Onshore concessions are operated by Pertamina, the Indonesian national oil company, and offshore concessions are operated by companies in production-sharing agreements with Pertamina.  Concessions cover large areas and have been operated for many years; however, some parts of the Java Sea region remain virtually unexplored.

Tectonics
     The Sunda Shelf plate is confined on the east by oceanic crust and spreading centers, to the west by continental crust, and to the south by Cretaceous oceanic and continental crust and was emergent during much of the Tertiary (Pulunggono, 1985; Ponto and others, 1988).  Since the early Tertiary, the plate has generally tilted southward and subsided (Ponto and others, 1988).  The current subduction system, located offshore south of present-day Java, began in late Oligocene (Hamilton, 1979). Tectonic stress and extension, resulting from northward movement of the Australian and Indian plates and rotation of Borneo, formed rifts or half-graben complexes along much of the southern margin of the Sunda Shelf plate (now Sumatra and Java) in Eocene to Oligocene time (Fig. 3) (Hall, 1997a, b; Longley, 1997; Sudarmono and others, 1997).  These complexes are aligned north-south and are separated by faulted plateaus.  Onshore structural features comprise alternating basins and structural highs, from west to east these are the Tangerang High, Ciputat Basin, Rengasdengklok High, Pasir Putih Basin, Pamanukan-Kandanghaur High and Horst, Jatibarang Basin, and the Ceribon Trough (Fig. 2) (Reminton and Pranyoto, 1985; Adnan and others, 1991).  Offshore structures include the Sunda and Asri Basins, Seribu Platform, Ardjuna Basin, F High, Vera Basin, Jatibarang Basin, Eastern Shelf, Billiton Basin, Karimunjawa Arch, and the Banwean Trough (Ponto and others, 1988; Adnan and others, 1991). Some evidence suggests a combined symmetrical sag and half-graben history to the early tectonics of the Sunda and Asri Basins (Aldrich and others, 1995).

Deposition
     Half-graben style deposits began to fill individual and connected basins beginning with locally derived clastics from the shoulders of the half-grabens, some of which were sometimes occupied by fresh-water lakes (Fig. 4) (Bishop, 1988; Wicaksono and others, 1992).  Sediments eroded from the emergent Sunda Shelf entered the rift basins generally from the north.  Regionally derived clastic sediments were added to the half-grabens as subsidence continued, and larger basins developed, changing to the late-rift and post-rift sag phase (Wicaksono and others, 1992).  Lacustrine source rocks, reservoir facies, and migration paths described in the Sunda and Asri Basins are analogous to the distinct and predictable sedimentary facies associated with half-graben architecture described by Bishop (1988); Lambiase (1990); and Lambiase and Bosworth (1995).

     Clastics from the Sunda highlands formed shoreline and deltaic deposits that surrounded and filled the half-graben lacustrine basins (Ponto and others, 1988; Sudarmono and others, 1997).  Regional minor marine transgressions of late Oligocene age from the south, influenced the southernmost of these basins individually at differing times (Sudarmono and others, 1997).  The associated marine shorelines, which were roughly parallel with the present-day north coast, generally shifted north and south with changes in relative sea level (Ponto and others, 1988).  The relatively high areas of the Eastern Shelf and the Seribu Platform were separated by the subsiding Ardjuna Basin, where late Oligocene age deltaic and marine deposits accumulated (Ponto and others, 1988).  In the Ardjuna Basin area, south across Java and the Jatibarang area, thick coal deposits developed (Gordon, 1985; Ponto and others, 1988).  Shallow marine facies of latest Oligocene and earliest Miocene age are also shown to occur in the Vera Basin, separated from, and later connected to, the Ardjuna Basin (Ponto and others, 1988).  During early Miocene time, a marine transgression from the south flooded much of the south Sunda Shelf including all of the Northwest Java Province.  The transgression occurred latest in the Asri Basin (Aldrich and others, 1995).  Marine carbonates formed around paleohighs (Park and others, 1995).  High fault-block trends of the half-graben systems, aligned north-south, also were the sites of carbonate buildups that reinforced the north-south alignment well after the fault blocks were buried.  Intermittent lowstands exposed the reefs and associated facies producing areas of enhanced porosity for the development of later hydrocarbon reservoirs (Park and others, 1995; Wicaksono and others, 1995; Pertamina, 1996).  Widespread marine highstands in the Miocene deposited marine shales and marls that are considered to provide basin and regional hydrocarbon seals (Wicaksono and others, 1995; Pertamina, 1996).  Major regression in the middle Miocene resulted in the deposition of clastic shallow marine, shoreline, deltaic, and continental deposits alternating with the occasional phases of carbonate development (Pertamina, 1996).  Late Miocene transgressions again provided for development of carbonate bioherms and deposition of marine sediments (Pertamina, 1996).  During the Pleistocene, the Sunda Shelf was exposed intermittently, and shales, fluvial clastics, and volcanics were deposited (Pertamina, 1996).