Bulletin of the Geological Society of Malaysia, Volume 37, July 1995, pp. 285 – 299
1Birkbeck College and UCL Research School of Geological and Geophysical Sciences, Gower St., London WC1E 6BT
2LEMIGAS, PO Box 1089 Jkt, Jakarta
Abstract: Although Sumatra includes the site of Indonesia‘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 of the seismic data in some areas may be due to extensive shale diapirism, which must be recognised as a factor in future exploration.