Tectonics
In the center of the South Sumatra Basin Province, are Permian to Carboniferous metamorphic and igneous rocks crop out in a northwest to southeast trend. These consist of phyllites, slates, argillites, quartzites, gneisses and granites (Adiwidjaja and de Coster, 1973). In the northwest, and south of the Permo-Carboniferous trend, are areas of Mesozoic metamorphic rocks with local granite intrusions (Adiwidjaja and de Coster, 1973). In a broad trend south of the Permo-Carboniferous and Mesozoic rocks are Mesozoic metasediments and limestones, which have been dated as Jurassic or Cretaceous, together with mafic igneous rocks (Adiwidjaja and de Coster, 1973). To the north of the Permo-Carboniferous trend near the city of Palembang, is a northwest to southeast trending area described as micritic limestone of Cretaceous age (Adiwidjaja and de Coster, 1973).

The South Sumatra basin was formed by three major tectonic phases: 1) extension during late Paleocene to early Miocene forming north-trending grabens that were filled with Eocene to early Miocene deposits; 2) relative quiescence with late normal faulting from early Miocene to early Pliocene; and 3) basement-involved compression, basin inversion, and reversal of normal faults in the Pliocene to Recent forming the anticlines that are the major traps in the area (Suhendan, 1984). Many of the normal faults that formed the depositional basins in South Sumatra have been reactivated and some have been reversed during Miocene to Plio-Pleistocene compression and basin inversion (Sudarmono and others, 1997; Zeliff and others, 1985; Moulds, 1989). The emergent Sunda Shelf plate (platform, craton, or Malay micro-plate now mostly beneath the Java Sea) was 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 (Pulunggono, 1985; Ponto and others, 1988). Sundaland, or the Sunda Shelf Plate, is considered to be composed of a mosaic of continental and oceanic microplates accreted and sutured together in the Late Triassic (Pulunggono, 1985; Cole and Crittenden, 1997). Since the early Tertiary, the Sunda Shelf plate has generally tilted southward and has been subsiding (Ponto and others, 1988). The current subduction system, located offshore west of Sumatra and south of Java, began in the late Oligocene. Uplift of the Barisan Mountains, resulting from the subduction, began in late Miocene but primarily occurred in the Plio-Pleistocene (Hamilton, 1979; Sudarmono and others, 1997). In the Eocene to Oligocene, tectonic stress and extension, resulting from northward movement of both the Australian tectonic plate to the east and the India plate to the west, and rotation of Borneo, formed rifts or half-graben complexes along much of the southern margin of the Sunda Shelf plate (now Sumatra and Northwest Java) (Hall, 1997a, b; Longley, 1997; Sudarmono and others, 1997). These rift basins overlie an unconformity formed on a variety of pre-Tertiary rocks.

The grabens and major faults of the South Sumatra Basin Province are oriented north-northwest to south-southeast. This is a similar alignment to the grabens of Central Sumatra but they are deeper and larger basins (Fig. 3). The Palembang Basin in South Sumatra is greater than 4,500 m deep (Hutchinson, 1996). The fault-bounded Benakat Gulley connects the major basin complexes of the Lematang Depression and the Palembang Depression (Fig. 2) (Hutchinson, 1996; Moulds, 1989). The north—south Benakat Gulley is similar in trend to the Bengkalis depression in Central Sumatra, the fault zone that forms the eastern coast of Sumatra, the Sunda and Asri Basins offshore, and the grabens of Northern Sumatra (Hutchinson, 1996; Pulunggono and others, 1992; Moulds, 1989). A fault zone that trends southwest to northeast, the Tembesi Fault, forms the northwestern edge of the Jambi Depression (Fig. 2).

Deposition
The overall Tertiary depositional fill of the South Sumatra Basin began in the Eocene with deposition of continental sediments derived from local erosion (Cole and Crittenden, 1997; Courteney and others, 1990). Characteristic half-graben-style locally derived deposits began to fill these basins in response to the half-graben architectural style and subsidence of the basins (Bishop, 1988; Wicaksono and others, 1992). Additional synrift deposits of tuffaceous sands, conglomerates, breccias and clays were deposited in faulted and topographic lows by alluvial, fluvial, and lacustrine processes (Fig. 4). Marine transgression occurred in some areas possibly as early as the late Eocene (Courteney and others, 1990). Widespread marine transgression from the south and southwest in the late Oligocene to Miocene resulted in onlap of clastic deposits onto basement rocks, development of platform carbonates, and carbonate build-ups on fault-block highs. Carbonate and sands were also deposited around emergent islands (Cole and Crittenden, 1997; Courteney and others, 1990; Sitompul and others, 1992; Hartanto and others, 1991; Hutapea, 1981; Tamtomo and others, 1997; Hamilton, 1979). The overall transgression was punctuated by lowstands. This resulted in development of secondary porosity in some of the carbonates. Lowstands also resulted in submarine fans within the marine shale strata (Cole and Crittenden, 1997; Courteney and others, 1990; Sitompul and others, 1992; Hartanto and others, 1991; Hutapea, 1981; Tamtomo and others, 1997; Hamilton, 1979). Regional sediment sources were generally from the Sunda Plate to the north and Palembang or Lampung High to the east (Sitompul and others, 1992). Maximum transgression in the middle Miocene deposited the marine Gumai Shale Formation seal across the region before uplift and compression resulted in deposition of shallow marine and continental sandstones and shales (Fig. 4) (Courteney and others, 1990; Cole and Crittenden, 1997; de Coster, 1974). Development of the Barisan Mountains, and possible volcanic islands to the south and southeast, further decreased and then cut off and overwhelmed marine influences and added new clastic and volcaniclastic sources from those directions (de Coster, 1974; Cole and Crittenden, 1997; Hamilton, 1979). Erosion of northwest trending anticlines that were formed during compression resulted in local Plio-Pleistocene continental deposits within the intervening synclines (de Coster, 1974). Continued volcanic activity has covered much of the surface of the South Sumatra Basin (van Bemmelen, 1949).

History of Exploration
Early exploration was guided by surface seeps that were associated with anticlines, and led to the discovery of Kampung Minyak Field in South Sumatra in 1886 (Fig. 2) (Macgregor, 1995). This field reportedly contained reserves of 31.3 MMBOE in the deltaic Pliocene Muara Enim Formation (Fig. 4) (Zeliff and others, 1985). Numerous surface anticlines have been mapped in South Sumatra, generally with a northwest to southeast trend, and are more tightly folded in the north than the south (Fig. 2) (van Bemmelen, 1949). Until 1921 the exploration target was sandstone in the Air Benakat Formation and the deepest penetration had been the Gumai Formation (Zeliff and others, 1985). In 1921 Nederlandsche Koloniale Petroleum Maatschappij (NKPM), formed by Standard of New Jersey (SONJ), discovered the Pendopo/Talang Akar Field (Fig. 2) (Zeliff and others, 1985). This discovery in the Talang Akar Formation sandstone is the largest oil field in South Sumatra with estimated reserves of 360 MMBOE (Zeliff and others, 1985; Ford, 1985). More recent estimates have increased these reserves by more than 15 percent (Petroconsultants, 1996). This discovery reportedly occurred due to communication delays, since the drillers were being paid by the foot, they drilled ahead after reaching the target Air Benakat Formation, not having been told to stop (Ford, 1985). Royal Dutch Shell (BPM), Standard of New Jersey, Socony Vacuum (Standard of New York (Mobil)), and Pertamina were all companies involved in the early exploration of South Sumatra (Zeliff and others, 1985). In 1933 SONJ (Exxon) and Socony Vacuum each held 50% interest in Standard Vacuum Oil Company (Stanvac) that took over NKPM’s oilfields and refineries and Socony’s marketing in the Asia Pacific region (Ford, 1985).