Hydrocarbon basins in SE Asia: understanding why they are there

There are numerous hydrocarbon-rich sedimentary basins in Indonesia, Malaysia and southern Thailand. Almost all these basins began to form in the Early Cenozoic, they are filled with Cenozoic sediments, most are rifted basins that are the product of regional extension, and they formed mainly on continental crust. Many different models have been proposed to account for their formation and age. Understanding basin development requires a better knowledge of the Mesozoic and Early Cenozoic history of Sundaland, which mainly lacks rocks of this age. Sundaland is a heterogeneous region, assembled from different continental blocks separated by oceanic sutures, in which there has been significant Mesozoic and Cenozoic deformation. It is not a ‘shield’ or ‘craton’. Beneath Sundaland there is a marked difference between the deep mantle structure west and east of about 100°E reflecting different Mesozoic and Cenozoic subduction histories. To the west are several linear high velocity seismic anomalies interpreted as subducted Tethyan oceans, whereas to the east is a broad elliptical anomaly beneath SE Asia indicating a completely different history of subduction. Throughout most of the Cretaceous there was subduction north of India, preceding collision with Asia. However, north of Australia the situation was different. Cretaceous collisions contributed to elevation of much of Sundaland. Subduction beneath Sundaland ceased in the Cretaceous after collision of Gondwana continental fragments and, during the Late Mesozoic and Paleocene, there was a passive margin surrounding most of Sundaland. When Australia began to move northwards from about 45 Ma, subduction resumed at the Sunda Trench. Basins began to form as the region went into compression at the time of subduction initiation. There was widespread extension, broadly orthogonal to the maximum compressive stress but modified by pre-existing basement structure. After subduction resumed, the weakness of the Sundaland lithosphere, unusually responsive to changing forces at the plate edges, meant that the basins record a complex tectonic history.