U. S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY
Petroleum Systems of the Northwest Java Province, Java and Offshore
Southeast Sumatra, Indonesia
by Michele G. Bishop
Open-File Report 99-50R
2000
JATIBARANG/TALANG AKAR-OLIGOCENE/MIOCENE
(382402) ARDJUNA ASSESSMENT UNIT (38240201)
Reservoir Rocks
Almost 58% of the oil and gas
in the Ardjuna Basin portion of the assessment unit is from the Main and
Massive Formations and 23% is from the Talang Akar Formation and Batu Raja
carbonates (Gresko and others 1995). The oldest reservoir is within
a weathered or karstified basement limestone remnant of middle Eocene age
(Pertamina, 1996). This reservoir is found in KLS-1 (Fig 7) and is
thought to be sourced by rocks in the downdip, deeply buried Talang Akar
Formation (Pertamina, 1996).
Jatibarang Formation
The Eocene to Oligocene synrift
Jatibarang Formation is known from the north edge of the Bogor Trough across
central Java, east to Bogor and north to Jakarta. This formation
is thick in subbasins, being particularly well developed in the Jatibarang
subbasin. It probably occurs within the half grabens of the Ardjuna
basin, but is thin or missing on structural highs (Gresko and others, 1995).
The formation overlies pre-Tertiary basement, which is granite to the northeast
and low-grade schist to the northwest (Nutt and Sirait, 1985) and is time
equivalent to the Banuwati Shale of the Sunda Basin (Fig. 4) (Pertamina,
1996). The Jatibarang strata in the Jatibarang field area were folded,
faulted and eroded prior to deposition of the subsequent Talang Akar Formation
(Kalan and others, 1994). This erosional unconformity is recognized
in the Ardjuna subbasin (Gresko and others, 1995).
The Jatibarang Formation consists
of andesite lavas at the base and dacite basaltic lavas interbedded with
clays, sandstone, conglomerate, and pyroclastics in the upper parts (Nutt
and Sirait, 1985). Andesitic volcaniclastic flows and tuffs and reworked
volcanics and basement-derived sediments have also been described (Pertamina,
1996). Clastic facies change rapidly both vertically and laterally
and are mostly fluvial in origin (Adnan and others, 1991). The formation
is more than 3,900 ft (1,200 m) thick in the onshore Jatibarang field and
thins to the west (Adnan and others, 1991). Depth to the top of the
formation ranges from 9,000—13,000 ft (2,700—4,000 m) (Pertamina, 1996).
Effective porosity is due to
fractures with some intergranular and vessicular porosity (Nutt and Sirait,
1985). Porosity in some of the best producing intervals is as much
as 20% (Kalan and others, 1994) as measured by well logs.
Oil and gas are produced from the Jatibarang Formation
in the Jatibarang field (Courteney and others, 1989) and several non-commercial
hydrocarbon accumulations have been tested in the offshore area (Pertamina,
1996).
Talang Akar Formation
The Talang Akar Formation of
Oligocene age (Talangakar, Lower Cibulakan Formation) overlies the Jatibarang
Formation and basement rocks. The formation is characterized as synrift
to late rift continental style deposition (Pertamina, 1996). The
lower part represents continental deposition and the upper part represents
an increasing marine transgression (Gresko and others, 1995; Pertamina,
1996). The formation has also been divided into three units:
in ascending order, Talang Akar Grits, Deltaic Talang Akar, and Marine
Talang Akar (Kaldi and Atkinson, 1993). The basal unit is generally
of poor reservoir quality, the deltaic interval contains both the source
rock and good reservoirs, and the marine interval contains good reservoir
rocks. For example, the Talang Akar in the Jatibarang Basin area
includes carbonaceous shales in the lower unit that contain TOC of 0.5—2.0
wt% and alternating shales and limestones in the upper unit that produce
oil, gas, and condensate (Adnan and others, 1991).
The lower Talang Akar in the
Ardjuna area is time equivalent to the Zelda of the Sunda Basin (Fig. 4).
It is relatively confined to subbasin areas that had developed during deposition
of the Jatibarang Formation (Gresko and others, 1995). Paleodepositional
maps published by Ponto and others (1988) illustrate an eroding Sunda Plate
occupied by lake-filled grabens of the Ardjuna area, and an east to west
shoreline related to a marine transgression along the Bogor Trough (Suria
and others, 1994) that ran from Semarang to Cirebon and on to south of
Jakarta during earliest Talang Akar deposition. The lower Talang
Akar is dominated by continental deposits, which are immature, fine- to
coarse-grained, lithic-rich, and poorly sorted (Gresko and others, 1995;
Pertamina, 1996). They consist of sandstones, mudstones, minor coals,
and tuffs of alluvial to deltaic origin that total an average thickness
of 1,500 ft (450m) with local thickness estimated at 2,000 ft (600 m) (Gresko
and others, 1995). The sandstone reservoir is mostly poor and highly
variable in quality (Gresko and others, 1995; Pertamina, 1996). Carbonate
cement reduces porosity along with authigenic kaolinite and compaction
of the immature igneous and metasedimentary rock fragments that make up
the clastics (Gresko and others, 1995; Pertamina, 1996). Porosity
ranges from 7—28% with poor permeability (Pertamina, 1996).
The Upper Talang Akar Formation
consists of nonmarine to deltaic and marginal marine to shelf sediments
deposited during late Oligocene to early Miocene time (Ponto and others,
1988). Paleodepositional maps published by Ponto and others (1988)
show the migration of the shoreline toward the north to a position offshore
of the modern shoreline between Semarang and Cirebon, and to a later position
closer to the modern shoreline between Cirebon and Jakarta during the next
stage of deposition. Embayments extended north across the Jatibarang
subbasin depositing shoreline facies and across the Ardjuna subbasin where
major delta complexes and shoreline facies were deposited (Ponto and others,
1988). The Jatibarang subbasin, Ardjuna subbasin, and the low subsiding
area located offshore of the city of Jakarta, continued to be the focus
of marine incursion and deposition throughout deposition of the Talang
Akar Formation (Ponto and others, 1988).
Reservoir facies that have been
identified include estuarine and distributary channels, distributary mouth
bars/tidal bars, and delta front bars (Kaldi and Atkinson, 1993; Suria
and others, 1994; Pertamina, 1996). The formation may be as much
as 1,000 ft (300 m) thick, with interbedded shale, limestone, coal, and
sandstone in an overall transgressive sequence where flooding surfaces
and channel-fill have been identified using seismic data (Suria and others,
1994).
The best reservoir quality is
in 40—60 ft (12—18 m) thick estuarine distributary channel sandstones interpreted
as incised valley fill (Pertamina, 1996). These widely distributed,
stacked sandstones have porosity of 22—28% and permeability of 1—3 Darcies
(Pertamina, 1996). Sandstones interpreted as delta lobe switching
distributary channels are 20—30 ft (6—12 m) thick, locally cemented by
kaolinite, limited in extent, and have 22—28% porosity (Pertamina, 1996).
Sandstone reservoirs deposited
as distributary mouth bars are 3—15 ft (1—5 m) thick and cemented by quartz
overgrowths, illite, and kaolinite (Kaldi and Atkinson, 1993; Pertamina,
1996). Reservoir quality is considered to be good with 21—25 % porosity
and 20—526 mD permeability (Pertamina, 1996).
Burrowed delta front sandstones
are generally poor reservoirs depending on diagenesis (Kaldi and Atkinson,
1993; Pertamina, 1996). These 1—5 ft (less than 1.5 m) thick sandstones
are cemented with dolomite and kaolinite resulting in porosity of 6—14%
and permeability of 0.02—0.4 mD (Pertamina, 1996). Wave dominated,
delta front sand bars were subjected to early marine ferroan dolomite cementation
that reduced porosity to 5% and resulted in poor quality reservoir sandstones
(Pertamina, 1996).
Batu Raja Formation
As the early Miocene marine
transgression continued, and tilting of the Sunda Plate submerged sources
of clastics, carbonate development increased in the marine member of the
Talang Akar Formation. This formation was eventually conformably
overlain by the lower Miocene Batu Raja Formation (Lower Cibulakan Formation)
(Ponto and others, 1988).
In the Ardjuna Basin, the Talang
Akar Formation consists of well-developed limestones on the Seribu platform,
along fault-controlled basement highs, and around basement highs (Pertamina,
1996). The best reservoirs are reef buildups around basement highs
that were exposed during sea-level lowstands where secondary moldic porosity
resulted from leaching of aragonite grains (Pertamina, 1996). The
reefs vary in thickness from 100—150 ft (30—45 m). The main pay zones
are from 5—25 ft (2—8 m) thick with porosities of 31—36% and permeabilities
of 100—1,000 mD (Pertamina, 1996). In the Jatibarang Basin area,
the limestone with shale and marl interbeds of the Batu Raja Formation
reaches 165 ft (50 m) in thickness and produces oil and gas with high CO2
content (Adnan and other, 1991). These rocks contain approximately
5% of the identified oil equivalent reserves (Petroconsultants, 1996).
Upper Cibulakan Formation
The lower to middle Miocene
Upper Cibulakan Formation was deposited in inner to outer shelf and deltaic
environments and is divided into the Massive, Main and Pre-Parigi units.
It is equivalent to most of the Gumai Formation and some of the Air Benkat
Formation in the Sunda Basin (Fig. 4) (Butterworth and others, 1995; Reksalegora
and others, 1996). The Main and Pre-Parigi intervals are major hydrocarbon
reservoirs in the Ardjuna assessment unit; sandstone reservoirs contain
58% of the known oil equivalent reserves with the majority of the reserves
in the Main interval (Petroconsultants, 1996). Limestone reservoirs
within these intervals contain 10% of the known reserves (Petroconsultants,
1996).
The Massive and Main intervals
of the Upper Cibulakan Formation consist mainly of sandstones and limestones.
Deposition was on a marine shelf that occupied the area of the Ardjuna
Basin east of the Seribu Platform (Fig. 2) (Purantoro and others, 1994;
Reksalegora and others, 1996); marine waters transgressed from the south
and clastic sediments were derived from the north. The shoreline
trended northwest to southeast offshore of the modern coastline (Purantoro
and others, 1994; Pertamina, 1996). Multiple sea-level highstands
and lowstands have been recognized in this generally transgressive succession
(Purantoro and others, 1994).
The Main interval consists of
approximately 2,300 ft (700 m) of interbedded shales, sandstones, siltstones,
and limestones (Butterworth and others, 1995; Reksalegora and others, 1996).
Two distinct sandstone geometries that occur within this interval are discussed
by Reksalegora and others (1996): (1) north to south elongate, discrete
sandstone bodies, interpreted as filling lowstand erosional features; and
(2) extensively distributed cleaning-up sandstones interpreted as shoreface
deposits.
The strata interpreted by Purantoro
and others (1994) as lowstand sandstones and valley fill within the Main
interval are quartzose and highly burrowed. Stacked sandstones are
as much as 50—100 ft (165—330 m) thick and are separated by as much as
200 ft (60 m) of highstand tuffaceous marine shales (Butterworth and others,
1995). These reservoir sandstones have porosity of 16—33% and permeability
of 7—3,000 mD (Purantoro and others, 1994). Strata interpreted as
transgressive sandstones are glauconitic and highly burrowed with local
calcite cement (Purantoro and others, 1994). The porosity of these
reservoir sandstones varies from 21—36% and permeability ranges from 2—2,000
mD (Purantoro and others, 1994). Strata interpreted as highstand
sandstones are described as calcareous with siderite cement (Purantoro
and others, 1994). Reservoir quality is poor to moderate with porosity
of 12—30% and permeability from 0.2—800 mD (Purantoro and others, 1994).
Carbonates in the middle part
of the Main interval are north- to south-oriented build-ups on basement
highs and on the Seribu Platform (Pertamina, 1996). This interval
reaches 340 ft (100 m) in thickness with secondary solution porosity ranging
from 16—32% in pay zones that are as much as 92 ft thick (28 m) (Pertamina,
1996).
The Pre-Parigi interval of the
Upper Cibulakan Formation consists of localized carbonate bioherms formed
in middle to late Miocene and distributed over a large area northeast of
Jakarta (Yaman and others, 1991; Pertamina, 1996). It is composed
of partially dolomitized wackestone to grainstone that grade laterally
into claystone with limestone stringers (Pertamina, 1996). In well-developed
areas these strata are as much as 700 ft (210 m) thick, and the bioherms
are oriented north to south on shallow marine platforms with structural
control of basement highs or prior Batu Raja carbonate buildups (Yaman
an others, 1991; Carter and Hutabarat, 1994; Pertamina, 1996). Reservoir
quality is excellent, with preserved porosity averaging 30% and permeability
of 2 Darcies (Yaman and others, 1991). The reservoir gas, 98% methane,
is dry; (Yaman and others, 1991).
Parigi Formation
The late Miocene Parigi Formation
developed on structurally stable shallow marine platforms as bioherms associated
with paleohighs but not necessarily basement highs (Fig. 4) (Yaman and
others, 1991). It is widespread, being distributed onshore and offshore
across an area overlapping the eastern portion of Pre-Parigi distribution
and continuing to the east (Yaman and others, 1991). Offshore, north-
to south-oriented Parigi bioherms are more than 400 ft (120 m) thick (Yaman
and others, 1991; Pertamina, 1996). Separated from this trend, to
the south in both onshore and offshore areas, are northeast- to southwest-oriented
Parigi bioherms that are as much as 1,500 ft (450 m) thick (Yaman and others,
1991; Pertamina, 1996). The orientation of the bioherms is interpreted
to be the result of a combination of paleogeographic features and paleocurrent
directions; the separation of the two trends may have been caused by a
deeper water reentrant from the east (Yaman and others, 1991). Bioherms
in the northern trend are composed of skeletal-foraminiferal packstone
with little coral and generally no framework whereas bioherms in the southern
trend are composed of coral-algal reefs (Yaman and others, 1991).
In the Jatibarang Basin area, the Parigi consists of buildups composed
mostly of reef limestone that reach a thickness of approximately 490 ft
(150 m) (Adnan and others, 1991).
Reservoir quality varies from
tight to very good, due to cementation by calcite and development of secondary
porosity (Yaman and others, 1991). Porosity is as much as 30% and
permeability 2 Darcies (Yaman and others, 1991). This reservoir has
tested from 14.5 million cubic feet of gas per day (MMCFGPD) to 58.94 MMCFGPD
(Pertamina, 1996). Oil is produced in wells JTB-43 and -45 (Adnan
and others, 1991).
|