Abstract: Reservoir Development in Miocene Carbonates, Central Luconia Province, Offshore Sarawak

Mohammad Yamin Ali

The broad platform of the central Luconia province of offshore Sarawak is characterized by extensive development of Miocene carbonates. Their distribution was controlled by paleostructures that formed earlier during Oligocene-middle Miocene faulting. Platform-type buildups formed on fault-bounded regional highs, whereas pinnacle types formed in basinal areas. Relief of the pinnacles was controlled by fault reactivation of the structures during deposition, and some buildups in structurally stable areas ceased to develop because they could not keep pace with rising sea levels.

Seismic, well-log, and rock data of several buildups from different stratigraphic levels (Cycles III-VI) were integrated in formulating a model for reservoir development in the Luconia carbonates. Depositional and diagenetic cycles due to eustatic sea level changes exerted strong influence on the internal architecture of the buildups. Cyclic depositional patterns, represented by alternating layers with good and poor porosities, include small-scale units (<3 m thick) recognized in cores and reservoir-scale successions (>63 m thick) identifiable in well logs and seismic data. Cycle V carbonates, for instance, consist of three Tortonian-Messinian depositional sequences. Two layers of dolomite formed in all buildups, each contemporaneous with a period of emergence. The Messinian dia enetic cycle, related to the 5.5 Ma sea level lowstand, partially overprinted the Tortonian diagenetic cycle that occurred during the 6.3 Ma lowstand. Superposition of these diagenetic cycles, with leaching, chalkification, and dolomitization, is responsible for a complex pore geometry in the carbonate reservoirs. This model can be applied to map the reservoir units in the carbonate buildups within the entire basin.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994