Basin formation in the Nias area of the Sumatra forearc, western Indonesia

Although Sumatra includes the site ofIndonesia's first oil field and continues to be one of that country's most important hydrocarbon producing regions, the forearc basin to the west of the island remains a frontier exploration area. The possibility of commercial reserves hinges on the presence of a complex of deep sub-basins, but the development of these basins, and their relationship to structures onshore Sumatra, is stIll unclear. Although geological observations on the forearc islands are acquiring increasing exploration importance with the recognition that the sediments exposed were deposited dominantly or entirely within the forearc basin, seismic reflection data remain the key to geological understanding. Reconnaissance surveys in the basin near Nias, the largest of the forearc islands, have defined two major depocentres but interpretation has been hampered by poor data quality in some areas. Measurements of gravity field point to remarkable structural variations along the axis of the forearc basin and can be used to amplify and extend seismic interpretation. The combined analysis demonstrates that although some of the modern structural highs that transect the forearc basin have been positive elements for considerable periods, at least one, in the vicinity of the Banyak Islands, overlies a deep depression. Formation of this basin appears to be related to the partitioning of strike-slip motion between the main Sumatra fault system and the Mentawai Fault which defines the outer margin of the forearc basin. The poor quality ofthe seismic data in some areas may be due to extensive shale diapirism, which must be recognised as a factor in future exploration.