Abstract

In the Bengkulu PSC of onshore and offshore Southwest Sumatra, localized basins containing four distinct seismic megasequences are recognized.

The basal, Paleogene, megasequence was deposited as a syn-rift unit within a series of northeast-trending half graben, probably segmented by northwest-trending transfer faults. A major unconformity separates this unit from a late Paleogene to early Miocene megasequence and appears to mark a change in basin-forming mechanism from orthogonal extension to possible oblique slip. According to this model, the transfer faults of the rift system were rejuvenated by right-lateral oblique slip in the late Paleogene to early Miocene, thereby superposing local pull-apart basins on the underlying graben.

These units are succeeded with strong unconformity by a middle to late Miocene megasequence marking the onset of open marine deposition within a unified forearc basin. Finally, this unit was overlain by a dominantly regressive Pliocene to Recent syn-orogenic megasequence resulting from the main period of uplift and erosion of the Barisan Mountains. The associated basin inversion of the older megasequences increases in intensity from offshore toward this mountain belt.

These results imply that far from accommodating a simple, homogeneous fore-arc basin, the fore-arc is tectonically heterogeneous with considerable potential for localised Paleogene and early Neogene basins.

Recent exploration of the Bengkulu PSC, targetting the lower two megasequences of Paleogene to early Miocene age, implies that such localized basins within the fore-arc can be prospective for hydrocarbons. Well results indicate the presence of mature source rocks and migrated hydrocarbons, and therefore appear to contradict the widespread assumption that heat flow values in fore-arc areas are insufficient to allow expulsion and migration of hydrocarbons.