Abstract

Surface geology and gravity data reveal that the structural pattern of the onshore portion of the Bengkulu Basin is formed of a transtensional duplex of NW - SE striking basin-bounding faults and N-S trending step-like tensional faults. Two asymmetrical grabens occur within the study area, i.e. the Pagarjati Graben in the northwest and the Kedurang Graben in the southeast. These are separated by the N-S trending Masmambang High.

Two phases of graben development can be recognized. NE-SW oriented Paleogene grabens were overprinted by N-S oriented Neogene grabens. The N-S trending Neogene grabens were formed during the Oligo-Miocene and are related to dextral motions along the Sumatra Fault System (Ketaun-Tanjung Sakti Fault and Manna Fault). The first such transtensional episode occurred in the Oligo-Miocene during which the sandstones, breccia-conglomerates, tuffaceous shales and limestones of the Seblat Formation were deposited in a fluviatile to shallow marine environment.

Rejuvenation of the pre-existing tensional faults took place during Middle-Late Miocene times. Sandstones and claystones with coal intercalations (Lemau Formation) were deposited in shallow marine and lagoonal environments. Basin subsidence continued during the Late Miocene-Pliocene times, accompanied by deposition of the littoral sediments of the Simpangaur Formation. The shallow marine deposits of the Plio-Pleistocene Bintunan Formation were deposited during a phase of basin uplift and volcanic activity related to the Barisan orogeny.

Hydrocarbon exploration potential in the Pagarjati and Kedurang Grabens appears to be attractive. Reservoirs are identified in the Seblat formation sands and Middle Miocene limestones while potential source rocks occur in the organically rich Lemau Formation. If a model of basin initiation in the Paleogene is accepted, there may be potential for lacustrine source rock developments, the presence of which would considerably upgrade the prospectivity of the basin.