Bulletin of the Geological Society of Malaysia, Volume 49, April 2004, pp. 107 – 110
Sarawak Shell Berhad, Locked Bag No. 1, 98009 Miri, Sarawak
Abstract: The Half-Graben sub-province in West Luconia, offshore Sarawak is characterised by a series of NNW trending, SW dipping extensional faults, creating significant sub-basins within the half-grabens. Structural extension is interpreted to have taken place mainly during the Middle Miocene. The timing is supported by age dating of tilted syn-rift carbonate wedges at the base of the half-graben. These carbonates probably drowned as a result of the half-graben formation and the related rapid subsidence and influx of clastics. Middle Miocene fluviomarine to shelfal sediments later filled the half-grabens. The beginning of the Upper Miocene coincides with the end of rifting. Post-rift sediments in this area are separated by two main hiatuses: the Upper Miocene (SB 3.1, ~10.6 Ma) and the Lower Pliocene (SB 3.4, ~5.6 Ma) unconformities, which are consistently observed throughout the study area. They can be identified by the presence of lowstand features such as erosional truncations and channel incisions. Thick, seismically transparent transgressive shales lie above the unconformities. Pliocene to Recent fluviomarine to shelfal sedimentation is dominated by sea-level fluctuations; evidence of the latest lowstand is still present in the form of a sea-bottom trough/channel, which is interpreted as the Proto Rajang/Lupar River. The Miocene-Pliocene boundary was also the time of the last major structural deformation during, which large, highly faulted, anticlinal structures formed locally within the NW part of the area. The origin of these anticlines may be attributed to wrench related inversion on some of the extensional faults. The structural and sedimentation history of the area produced a variety of trapping configurations and a diverse portfolio of leads.