshore and all of the

offshore areas.

The Lower Miocene Bebulu Group overlays the Pamaluan Formation and

consists of a shelf edge bioclastic limestones in the onshore area (Maruat Fm)

and a slope to bathyal sandstones, siltstones and shales sequence offshore

(Pulau Balang Fm).

In the Lower Kutai Basin/North Makassar Basin, the Lower Miocene sequence

is mainly formed as carbonate and marine shales deposition. Carbonate was

mainly formed in the shallow part of the basin and laterally passes to the open

marine shales in the offshore part of the Kutai Basin.

The Middle Miocene was marked by eastward an initial out building of the delta

systems over shelf to slope sediment, with carbonates developed locally on the

shallow marine shelf. The Middle Miocene deltaic sediments of the Balikpapan

Group overlays the Bebulu Group and is composed of two units: a paralic-

deltaic sequence of massive sandstones with occasional shales interbeds and a

sequence of shales, siltstones with occasional shales interbeds and a sequence

of shales, siltstones, limestones and rare sandstones which represent a shelf-

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slope-bathyal marine transition in the offshore areas. The Middle Miocene

deltaic sequences that developed in the onshore Kutai Basin laterally change to

the distal outer shelf, slope, and basin floor fan. The development of the Middle

Miocene lowstand sediments the offshore part of the Kutai Basin/North

Makassar Basin were closely related to the major sea level drop in the base of

Middle Miocene (16.5 ma) (Baillie et al., 2000)

During the Late Miocene-Pliocene times, the eastern part of Kutai Basin

contained deltaic to shallow marine facies laterally changes to the distal outer

shelf, slope and basin floor sediments of Late Miocene-Pliocene lowstand

deposits.

The Late Miocene–Pliocene Kampung Baru Group supersedes the Bebulu

Group and continues the west to east basin fill progradation. Onshore and near

offshore sediments consist of interbedded sandstones, siltstones, shales and

coals, interpreted as a paralic-deltaic sequence (Tanjung Batu Fm). Distant

offshore areas contain shales and siltstones with some thin sandstones and

limestones interpreted as shallow marine shelf sequences (Sepinggan Fm).

The Mahakam Group (Handil Dua and Attaka Formations) overlays the

Kampung Baru Group and its upper surface form the present-day seafloor. The

Handil Dua Fm represents the onshore sub aerial delta plain of mixed fluvial

and tidal-marine delta environments. The lithologic composition is intercalated

sands, silts, clays and lignites. Offshore, the Attaka Formation consists of

interbedded fossiliferous clays, coarse-fine, unconsolidated sands and some

bioclastic shell beds interpreted as an open to restricted shallow marine shelf

deposits (Marks et al, 1982).

X.3 Petroleum System

X.3.1 Source rock

The source rock mainly paralic/delta/restricted shallow marine carbonaceous

mudstones of the Tanjung Formation, shales of the Bongan Formation and the

claystones, mudstones and coals of bay fill estuarine, deltaic and shallow

marine facies of the Miocene section. Geochemical analysis of outcrop samples

indicates the Miocene shales, claystones and coals of the delta and pro-delta

are fair to good waxy oil and gas prone source rock.

X-5

The Miocene shales, claystones and siltstones have a Total Organic Content

(TOC) varying from 0.14% to 15.37% with the majority between 0.5% and 1.0%.

The sediments contain only terrestrial derived organic matter and are waxy oil

and gas prone from mixed kerogen types or exclusively gas prone. It is

concluded that terrestrially derived organic matter within the Miocene deltaic

sequences have generated hydrocarbons prior to the section being uplifted

4,500 feet and eroded away.

In the Oligocene section, only few samples have TOC between 0.65% and

0.85%, this is generally considered as an organic lean source. Kerogen types

are oil and gas prone sapropelic/humic kerogen.

The Eocene samples yields poor to fair TOC (0.06% to 0.60%) for marine

shales/claystones and poor to excellent (0.08% to 16.96%) for the deltaic/non

marine shales/claystones. A few samples in the mature areas with Ro 0.45 to

1.2 have poor to good TOC. Kerogen types generally range from gas prone

vitrinitic kerogen to mixed oil and gas prone liptinitc/vitrinitic kerogen. From the

data available it is concluded that the Eocene section contain mature to over

mature oil and gas prone source rocks.

X.3.2 Reservoir

From the sedimentary section present in the Kutai Basin the following facies

from the Eocene Tanjung Formation are prone to hydrocarbon accumulation;

they are the basal sands of fluvial channel, high energy and coarse-grained

sandstones of estuarine/deltaic environment and shallow marine sandstones

and limestones. Shallow core samples have measured porosity of 13 to 25%

and permeabilities of up to 450 mD suggesting that these sands may have

excellent reservoir properties for both oil and gas (Guritno & Chambers, 2000).

Other Palaeogene reservoir interval is the Oligocene deep-water sediments,

which consists of monotonous grey mudstones with thin sandstones, siltstones

and limestones. Surface porosity of litharenithic sands range from less than 5%

to 25%, and permeability ranging from less than 10 up to 200 mD.

The main productive reservoirs in the East Kutai Basin are fluvial deltaic and

near-shore marine sandstones of Middle Miocene-Pliocene age. These sands

are part of a series of prograding deltaic sequences that filled an ancient deep-

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water basin. Petrographically, the sandstones are classified as feldspathic

sandstones in the young reservoirs. Porosity and permeability show a linear

decline with depth due to physical diagenesis.

X.3.3 Seal

The Oligocene shales provide an effective and regional vertical seal. However,

the fact that all Oligocene sands seen Tengkawang-1 had oil and gas shows

must indicate that the seal in the seal has leaked. The Possibility of an

intraformational seal in the Beriun Formation is unlikely due to a high sands

percentage, hence only areas with an intact Oligocene to Miocene cover section

are considered prospective.

X.3.4 Trapping Mechanism

Exploration plays for the Miocene and Eocene objectives are dominated by

structural trap particularly four-way closure bounded by fault. However, on flank

of major structures or basement high or even along monoclinal dips

stratigraphic traps may occur in fluvial-deltaic reservoirs of the Miocene and

Eocene. The late Miocene tectonic movement that thrust the basement and the

sedimentary cover westwards against the stable Barito shelf is responsible for

the current structural configuration. In the Bangkanai area the thrust resulted in

a major uplift of up to 10,000 feet and eroded most of the Miocene and

Oligocene sequences. Therefore it is presumed that traps formed prior to the

uplift might not have survived and have been breached except the Oligocene

carbonate build-up, which is principally a stratigraphic/structural trap. Therefore,

carbonates build-ups or clastic stratigraphic traps are primary target that

promises a higher chance of success than the structural traps.

X.4 Hydrocarbon Play

Palaeogene Play

The main play type with the area involves structural closure containing Middle-

Upper Eocene deltaic sediments that make a ‘shelf sourcing system’. Basement

involved faults also act as migration pathway from the kitchen areas to traps

stratigraphically higher in the section, but the value of such targets are

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discounted in light of the poor reservoir properties in limestones and Oligocene

turbidites throughout the region.

Primary migration hydrocarbon from the Middle to Upper Eocene source rocks

occurs vertically and laterally. In the offshore part of the Kutai Basin vertically

migration pathway from the mature Palaeogene kitchen occurs through network

of the NNE-SSW trending faults to the Middle and Upper Miocene lowstand

reservoirs. The lateral migration from the mature kitchen area can also be

facilitated through eastward dipping lowstand reservoir to either stratigraphic or

structural traps that are available in this area.

Neogene Play

The timing of hydrocarbon migration from the Early-Middle Miocene source

rocks is post Middle Miocene. Traps formation has been formed since Middle

Miocene to present day. Migration pathway primarily vertical, and may have

some moderate horizontal migration from basin centres (Figure 4).

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References

Darman H., and Hasan Sidi F., 2000, An Outline of The Geology of Indonesia,

Published by IAGI-2000, pp 73-75.

Fukasawa H., R. Sunaryo, and P.H. Napitupulu, 1987, Hydrocarbon Generation

and Migration in the Sangatta Area, Kutai Basin, Proceed. Indon.

Petrol. Assoc.16h Ann. Conv. pp 123-139.

Guritno E.E., and Chambers J., 2000, North Runtu PSC: The First Proven

Eocene Petroleum Play in the Kutai Basin, Proceed. Indon. Petrol.

Assoc.27th Ann. Conv. pp 361-380.

LEMIGAS, 2000, East Kutai-North Makassar Basins: Regional Geology Study,

unpublished report.

Marks E., Sudjatmiko, L. Samuel, H. Danutirto, T. Ismoyowati, and B.B. Sidik,

1982, Cenozoic Stratigraphic Nomenclature in the East Kutai Basin,

Kalimantan, Proceed. Indon. Petrol. Assoc.11th Ann. Conv. pp 147-

179.

N

Kilometers

0 250

Pontianak

Palangkaraya

Banjarmasin

FIGURE1. Location Map of Kutei Basin

Kutei Basin

Balikpapan

Samarinda

Palu

Palangkaraya

Balikpapan

Samarinda

Palu

115 Eo

120 Eo

0 125

Kilometers

115 Eo

120 Eo

STRUCTURALHIGH

BASINMARGIN/TERRACE

STRUCTURAL/BASINALLOW

ULTRABASICTERRAINIGNEOUSTERRAIN

NORMALFAULT

STRIKESLIPFAULT

TRUSTFAULT

FAULTZONE

LEGEND :

FIGURE2. Tectonic Element of the Kutei Basin

FIGURE3. Regional Stratigraphy of Kutei 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

P14P15P16P17

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)

LITHOSTRATIGRAPHY

W E

MANGKUPA

BERIUN

ATAN

KEDANGO

SEMBULU

MARAH

PAMALUAN

PULAUBALANG

KAMPUNGBARU

ATTAKAHANDILDUA

S

R

S

S

S

S

S

R

R

R

R

R

R

S

S

S

S

S Source R ReservoarS Seal

FIGURE4. Hydrocarbon Play Model of Kutei Basin

Tambora Tunu Sisi

Migration paths

Hydrodynamicflow