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
JATIBARANG/TALANG AKAR-OLIGOCENE/MIOCENE (382402) ARDJUNA ASSESSMENT UNIT (38240201)
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.
Talang Akar Formation
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
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 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).
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).