basin evolution of the ardjuna rift system and its implications for hydrocarbon exploration, onwj

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PA9

1.1 -

078

PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION

Twenty Fourth Annual Convention, October 1995

BASIN EVOLUTION OF

THE

ARDJUNA

RI lr SYSTEM

AND

lTS

IMPLICATIONS

FOR

HYDROCARBON EXPLORATION, OFFSHORE NORTHWEST JAVA, INDONESIA

Mark

Glesko*

Chandra

ria*

Steve SinclaiP

ABSTRACT

The Ardjuna Basin, which lies approximately 90 km

northeast of Jakarta, is one of a series of hydrocarbon-

bearing basins on the southern edge of the Sunda

craton that originated during a major Eocene-

Oligocene rifting event. The Ardjuna Basin is the

name given to a large sag basin located over three

precursor rift sub-basins that comprise the Ardjuna rift

system: Northern Ardjuna, Central Ardjuna, and

Southern Ardjuna sub-basins. The Ardjuna Basin as a

whole covers an area of approximately 3000 km2, with

each sub-basin comprising an average area of 800

km2. Each sub-basin is comprised of at least one half-

graben system and contains, in varying amounts and

facies, the primary hydrocarbon source rocks and a

major reservoir facies within ARCO Indonesia's

Offshore Northwest Java (ONWJ) Production Sharing

Contract (PSC) area, the Oligocene Talang Akar

Formation.

This paper is a review of a geological and geophysical

study of the Ardjuna basin as it affected the

distribution and character of the Talang Akar

Formation. The study utilized seismic, well log, core,

and biostratigraphic data of the Talang Akar

Formation and older units. Structural depth and

isopach maps are used to describe the structural

history of these basins and how the timing of graben

development effected the accumulation and

distribution of hydrocarbon source and reservoir

facies. The first prospect developed from this study,

the LU-1 well located in the center of the Southern

sub-basin, was spudded in February 1995 and decked

AtIantic Richfield Indonesia,

Inc.

RCO

International

Oil

and

Gas

Co.

a suspended oillgas discovery in May 1995, after

testing a cumulative flow of 1400 BOPD and 12

MMCFGPD from three intervals. In addition to the

hydrocarbon tests, the well confirmed the presence of

a thick, mature, source facies comprised primarily of

coals and organic-rich fine-grained sediments in the

Southern sub-basin. The presence of these mature

source facies confirms that the Southern Ardjuna sub-

basin was the likely source kitchen for much of the

oil and gas discovered in the Ardjuna basin to date.

INTRODUCT ION

ARCO Indonesia's Offshore Northwest Java (ONWJ)

Production Sharing Contract (PSC) area contains

Java's largest hydrocarbon producing basin, the

Ardjuna basin, with nearly 600 million barrels of oil

produced since 1967. While the majority

of

the

hydrocarbons (80 ) within ONWJ are reservoired

within the Miocene Upper Cibulakan Formation, 20

are reservoired within the deeper, Oligocene Talang

Akar Formation. The Talang Akar Formation is

relatively under-explored in the ONWJ area; less than

20% of the 1000 wells drilled in ONWJ reach the

Talang Akar. The Talang Akar is comprised of a thick

section (150-1500 meters) of interbedded sandstones,

shales, siltstones, coals and limestones, deposited in

an overall transgressive setting. The Talang Akar was

deposited in a syn-rift to post-rift setting and is the

primary source intervaI for all the oiI and a majority

of the gas discovered within the Ardjuna basin to date

(Gordon, 1985). The Talang Akar Formation is an

attractive exploration target because it contains high-

quality reservoir rocks that deliver hydrocarbons at

relatively high rates. The average Talang Akar field

in ONWJ has reserves of 25 to 30 MMBOE, normally

consisting of approximately 50 gas and 50% oil.

© IPA, 2006 - 24th Annual Convention Proceedings, 1995sc Contents

Contents

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148

In early 1992, a team was formed to conduct a

regional study in the Offshore Northwest Java

(ONWJ) contract area to evaluate the remaining

potential of the Talang Akar Formation. The results

suggested that the relatively unexplored Southern

Ardjuna sub-basin was the source kitchen for much of

the billions of barrels of oil within the Ardjuna basin.

Play concepts were developed to explore for oil and

gas reservoired within the Talang Akar deep within

the Southern sub-basin. The first well to test these

play concepts was the LU-1, a Talang Akar discovery

drilled in early 1995.

This paper reviews the results of the Talang Akar

study as it pertains to the development and evolution

of the rift sub-basins within the Ardjuna rift system.

DATA BASE AND

METHODOLOGY

Although over one thousand wells (both exploration

and development) have been drilled in the ONWJ

PSC, less than 20 percent penetrated as deep as the

Talang Akar Formation. One hundred twenty-one

wells that penetrated the Talang Akar and older

sediments were included in this study. The logs from

these wells were used for correlation purposes as well

as for lithologic and depositional environmental

interpretations. The depositional facies interpretations

from well logs were tied back to the wells using more

than 1100 meters of available conventional core from

36 wells. Detailed biostratigraphic analyses from 19

wells were integrated with. the well correlations to

improve and confirm chronostratigraphic correlations.

Geochemical interpretations were made based on rock

data from 67 wells (915 samples) and oil analyses of

121 samples. The results of 1,859 drill stem tests

(DST’s) were used to calculate geothermal gradients

and temperatures for thermal maturity modeling,

In preparation for the regional study, seismic data

were acquired specifically for regional mapping

of

the Talang Akar and deeper objectives. These data,

designated as the DP-92 survey, consist of

approximately 6000 km of high quality 2 seismic

acquired roughly

on

a l-km dip-oriented grid

(predominantly E-W) with north-south cross-lines

spaced every 5 to 8

km

These data were

supplemented by previous 2D and 3D seismic data.

One hundred twenty-one wells with geologic tops

were tied to the seismic data using velocity

checkshots, VSP’s and synthetic seismograms. Nine

horizons were mapped within the Jatibarang and

Talang Akar Formation; depth and isopach maps were

generated for all horizons.

REGIONAL SETTING

The Ardjuna basin is located within the central part of

the ONWJ

PSC

area (Figure 1). This basin is one of

a series of basins (Palembang, Sunda, Asri, etc.) on

the southern edge of the Sunda craton that originated

during a major Eocene-Oligocene period of dextral

wrenching (Daly et al., 1987). The Ardjuna basin is

the name given to a large sag basin located over an

older rift system containing three sub-basins

(Northern, Central and Southern) (Figure 1). The

Ardjuna basin covers an area of approximately 3000

km2

(100

km by

30

km or 740,000 acres). The sub-

basins average 800 km2 and are composed of at least

one half-graben system. Each sub-basin is separated

from the adjacent sub-basin by an accommodation

zone.

The stratigraphic succession in this basin ranges in

age from Late Paleocene(?)-Early Oligocene to

Holocene (Figure 2). This study focuses on the pre-

Miocene section of the Jatibarang and Talang Akar

Formations (Figure 2). Thickness of this interval

ranges from 7700 feet (2350 m) in the southern rift

to less than 500 feet (150 m) on the western flank of

the Ardjuna basin. More detailed descriptions of the

stratigraphy are given in Suria et al., 1994; Kaldi and

Atkinson, 1993; Suria, 1991; Ponto et al., 1988;

Gordon, 1985.

TECTONIC HISTORY AND STRATIGRAPHIC

FRAMEWORK

Five major tectonic events effected the structural

development of the Ardjuna basin. I n order, from

oldest to most recent:

Late Cretaceous to Eady Eocene 100-56 Ma)

Regional metamorphism generated by subduction

and development of the Meratus arc. Deformation,

uplift, erosion and cooling occurred in the

Paleocene. Calc-alkalic magmatism occurred

throughout the area that is now onshore and

offshore Java due to normal subduction related

processes. Andesitic magmatism continued into

the Early Eocene.



149

Eocene 50 -

40

Ma)

India plate collided with

the Eurasian plate which gave rise to major

dextral wrenching of the Sunda craton's southern

margin (Daly et al., 1987).

Oligocene

34

-

30

Ma)

South China Sea rifting

and accretion in northern Kalimantan. The

Australian plate margin (New Guinea) collided

with several arc complexes (Daly et al., 1987).

Mid Miocene (17

-

10 Ma)

- South China Sea

rifting ceased with the collision of continental

fragments derived from Gondwana (northern

Australiahrian Jaya) against the eastern Sunda

margin (Daly et al., 1987).

Late Miocene

7-5

Ma)

- NW Australia collided

with Sunda trench (Daly et al., 1987).

Regional metamorphism of carbonates and

siliciclastics of the Sunda continental passive margin

sediments occurred in the Late Cretaceous. These low

to moderate-grade metamorphics, as well as

Cretaceous to Paleogene calc-alkalic intrusions, form

the basement rocks present throughout most of the

ONWJ area (Figure 2).

An extensional tectonic episode (Eocene?) initiated the

first phase of rifting within the Ardjuna basin,

designated as Rift

I

phase (Figure 2). Sediments

deposited during Rift

I

phase are designated as Syn-

rift

I

sediments (Figure

2).

The initial rift basins (Rift

I phase) formed during fragmentation, rotation and

lateral migration of the Sunda craton. These volcanic-

rich extensional basins are concentrated along a line

that trends across the contract area from the Jatibarang

basin in the southeast, across the southern central

Ardjuna sub-basin to the North Seribu trough in the

northwest (Figure

3).

Two normal fault trends affected

the Rift I development, one approximately N60 W to

N40 W and the other trending nearly due north-south.

An overall N3 0 -70 E extension direction agrees well

with regional observations by Daly et al. (1987, 1991)

of north-northwest trending extensional basins in

Sumatra being related to northwest-southeast

compression (Figures 3A and 3B).

During the early Oligocene, volcanism and rifting

ceased in the Ardjuna area. This period of tectonic

quiescence in the Ardjuna area stands in contrasts

with the collisional events recorded in the Java and

Sumatra forearcs during this time (Daly et al., 1987).

These collisional events may have led to a major

reorientation of the regional stress fields that

generated significant regional uplift and erosion along

the southern margin of the Sunda craton. An angular

unconformity is observed on seismic data and in well

logs in all the nearby basins (Vera graben, Jatibarang

sub-basin, Ardjuna, Sunda and Palembang basins) and

is noted on a seismic line in Figure

5 .

Renewed rifting and reactivation of faults occurred at

the end of the early Oligocene (Rift I1 phase) that is

likely related to an increased rate of lateral movement

of the Indochina block and opening of the South

China Sea (32 to 30 Ma)(Daly et al., 1987, Figure

3B).

During the late Oligocene, displacements along major

fault systems in the Malay and Thailand peninsula

area ceased (Daly et al., 1987). Uplift and exposure

of the northern Sunda igneous platform at this time

caused a significant provenance change in sediments

directed into the Ardjuna basin: Syn-rift I and Syn-rift

I1 sediments are locally derived from basement while

later sediments are from the denudation of the Sunda

craton.

The end of the Oligocene and the earliest portion of

Miocene time was marked by tectonic quiescence

throughout the Ardjuna basin. This tectonic

quiescence may also have coincided with a eustatic

sea-level highstand during which the thick limestones

of the Batu Raja Formation were deposited (Figure

2).

STRUCI URAL FRAMEWORK

The basement assemblage in the offshore Northwest

Java Sea is composed of metamorphic and igneous

rocks, primarily of Cretaceous and older ages, and

subordinate indurated limestones and clastic sediments

of possible early Tertiary age (Figure

2).

Based on

basement age dates, regional metamorphism ended

during the late Cretaceous while deformation, uplift,

erosion and cooling continued into the Paleocene.

A

depth map generated from seismic data integrated

with well penetrations show the three main precursor

sub-basins within the Ardjuna basin: the Northern,

Central and Southem sub-basins (Figure 4). These

basins alternate from west-facing (down-thrown to the

west) in the Northern and Southern sub-basins, to



150

east-facing in the Central sub-basins with

accommodation zones separating the half-grabens. The

basement within the Ardjuna basin ranges in depth

from approximately -3,000 feet (914 m) (subsea) on

the northeastern margin of the basin, near W-1 well,

to more than

-18,000

feet (5490 m) in the South

Ardjuna sub-basin.

Of the three sub-basins, the Northern is aerially the

smallest as well as the shallowest. It is comprised of

a single, west-facing, asymmetric half-graben,

approximately 10 km wide (east-west) and 15-km long

(north-south). Although there are no basement

penetrations in this sub-basin, depth to basement is

estimated to be a maximum of -11,000 feet

(3350

m).

The basin-bounding fault displays approximately 8,000

feet (2440 m) of throw, the largest throw of any single

fault in the Ardjuna basin. A seismic line through the

Northern sub-basin (Figure 5) demonstrates the west-

facing half-graben nature of the Northern sub-basin as

well as the seismic reflection character of the

sediment fill.

The Central sub-basin is comprised of at least 6 half-

grabens, most of which are east-facing with minor

intervening west-facing faults. The complex pattern of

the basement faults is likely caused by the merging of

a series of smaller faults by fault-tip migration. The

Central sub-basin is the aerially largest sub-basin and

covers an area 40 km x 20 km. The deepest portion of

the Central sub-basin is located near the SS-1

well at

a depth of approximately -13,000 feet (3960 m ), The

asymmetric nature of the grabens form structures that

support the only two hydrocarbon accumulations in

the Central sub-basin, the SC and the SB fields. Both

are located in footwall closures on half-graben

bounding faults. A seismic line over the Central sub-

basin is shown in Figure

6 .

The Southern Ardjuna sub-basin is comprised of a

single half-graben covering an area

of

approximately

400 km2. It is the deepest half-graben with basement

estimated in excess of -18,000 feet (5490 m). Previous

seismic mapping in this area suggested that this basin

was’ shallower than the Central sub-basin; however,

with improved seismic data, it became clear that the

depth to basement in this sub-basin was grossly

underestimated. A seismic line over the Southern sub-

basin is shown in Figure

7.

The sub-basin is bounded

by

two

orthogonal faults; one trends to the northwest

and is downthrown to the southwest, and the other

trends north-northeast and is down-thrown to the

west-northwest (Figure 4). Maximum throw along the

fault is difficult to estimate due to the step-wise

nature of some of the faulting, but is likely in excess

of

5,000

feet (1500 m). Until recently, there were no

Talang Akar penetrations within the Southern

suh-

basin. However, the LU-1 well was drilled nearly in

the center of the basin, giving the first geologic

information in this important sub-basin.

LITHOSTRATIGRAPHY

The pre-Miocene sedimentary section in ONWJ is

subdivided into three distinct units: the Late

Paleocene(?)-Lower Oligocene Jatibarang Formation

(Syn-rift I), the Lower Talang Akar Member (Lower

Oligocene, Syn-rift 11) and the Upper Talang Akar

Member (Upper Oligocene, Post-rift sag)

of

the

Talang Akar Formation (Figure 2). In ONWJ, the

Oligocenehliocene boundary occurs at the top of the

Talang Akar Formation at its contact with the

overlying Batu Raja limestones (Figure 2). Within the

pre-Miocene section

of

the Ardjuna basin, the Upper

Talang Akar (source and reservoir) and the Batu Raja

(reservoir) are primary exploration targets, while the

Jatibarang and Lower Talang Akar are secondary

targets for both potential reservoir and source facies.

Syn-Rift

I

-

Jatibamng F ormation

The Jatibarang Formation, as defined in this study,

comprises the predominately continental sediments of

Latc Paleocene(?) to early Oligocene age (-60-34 Ma),

deposited in a syn-rift setting above basement and

below an angular unconformity (34 Ma?) recognized

on seismic data (Figure

5 .

A similar, but likely non-

synchronous, angular unconformity is recognized in

all of the Ardjuna sub-basins and in other nearby

basins that include the Vera graben, and the

Jatibarang, Sunda, and Palembang basins (Van de

Weerd and Armin, 1992).

The Jatibarang Formation is typically composed of

alternating lacustrine clastics and volcaniclastics

deposited in isolated half-grabens during the Rift I

phase. The Jatibarang volcanics are predominately

andesitic volcaniclasitic flows and tuffs interspersed

with reworked volcanics and basement-derived

sediments. An isopach map of the Jatibarang,

generated from seismic data and available well control

is

shown in Figure

8 .

The Jatibarang overlies the



151

basement within most half-grabens and is absent on

most structural highs. It is unknown whether the highs

were

areas

of non-deposition or were denuded

subsequent to Jatibarang deposition.

Where it can be adequately imaged on seismic data,

the Jatibarang is characterized by a series of parallel

to sub-parallel, moderate amplitude, continuous

reflections that are typically concordant with the

basement reflection and show slight thickening into

the basin (Figure

5).

The top

of

the Jatibarang has a

distinctive erosional unconfonnity best observed on

the hanging wall margin (western margin) of the

Northern sub-basin (Figure

5).

The unconformity is

less apparent and possibly conformable toward the

center of the basin.

In areas adjacent to the Ardjuna basin, the Jatibarang

is both a hydrocarbon source rock (Jatibarang sub-

basin) and a reservoir (onshore Jatibarang Field,

production from fractured volcanic tuffs; Figure 3).

Wells drilled into the Jatibarang in ONWJ have

commonly encountered shows, but no production

exists offshore from this typically reservoir-poor

interval. The Jatibarang apparently is not the source of

Ardjuna oils, which are typed to a non-lacustrine

source, likely the coals and organic-rich shales within

the Upper Talang Akar (Gordon, 1985). While the

Jatibarang has not proven

to

be a significant

exploration target within ONWJ, exploration

opportunities remain in areas such

as

the Northern

Ardjuna basin where the Jatibarang is significantly

shallower and likely has better reservoir qualities

compared to other areas.

Syn-Rift

II - LowerTalang Akar

Formation

Overlying the Jatibarang, or overlying basement where

the Jatibarang is absent, is a thick section of

Oligocene-aged, interbedded shales, sands, coals and

. thin limestones of the Talang Akar Formation (Figure

2). Based on lithologic characteristics and on

chronostratigraphy, the Talang Akar is further

subdivided into two members: the Lower Talang Akar

and the Upper Talang Akar. The Lower Talang Akar

is predominantly non-marine, massive bedded

conglom mates and sandstones with interbedded fine-

grained 13custrine shales and minor coals. The Upper

Talang Akar is characterized by medium to fine-

grained sandstones, mudstones, and

coals

near the

base to sandstones, marine shales and limestones in

the upper part of the member. The coals and other

fine-grained organic-rich sediments within the Talang

A k a have been typed to the oils discovered within

the Ardjuna

basin

(Gordon, 1985). Sandstones within

the Upper Talang Akar are the producing reservoirs

within the Talang Akar fields in the Ardjuna basin.

The Lower Talang Akar consists

of very coarse-

grained, massive, pebbly conglomerates and medium

to coarse-grained litharenite sandstones to fine-grained

lacustrine mudstones, paleosols and air-fall tuffs.

These sediments were previously referred to as the

continental member of the Talang Akar (Ponto et

al., 1988). The age of this interval is poorly

constrained

as

it is primarily a continental deposit that

contains few datable taxa; however, a few, sparse

nannofossils suggest the upper section of the Lower

Talang Akar is Early Oligocene (NP23) age.

The source for the coarse-grained clastics within the

Lower Talang Akar was the nearby uplifted igneous

and metasediments of the basement. Reservoir quality

in the Lower Talang Akar clastics is generally poor

and overall quality decreases with depth due to

decreasing sandstone compositional maturity and

increasing burial-related compaction.

An isopach map of the Lower Talang Akar including

basement faults (Figure 9) shows that the thickness of

the Lower Talang Akar is roughly equivalent in all

the sub-basins, averaging approximately 1500 feet

(450 m). There are, however, localized thicks in the

Central sub-basin near the SH-1 well and west of the

SB-1 Field where the thickness of the Lower Talang

Akar is estimated to exceed 2000 feet (600 m) thick.

The Lower Talang Akar is absent due to onlap onto

the western margin of the Ardjuna basin (toward the

APN area)

as

well as on basement highs in the B and

K

Field areas. In the Ardjuna basin, the top of the

Lower Talang Akar averages approximately -8,000

feet (2440 m) and has a maximum depth in the sub

-

basins of -7000 feet (2130 m) in the Northern, -8500

feet (2600 m)in the Central, and -12,000 feet (3650

m) in the Southern.

The seismic character

of

the Lower Talang Akar

typically consists of relatively low amplitude,

discontinuous reflections (Figures

5

6 and

7).

This

reflective character of the Lower Talang Akar is likely

due to the relatively homogeneous nature of the



152

continental deposits with respect to seismic imaging,

and depth of burial, which limits the bandwidth of the

seismic signal.

Post-Rift Upper

Tdang

A h

The thick section of interbedded sands, shales,

siltstones, coals and limestones, deposited in an

overall transgressive setting, and bounded below by

massive, coarse-grained clastics of the Lower Talang

Akar and above by the massive limestones of the Batu

Raja are designated as the Upper Talang Akar

member of the Talang Akar Formation (Figure 2).

This interval contains the primary hydrocarbon source

rocks for the oils and much of the gas found in the

Ardjuna basin, as well as high reservoir quality

sandstones that have produced over 50 MMBOE from

8

Ardjuna fields.

There is considerable lithologic variation in the Upper

Talang Akar; medium to tine-grained quartz-rich

sandstones, coals, shales, marine limestones and airfall

tuffs (Figure

2).

In general, coals, shales and

sandstones are the dominant lithologies within the

lower section of the Upper Talang Akar, while

limestones and shales are dominant in the upper

section.

The sandstones within the Upper Talang Akar

document a significant change from the underlying

Lower Talang Akar continental deposits. Generally,

the Upper Talang Akar sandstones are more mature,

better sorted, and finer-grained than the Lower Talang

Akar sandstones. The typically black, amorphous coals

within the Upper Talang Akar have sharp, lower basal

contacts that are generally rooted. The coals range in

thickness from a few inches (4-6 cm) up to

20 feet

6

m). Coals are thickest and more frequently occur

near the base of the Upper Talang Akar, near the

Basal Coal Marker (Figure

2),

and become thinner and

less frequent upward.

The Upper Talang Akar member has variable seismic

character, which

is

dependent on the presence or

absence of the dominant lithologies that affect the

seismic response: limestones and coals. The

limestones within the Upper Talang Akar have high

acoustic impedance (high velocity, high density) and,

therefore, have a resulting high amplitude positive

reflection. Conversely, coals have a low acoustic

impedance (low velocity, low density), due to their

high organic content, and, therefore, have a resulting

high amplitude negative impedance. These seismic

characteristics were used to aid in predicting the

presence

or

absence €potential source rocks within

each of the sub-basins in this study. The seismic line

in

Figure

7

shows high amplitude reflections near the

base of the Upper Talang Akar. This seismic

signature was used to predict that the Southern

Ardjuna sub-basin contains thick coal sequences. This

was later confirmed by the drilling

of

the LU-1 well

that penetrated a 2,000-feet (600-m) thick section of

coals, carbonaceous shales, mudstones and thin

sandstones. These coals and other organic-rich, fine

-

grained sediments are thermally mature in the

Southern sub-basin and were the likely source for the

Ardjuna oils found in the surrounding fields.

BASIN EVOLUTION

One of the goals of this study was to analyze the

timing and nature

of

sediment

fill

within each of the

sub-basins. This is best described by comparing

isopach maps as a function of time,

so

that variations

in fault movement and basin development can be

recognized. The results of this study show that the

orientation of the basin margin faults with respect to

the dominant extensional direction was the key to

basin development.

The Jatibarang isopach (Figure

8)

shows that

Jatibarang sediment thicks have a well-defined NW-

SE trend parallel to ~ the basin-bounding faults,

especially within the Central and Southern sub-basins.

This trend suggests that the NW-SE trending

extensional faults were active during the deposition of

the Jatibarang, which agrees with observations by

Daly et al. (1987) of regional N3O0-70 E extension

direction during this time (Figure 3B).

Comparison of the Jatibarang isopach (Figure 8) with

the Lower Talang Akar isopach (Figure 9) suggests a

change in the dominant extension direction between

the Jatibarang and the Lower Talang Akar deposition.

Some half-grabens that were active during Jatibarang

time, such as the northwest-southeast trending faults

east of the TZ-1 well in the Central sub-basin, show

no movement during Lower, Talang Akar time.

Conversely, the roughly north-south trending fault

west of the BTS-1 well, whick exhibits only slight

movement during the Jatibarang, shows the most

movement of any fault during the Lower Talang Akar



53

(Figure 9). In an example from the Southern sub-

basin, the northern basin margin fault, which trends

northwest-southeast and was active during Jatibarang

time (Figure 8), apparently was inactive during the

Lower Talang Akar. However, during Lower Talang

Akar time, movement along the eastern margin fault,

which trends north-northeast to south-southwest,

became dominant. The differences in isopachs suggest

that NW-SE trending faults, more active during

Jatibarang time, became subordinate to more northerly

trending faults.

The isopach map of the Upper Talang Akar (Figure

10) shows dramatic thickening of this interval in the

Southern Ardjuna sub-basin (3000 feet or 900 m)

compared to the other sub-basins (maximum 1800 feet

or

550

m), a difference of approximately 30%. Other

basins were active, but not

as

much

as

the Southern

sub-basin because their basin margin faults were not

optimally aligned to the principle extension direction.

Based on additional detailed mapping, the thickening

in the Southern Ardjuna occurred during the earliest

part of the Upper Talang Akar, a period of widespread

coal development throughout the whole Ardjuna basin.

This thickened package also contains high amplitude

seismic reflections, indicative of a coal-rich section

(Figure 7). Geothermal modeling, based on predicted

depths to the top of this proposed coal-rich section,

suggested that these sediments are now within the late

oil/early gas maturity window. This direct

identification of

a

thick sequence of thermally mature,

potentially organic-rich sediments helped in

redirecting exploration from the Central sub-basin

toward the southern sub-basin.

By comparing all three isopach maps, a continued

clockwise rotation of the dominant extension direction

is apparent in the Ardjuna basin from the Eocene to

the upper Oligocene. The dominant extension direction

during deposition of the Jatibarang Formation (N45 E)

changed to approximately N60 E during deposition of

the Lower Talang Akar, which then changed to

approximately N90 E during Upper Talang Akar

deposition, roughly 45 degrees

of

total rotation. Based

on additional, shallower, interval isopach maps and in-

situ break-out studies in wells,

this

E-W extension

appears to have been the dominant extension direction

within ONWJ from the end of the Oligocene to

present day.

The Ardjuna sub-basins originated

as

a series of pull-

apart basins in the Eocene due to major fault

movement along strike-slip faults in the Malay

peninsula and Thailand areas (Daly et al., 1987).

Displacement along these faults ceased in the

Oligocene and the Ardjuna sub-basins appear to have

been dominated by the oblique, compressive

subduction of the Indian Ocean plate beneath Sumatra

and Java. Subduction imparted a shear component and

related extension

in

the Ardjuna area that initiated

during the early Eocene and is still observed today.

The E-W extension during Upper Eocene led to the

deposition and preservation of a thick section of

organic-rich sediments within the South Ardjuna sub

-

basin during early Upper Talang Akar time. Continued

subsidence moved these sediments into the oil

generation window and they are now in the late

oil/early gas maturity window. The presence of these

thermally mature source facies juxtaposed to good

-

quality reservoir sandstones of the Upper Talang Akar

creates a new play type in the Southern Ardjuna sub-

basin. With success of the LU-1 well, the first test of

this play concept, additional wells are planned for this

new play fairway in a mature hydrocarbon province.

CONCLUSIONS

The results

of

a Talang Akar regional study suggested

that an underexplored sub-basin within the Ardjuna

basin was the principle source for the Ardjuna oil and

most of the gas found to date in the basin. This

analysis, based on detailed structural maps, isopach

maps, and the identification of the seismic signature

of organic-rich sediments, was proven successful with

the drilling of a well in the center of the Southern

Ardjuna sub-basin, the LU-1. The LU-1 well tested

over 1400 BOPD and 12 MMCFGPD combined from

3

intervals. This sub-basin, which had previously

been overlooked due to the misinterpretation of

basement on poor quality seismic data, will be the

focus for future Talang Akar exploration.

ACKNOWLEDGMENTS

The authors would like to thank the managements of

ARCO Indonesia, Pertamina, and all other ONWJ

partners for their permission to publish this report.

The authors would also like to thank the ARCO

Indonesia Drafting Group, especially Hartanto,

Hendartoyo, and Asep for drafting the illustrations.



154

REFERENCES

Daly, M.C., Hooper, B .G.D ., and D.G. Sm ith, 1987,

Tertiary plate tectomics and basin evolution in

Indonesia, Proc. IPA 16th Ann ual Convention,

3 99-428.

Gord on, T.L., 1985, Talang Akar coals

-

Ardjuna

subbasin oil source., Proc. IPA 14th Annual

Convent ion, 9

-

120.

Kaldi, J.G., and C.D., Atkinson, 199 3, Sea l potential

of the Talang Akar Formation, BZZ area Offshore

N W Java Indonesia. , Proc. IPA 22nd Annual

Convent ion, 3 73 -3 94.

Ponto, C .V., Wu, C.H., Pranoto, A., and Stinson,

W.H ., 1 98 8, Improved interpretation of the Talang

Akar depositional environment as and aid to

hydrocarbon exploration in the ARII Offshore

Northwe st Java Contract Area., Proc. IPA 17th

Annual Convention, 397-422.

Suria, C., 1 991, Development strategy in the BZZ

field and the importance of detailed depositional

model studies in the reservoir characterization of

Talang Akar channel sandstones., Proc. IPA 20th

Annual Convent ion, p .

Suria, C., Atkinson, C.D., Sinclair, S.W., Gresko,

M.J., and B ima M ahaperdana, 1994, Application of

integrated sequence stratigraphic techniques in non-

mar indm arginal mar ine sediments ; An example f rom

the Upper Talang Akar Formation, Offshore

Northwest Java., Proc. IPA 23rd Annual Convention,

145-159.

Van d e Weerd, A.A ., and R .A. Armin, 1992, Origin

and evolution of the Tertiary hydrocarbons-bearing

basins in Kalimantan (Borneo), Indonesia: AAPG

Bulletin, v . 76, 1778-1803.



I55

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157

FIGURE 4 -

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161


basin evolution of the ardjuna rift system and its implications for hydrocarbon exploration, onwj

Documents

ardjuna basin

southern ardjuna sub

southern subbasin

basin evolution

southern ardjuna subbasins

central ardjuna

northern ardjuna

ardjuna rift system