Petroleum Occurrence 
     The Jatibarang/Talang Akar-Oligocene/Miocene petroleum system consists of several pods of active Jatibarang and Talang Akar Formation source rocks (Fig. 7).  The areas are combined into one petroleum system and one assessment unit in this report, although geochemical distinctions can be made among hydrocarbons derived from these mature areas of the same source-rock interval (Noble and others, 1997).  There are five major stratigraphic intervals that produce hydrocarbons in the Ardjuna assessment unit each with associated trap types: 1) Jatibarang tuff, 2) Talang Akar sandstones, 3) Batu Raja carbonates, 4) Upper Cibulakan clastics and carbonates, and 5) Parigi carbonates (Fig. 4).  Some of these stratigraphic intervals form geographic trends; the marine carbonate reefs, for example, are located on topographic highs (frequently north/south trending fault blocks) and occupy areas of favorable paleowater depth.

     The oldest producing stratigraphic horizon is volcanic tuff of the Jatibarang Formation.  Hydrocarbons accumulated in fault blocks adjacent to downfaulted areas of mature source rock primarily in the Jatibarang Basin near Cirebon (Fig. 8) (Adnan and others, 1991).  The Jatibarang field had produced 72 MMBO by 1989 from the volcanic reservoirs.  Additional reserves are in overlying reservoirs (Adnan and others, 1991), with production from Talang Akar clastics, Batu Raja carbonates, Upper Cibulakan clastics, and Parigi carbonates (Adnan and others, 1991).  Lacustrine rocks in the Jatibarang Basin, in both onshore and offshore areas, are the source of large hydrocarbon accumulations in all directions (Adnan and others, 1991; Noble and others, 1997).  This mature lacustrine source rock is included in the Jatibarang/Talang Akar-Oligocene/Miocene petroleum system (382402).  Oils identified as sourced by lacustrine source rocks are mixed in some fields and distinct in others in this area (Noble and others, 1997).  The Jatibarang field is the largest field discovered in onshore Northwest Java (Adnan and others, 1991).

     Sandstone reservoirs of the fluvial-deltaic, shoreline, shallow marine Talang Akar Formation produce oil and gas in traps distributed onshore and offshore in the Ardjuna assessment unit (Fig. 8) (Pertamina, 1996).  Marine channel sandstone and reef limestone of the Batu Raja Formation, which formed during the post-rift sag phase as the result of a widespread marine transgression from the south, are also important producing reservoirs in the assessment unit (Pertamina, 1998).

     The Upper Cibulakan Massive and Main reservoirs consist of deltaic and nearshore clastics and platform carbonates.  They were deposited during a rise in sea level that flooded the Sunda Platform (Yaman and others, 1991).  North- to south-oriented Mid-Main carbonate buildups are located in a small area northeast of Jakarta (Yaman and others, 1991).

     Pre-Parigi and Parigi carbonates were developed in middle to late Miocene time (Fig. 4), when a carbonate platform with north-south oriented bioherms developed across the area (Yaman and others, 1991).  Pre-Parigi buildups are located in a large area that extends 30 miles (50 km) offshore northeast of Jakarta (Carter and Hutabarat, 1994; Yaman and others, 1991).  From east of Jakarta to north of Cirebon, Parigi carbonate buildups occur, offset to the east of the Pre-Parigi buildups, over a large area both onshore and offshore (Carter and Hutabarat, 1994; Yaman and others, 1991).

     Oils analyzed by GeoMark (1998) from the Ardjuna assessment unit show similarities of stable carbon isotope composition of the saturate hydrocarbon fraction (13Cs) plotted against the pristine to phytane ratio (Pr/Ph) (Fig. 6).  The unit has oils with high oleanane concentrations, C27/C29 sterane > 1, hopane/sterane > 4 and dominated by medium weight n-alkanes (Haposan and others, 1997).  These oils are attributed to source rocks formed in a deltaic, coal-rich depositional environment (Haposan and others, 1997).  Sulfur concentrations of 0.02—0.42% were reported by Haposan and others (1997). There are also oils that have an abundance of botryococcane, hopane/sterane > 8, sterane distribution of C28 is > C29 is greater than C27, and n-alkanes of C21-C33 indicating a lacustrine origin for some of the source rocks in the assessment unit (Haposan and others, 1997).  Sulfur concentrations of these oils were reported as 0.11—0.16% (Haposan and others, 1997).  These accumulations have been included with the petroleum system.  A carbonate-derived oil, possibly mixed with the deltaic/coal sourced oils, was also recognized by Haposan and others (1997). 

     Gas in the Jatibarang/Talang Akar-Oligocene/Miocene petroleum system, is interpreted to be derived from the gas phase of the same source rocks as discussed above.  Carbon dioxide content in gas is low except in some onshore areas where CO₂ content is higher than 50% and attributed to metamorphism of carbonate minerals (Noble and others, 1997).

     Petroleum migration occurs both vertically and laterally.  Vertical migration paths are primarily faults that allow migration from the source rock to overlying reservoir rocks (Noble and others, 1997).  Lateral migration paths occur along basement surfaces and unconformities, or through porous beds, to reservoir rocks distant from the location of the mature source rock.  Clastic channels of the Talang Akar Formation, aligned north to south, are believed to be especially important migration paths in the region (Noble and others, 1997).  These channels connect the several basins that contain mature source rock onshore to reservoirs that now are located offshore, up dip and to the north (Fig. 1) (Noble and others, 1997).  Combinations of vertical and lateral migration are important mechanisms in this region for sourcing multiple stacked reservoirs (Noble and other, 1997).  Gas in the shallow carbonate fields of Pre-Parigi and Parigi Formations may be derived from Talang Akar source rocks in as much as many of these buildups occur on faults that controlled subsidence of some of the mature subbasins.  Carbonate buildups that lack a fault system, on the other hand, contain little or no gas (Yaman and others, 1991; Carter and Hutabarat, 1994).  Other buildups that are located away from these subbasins and faults may contain gas from the biogenic petroleum system and assessment unit. 

     The average field that produces from the Talang Akar Formation in the offshore area contains 25—30 million barrels of oil equivalent (MMBOE) with equal volumes of gas and oil (Gresko and others, 1995). At the end of 1995 cumulative oil production was reported at 1.025 BBO and gas totaled 1.427 trillion cubic feet of gas (TCFG).