Introduction
Indonesia is the largest archipleagic state in the world comprising five major islands and about 300 smaller island groups. Altogether there are 13,667 islands and islets of which about 6,000 inhabited. The archipelago is situated on a crossroad between two oceans, the Pacific and Indian oceans, and bridges two continents, the Asian and Australian. Indonesia has a total area of 9,8 million sq km, of which more than 7,9 million sq km under water. Physiographically, the islands of Sumatra, Java and Kalimantan are attached to the Sunda Shelf of the Asian continent. On this landmass the water depth does not exceed 200 meters. To the east, Irian Jaya and the Aru islands lie on the Sahul Shelf, which are parts of the Australian continent. Located between these two shelfal is the island grop of Nusa Tenggara, Sulawesi, Maluku and Halmahera. These islands are encircled by deep seas which in many places reach 5,000 meters. About 60 Tertiary sedimentary basins, spread out from Sumatra in the west to Irian Jaya in the east, are identified in Indonesia. So far only 38 basins have been explored and drilled for petroleum and 14 of the are now producing oil and gas. Seventy three percent of these basins are located offshore, about one third of them in the deeper sea, with water depth exceeding 200 m.
The basic premise to explore the petroleum is the existence of sedimentary basin. Sedimentary basin is a depression on the earth surface the can accummulated the sedimentation material through time. Since most of the sediment transporting agent are water, the sedimentary basin usually the place where the water finally accummulate. It can be in the form of sea, lake, delta, or river.
Indonesia is a large country with various type of sedimentary basins. Most of the sedimentary basin in the western Indonesia are already explored since the end of 19th century. The recent years also saws the exploration on the frontier basins, especially in the eastern Indonesia. Unfortunately, the interest of the major oil companies lost their appetite to conduct explorationa activities in Indonesia probably due to the unfavorable fiscal regime. As a result, the oil and gas production is keep declining in Indonesia without any substantial reserve replacement.
The sediment which comprise of clay, sand, boulder or carbonate are being deposited in the sedimentary basin over a geologic time (million years). Due to the weight of the sedimentary deposit, the sedimentary basin experience subsidence to allow more sedimentation. This sedimentary process may accummulate sediment with cumulative thickness of several thousand meters.
Structural Geology
The tectonics processes in Indonesia formed major structures in Indonesia. The most prominent fault in the west of Indonesia is the Semangko Fault, a dextral strike-slip fault along Sumatra Island. The formation of this fault zone is related to the subduction zone in the west of Sumatra.
Palu-Koro fault is another major structural feature formed in the central part of Indonesia. This fault has similar orientation as the Semangko fault, extend from Koro in central part of Sulawesi, to Palu in the west coast of Sulawesi and extend across the Makassar strait to East Kalimantan.
Stratigraphy
The stratigraphy of the western part of Indonesia is relatively young, ranged from Paleogene to Quarternary. The eastern Indonesia has older stratigraphy compare to the western part. The stratigraphy range from Triassic to Tertiary. Devonian limestone were found in Telen River, East Kalimantan, as fragments within Paleogene clastic sediments. Ichthyosaur fossils were found in the mud volcanoes in Kai Island, indicated Mesozoic deposit in the subsurface.
Characteristics
About 60 Tertiary sedimentary basins, spread out from Sumatra in the west to Irian Jaya in the east, are identified in Indonesia. So far only 38 basins have been explored and drilled for petroleum and 14 of the are now producing oil and gas. Seventy three percent of these basins are located offshore, about one third of them in the deeper sea, with water depth exceeding 200 m.
Back-arc Basin
The most prolific sedimentary basin in Indonesia are belong to back-arc basin, whichis located on the 'back' of the volcanic-arc. Most of the back-arc basin located in the northeastern coast of Sumatra, northern Java, and continue to northern Lesser Island. Almost all of the back-arc basin are currently producing oil and gas, therefore categorized as proven petroleum province.
Fore-arc Basin
The fore-arc basin located to the 'front' of the volcanic arc, which is located along southwestern coast of Sumatra, southern coast of Java and Lesser Island. The fore-arc basin is mainly considered as a frontier basin and no commercial hydrocarbon found yet. However, there is strong indication of working petroleum system in the fore-arc basin, such as gas discoveries in the Nias area. Oil shows also reported to be found in offshore Bengkulu (Southern Sumatra). On the interesting note, the fore-arc basin in the southern part of Papua New Guinea already found commercial gas accummulation in the Pandorra Gas Field.
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