The Malayan Archipelago is the principal oil-producing area of the Far East. The oil fields of the southern portion of the East Sumatra basin—the South Sumatra or Palembang basinal area—are large contributors to its output. The South Sumatra basinal area is geologically the best known of the entire Archipelago. To the west this Neogene sedimentation environment is flanked by the Barisan geanticlinal borderland, and to the east it is hinged on Sunda land.
Early in Neogene time the sea began to invade South Sumatra—at that time a subsiding land with a rugged topography and subject to considerable volcanism. In the first stage of Neogene sedimentation the original topography of the basin floor caused important facies changes. Thick monotonous sequences of clay shale were laid down in the original topographical "lows" of the basin floor whereas terrigenous sands developed around the original "highs." Later in Neogene, sedimentation in the basin had been almost completed. In fact, towards the beginning of the Pliocene the sea was in full retreat (nearshore mud deposits with sandstone members overlain by sediments of a sub-continental character).
In Pleisto-Pliocene time rather weak folding movements took place. Three major anticlinoria, developed over and partly adjacent to areas of original high basement relief, can be distinguished.
Reservoir sands are present throughout the entire Neogene section. They contain commercial accumulations of oil where in structurally favorable position and are underlain by, or immediately adjacent to, thick sequences of monotonous shale deposits. The shale filling of the original "lows" of basement should be regarded as the chief source of the oil accumulated in the reservoir sands. In areas where the basement rises rapidly toward the Sunda platform, facies changes caused the development of sheets and lenses of sand as well as overlapping lime deposits with the possibility of the presence of major accumulations in up-dip pinchouts.
Nearly all oil fields are situated on well-exposed anticlinal structures. For many years, the mere presence of oil seepages or the discovery of anticlinal closure or a combination of both was the sole reason for drilling. Surface oil indications are numerous in beds of upper Miocene and Pliocene age. Practically no seepages are known in Neogene beds older than upper Miocene. Those beds, however, produced the bulk of the oil, from well-closed anticlinal accumulations.
The oils from the oldest reservoirs are heavy paraffinic with A.P.I, gravities ranging between 35° and 370. Oils from younger reservoirs are light to medium paraffinic (A.P.I, gravities between 45.30 and 54.60). Asphaltic oil—A.P.I. gravities between 22.30 and 25.70—are produced from one or two minor fields in the Djambi and the Palembang (north of the Musi River) district.