Tertiary response to oblique subduction and indentation in Sumatra, Indonesia: new ideas for hydrocarbon exploration

Abstract

Abstract The Tertiary basins of Sumatra, which lie along the leading edge of the Eurasian Plate, comprise a mature hydrocarbon province whose origin and subsequent evolution are closely related to the processes of oblique subduction and indentation that have resulted from the collision of the Indo-Australian and Eurasian plates. The Sumatran basins exhibit a general rift-sag geometry with broad plate-margin parallel sag basins overlying a series of N-S-oriented grabens. It is these N-S-oriented grabens, whose origin and subsequent development are still debated despite extensive oil industry data, that control the distribution of the lacustrine sediments which are the primary source-rock for hydrocarbons in Sumatra. The Ombilin Basin provides high quality outcrops of sediments of equivalent age and depositional setting that may be studied, providing new insights into petroleum exploration of the basin itself that may be applied to the more mature areas of Sumatra. The Ombilin Basin shows a three-fold evolution. Initial sedimentation in the Eocene appears to have been controlled by normal fault displacements and not strike-slip motions. This suggests a close genetic relationship with the N-S-oriented early Tertiary grabens currently located within the back-arc region (North, Central and South Sumatra Basins), and that the Ombilin Basin did not originate as a local pull-apart related to the Sumatra Fault Zone. Subsequent sedimentation in the basin (Oligocene) was dominated by fluvial deposition which is shown to have had a tectonic control at a time of active volcanism and strike-slip faulting. This is taken to indicate modification of the initial basin style by strike-slip activity along the Sumatra Fault Zone. The final phase of sedimentation in the basin (Early Miocene) is dominated by marine deposits which form the lower part of a transgressive-regressive wedge of regional extent. This wedge of sediments buries the earlier graben systems and is related to thermal subsidence. Uplift of the basin to its present intramontane setting and differentiation from the Central and South Sumatra Basins took place in the Mid-Miocene or later. The model for the Ombilin Basin proposed here is that of a wrench-modified rift basin and is quite different from previous models giving the possibility for development of new play concepts. A similar genetic origin to the Central and South Sumatra Basins is clearly suggested, highlighting the applicability of the Ombilin Basin as an exploration model for the Sumatra graben and other analogous basins in SE Asia.