Regional Geology

The West Natuna Basin is located in the South China Sea between the Malay Peninsula and the Natuna Islands. It is bounded by the Khorat Swell basement high to the north, the Natuna Arch basement ridge to the east and the Sunda Shelf basement high to the south. The basin is open to the Malay Basin and is essentially the eastern end of that basin.

The West Natuna Basin tectonic elements are both the NE-SW extensional trend and the NW-SE transtensional trend. The Natuna Basin was formed by multiple extension events in the Late Eocene and Oligocene. Post-rift subsidence in the Early Miocene was followed in the Mid Early Miocene by regional compression which persisted into the Late Miocene and to a lesser degree is still evident today.

Among the formations within the West Natuna Basin, some are potential hydrocarbon source rocks. They are the Barat Shale, Keras Shale and Benua Shale. Probably the most prolific hydrocarbon source rock is the Benua Shale, which exists deeper than the others

In eastern Kalimantan, marginal basins developed as the result of Miocene rifting of Sulawesi from Kalimantan (Katili, 1973; Hamilton, 1976; Koesoemadinata, 1978). After rifting, in the marginal basins develop deltas. Two large deltas prograded from the Sunda Landmass in the west over the oceanic crust of the Makassar Strait.  According to Samuel and Muchsin (1975), an overall westward marine transgression prevailed in the northwest Kutai Basin until Late Oligocene when orogenic activity and the initial emergence of the Kuching High reversed the direction of shoreline migration. In the central Kutai Basin, marine transgression was dominant through Early-Middle Miocene. Koesoemadinata (1978) suggested that from Middle Miocene to Recent, the axis of the Kutai Basin depocentre migrated eastward due to deposition of sediments derived from the emergent western source area. The deposition of coal occurred in a deltaic environment.

In the Kutai basin and Barito sub basin, sedimentation occurred during one complete cycle of transgression and regression. Transgression occur in Eocene to Middle Miocene age, and regression in Middle Miocene until quarternary.

Mahakam Delta lies east of the Kutai Basin of deposenter fluvial deltaic since Early Miocene.  Development of geomorphology and hydrology of the Mahakam River is influenced by the tertiary tectonics. Appointment fold path that began in the early Middle Miocene caused the Mahakam river incise anticlinorium Samarinda (Chambers & Daley, 1995; Ferguson & McClay, 1997). This results since the Middle Miocene Mahakam delta deposenter position remains on the beach.

Mahakam delta has formed since 5000 years ago, and it still accumulated sediments as thick as 50 to 70 meters as a regressive highstand delta system that downlap in sediments deltaic system transgresi Holocene and Late Pleistocene lowstand (Allen, et al., 1979).

In the triangular diagram of Galloway, Modern Mahakam Delta is an example of the delta formed by the balanced interaction between the river and tidal (Fig. 3). However, in reality certain areas of the Mahakam delta shows the process of tidal and wave in the zone may interdistributary abandoned bay and delta zone of northern Modern Mahakam Delta.

The spread of Indonesian oil line first regionally proposed by van Bemmelen (1949). Van Bemmelen dividing lines or arbitrary basin into 11 points:
1. East Sumatra Belt (north, mid, south Sumatra)
2. West Sumatra Belt (fore-arc basins of Sumatra today)
3. SE Sunda Belt (mean SE Sundaland, including Java and Madura north, SE Borneo, E and NE Borneo Borneo, Sabah)
4. Central Borneo (Melawi-Ketungau)
5. West Celebes (Lariang, Karama, South Sulawesi)
6. East Arm Celebes (Banggai-Sula)
7. Buton
8. Timor-Seram (Banda arc current),
9. South New Guinea (stretching from the Bird’s Head-Lengguru-Asmat, Merauke)
10. Median New Guinea (Central Range),
11. North New Guinea (Sarera-Rombeba-Jayapura).
Van Bemmelen make this point based on the events seepage, oil fields, and the possibility of geology.