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The Lower Miocene ‘Malaysian Great Barrier Reef’ of Sarawak: The Influence of Tectonic Setting on Along Strike Variability in Platform Growth Styles and Reservoir Characteristics


In SE Asia widespread carbonate sedimentation took place during the Oligocene and early Miocene. In Sarawak, the Middle to Upper Miocene Central Luconia Province carbonates are renowned as prolific reservoir units. However, both onshore and offshore, ‘older’ carbonates have been penetrated including recent high profile discoveries. A recent play-based exploration study refocused efforts on Lower Miocene carbonates in Sarawak and examined the along strike variability in platform growth styles and reservoir characteristics. The present-day Sarawak shelf is a relatively stable, regionally extensive shelf system that was established during the early Late Oligocene and reached its maximum extent during the earliest Miocene. In the Early Miocene (∼18-20 Ma), the clastic shelf system retreated to the southwest, and consequently widespread carbonate deposition occurred in the distal shelf area. Depositionally, two environments of deposition (EOD) developed with: 1) a pronounced high-relief ‘Barrier Reef’ platform margin along the shelf edge, and 2) a proximal mixed carbonate-siliciclastic back-barrier shelf system. Primary reservoir quality of these two EOD types are distinct with argillaceous limestones dominating the back-barrier shelf, whereas high energy deposits including grainstones and coral-rich deposits have been documented for the ‘Barrier Reef’ platform margin. Furthermore, the development of the Lower Miocene ‘Malaysian Great Barrier Reef’ was affected by varying tectonic histories. Extensional and transtensional faulting and major uplift occurred in northern Sarawak, whereas passive margin subsidence took place in the south-east. Consequently, carbonate deposition terminated as early as the Burdigalian in south-eastern Sarawak as a result of smothering by the advancing siliciclastic shelf. In the northeast, the development of rotated and uplifted fault blocks generated sites for major carbonate deposition with the production of high-relief platforms that continued growing during the entire Miocene and into the Pliocene. As a result, the carbonates in south-eastern Sarawak have been deeply buried, whereas in northern Central Luconia only shallow burial took place. Fracturing and diagenetic processes are linked with these varied histories and strongly impact reservoir characteristics. The present study demonstrates how play-based exploration and better understanding of the tectonic context helps to de-risk carbonate plays in complex settings.