The upper Parigi Formation reservoir consists of platform carbonates, developed as bioherms with enhanced solution porosity, that form good reservoirs sealed by transgressive shales of the Cisubuh Formation (Yaman and others, 1991; Noble and others, 1997).  These carbonates range in thickness from a few feet to as much as 400 ft (122 m) (Yaman and others, 1997).  The carbonate buildups are aligned north and south and are best developed in an east to west trend, being less well developed to the north (Yaman and others, 1997).

       The Tertiary-Parigi petroleum system contributes 15% of the energy equivalent of the total gas and 8% of the total hydrocarbons to the reserves of the Northwest Java Basin province (Noble and others, 1997).  Shallow targets that can be easily identified on seismic data, make this gas resource important to the total reserves of the province (Yaman and others, 1991).
 

TERTIARY-CENOZOIC (382404) BILLITON BASIN HYPOTHETICAL (38240401)
       This petroleum system is hypothetical and based on the presence of two basins, Vera and Billiton, in the northeastern part of the North West Java province (Fig. 1).  These sedimentary basins are mapped as containing more than 6,500 ft (2,000 m) of Cenozoic sediments (Hamilton, 1974).  Several wells have been drilled to basement and have encountered more than 8,200 ft (2,500 m) of sedimentary strata (Petroconsultants, 1996).  Marine shelf and shoreline deposits are shown by Ponto and others (1988) to occur in the Vera and Billiton Basin areas, and probably include both source rocks and reservoir rocks similar to those of the Talang Akar Formation in the Ardjuna Basin.  The basins also may contain lacustrine sediments or coals.  Extrapolating from the general depositional history of the province, these strata are probably overlain by clastic fluvial, shoreline, deltaic, and marine deposits as well as carbonate platform deposits.

     No possible source rock has been described but, based on maturation in other areas of the province, maturation might be expected between 3,300—6,600 ft (1,000—2,000 m) of burial depth.  Migration presumably could occur vertically and laterally into any clastic and carbonate reservoirs that might be present. 

     The Pliocene to Pleistocene Cisubuh Formation is composed of claystones that act as seals in the province.  No trap style has been described.  Basement highs have been encountered during drilling, suggesting that possible drape structures, stratigraphic pinchouts, or carbonate buildups are present (Petroconsultants, 1996).
 

UNDISCOVERED PETROLEUM
       The Northwest Java Basin includes highly productive source rocks and high quality reservoir rocks trapped in numerous structural and stratigraphic settings of interest for further exploration.  In the Sunda/Asri area, Wicaksono and others (1992) have modeled expelled hydrocarbons from lacustrine source rocks in the amount of almost 10 BBO.  Past production combined with estimated recoverable reserves in this assessment unit total 1,330 MMBOE (Petroconsultants, 1996), and the estimated quantities of conventional oil, gas and condensate that have the potential to be added to reserves by the year 2025 are 627 MMBOE (U.S. Geological Survey World Energy Assessment Team, 2000).  Faults trapping hydrocarbon that have migrated out of the mature area, similar to Widuri Field, are attractive targets.  Lacustrine fans, similar to the fans that produce at Yani and Janti fields, and fluvial channel and deltaic sandstones in stratigraphic and structural traps are also possible targets.  A thorough understanding of clastic deposition as it responds to rift tectonics could lead to the identification of likely prospects and it is probable that additional carbonate buildups will be discovered. 

     There are also numerous future exploration targets in the Ardjuna assessment unit within the Talang Akar and Upper Cibulakan sandstones.  Large deltas, shoreline deposits, and sea-level highstand and lowstand features may provide additional opportunities for development of reservoirs and traps.  Although most of the carbonate buildups have been identified, more subtle carbonate reservoirs and traps may yet be found.  The estimated quantities of conventional oil, gas and condensate that have the potential to be added to reserves by 2025 are 1,380 MMBOE (U.S. Geological Survey World Energy Assessment Team, 2000).