The Geology of Indonesia/Introduction
Indonesia is the largest archipelagic 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 are 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 are 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 shelves is the island group 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 them are now producing oil and gas. 73 percent of these basins are located offshore, about one third of them in the deeper sea, with water depth exceeding 200 m.