Tertiary tectonics and sedimentation history of the Sarawak basin, east Malaysia

Abstract

A seismic stratigraphic study of the regional lines for the offshore Sarawak area was undertaken with the aim of understanding the tectonics and sedimentation history of the hydrocarbon prolific Sarawak Basin. The aim here is to develop a workable stratigraphic scheme, a model of the sedimentation history of the basin, a model for Tertiary tectonics, and an analysis of the subsidence history of the basin. Six unconformities have been identified within the Tertiary sedimentary succession, based on seismic reflection and well data. Some unconformities coincide with eustatic sea-level falls; others are probably tectonic in origin. An alternative stratigraphic scheme for the Sarawak Basin was developed by subdividing the whole Tertiary succession into seven sequences. Palaeoenvironment maps of the basin document the interaction of tectonics and sedimentation commencing in late Oligocene times. Deposition started with a NW-SE coastline and a broad coastal plain, almost perpendicular to the present-day coastline (NE-SW) developed during late Miocene times. The maps illustrate the likely distribution of Sarawak Basin source and reservoir rocks which will help in effective planning for future exploration in the area. The Sarawak Basin formed as a result of NW-SE trending right lateral fault movement during late Oligocene to Pliocene times. This dextral movement was responsible for creating the NW-SE coastline and divided the offshore Sarawak area into two sub-basins. Deposition and preservation of coastal plain and shallow-marine sediments continued in the eastern area while the western area remained as a 'high' until late Miocene times and subsided during late Early to Middle Miocene. The dextral strike-slip movement which controlled the evolution of the Sarawak Basin is sub-parallel to a number of lineaments elsewhere in Sarawak. The timing of movement suggests a progressive younging in an eastward direction. Basin modelling suggests that the Sarawak Basin was characterised by rapid subsidence in the early stage of basin formation with a high stretching factor and episodic movements. This suggests that the basin did not form as a foreland basin nor as a typical rift basin, but indicates a strike-slip origin. Supplementary evidence for this is provided by the findings of the regional seismic stratigraphic study, which suggests that the whole onshore area of Sarawak and northern Borneo was subjected to strike-slip tectonism during Tertiary times. The driving force may have been initiated by the lateral movement between the Sundaland and South China Continental blocks, probably due to collision between Indian and Asian plates during the Middle Tertiary, continuing with the opening of the South China Sea during the Oligocene. The end result of tectonism in the region, however, is believed to be the combination of strike-slip movements and the counter-clockwise rotation of Borneo during the Oligo-Miocene. The superiority of the proposed strike-slip tectonic model over the present subduction model is the capability to explain most of the geological phenomena, including the absence of evidence for any subduction taking place in the area. The findings of this study should contribute towards a better understanding of the tectonics of the area which will be able to provide information on the development of structural traps for hydrocarbon plays that are believed to have formed by strike-slip tectonism.