The Geology of Indonesia/Sedimentary Basins

Introduction
The basic premise to explore the petroleum is the existence of sedimentary basin. Sedimentary basin is a depression on the earth surface the can accumulated the sedimentation material through time. Since most of the sediment transporting agent are water, the sedimentary basin usually the place where the water finally accumulate. 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 explorational 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 accumulate sediment with cumulative thickness of several thousand meters.

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.