Diagenetic and depositional controls on reservoir quality in a Middle Miocene carbonate buildup, Central Luconia, offshore Malaysia

Middle Miocene carbonate buildups in Central Luconia, offshore Malaysia, are major contributors to hydrocarbon production in Southeast Asia; however, the interaction between depositional facies and diagenetic processes in controlling porosity development remains poorly understood. This study utilized petrographic analysis, seismic interpretation, and isotopic measurements to provide new insights into the diagenesis that enhances reservoir quality of a deeply buried dominantly limestone buildup (3600–4400 m). The studied platform has a thick reservoir section and a gas column exceeding 600 m that was divided by low-permeability flooding layers into five zones (from top to bottom): 1, 2 Upper, 2 Lower, 3 Upper, and 3 Lower. Freshwater dissolution below subaerial exposure surfaces created high porosity intervals with variations in reservoir properties due to depositional facies. Coral boundstones at the reef margin have moderate to high porosity (4–32 %) and high permeability (1–988 mD). Bioclastic packstones deposited in backreef environments have high porosity (2–32 %) and moderate permeability (0.1–193 mD). Bioclastic wacke-packstones deposited in platform interior environments have moderate porosity (2–19 %) and low permeability (0.01–9 mD). Seismic coherency attributes revealed small dendritic features, suggesting brief subaerial exposure events. A thick gas column and overpressure in the platform carbonate and overlying shale are the key mechanisms for porosity preservation in the upper part of this deep-buried platform. Early replacive dolomitization preserved porosity of 23 % in the lower platform where adjacent limestones have <5 %. In contrast to other Luconia platforms, this aquifer is small and has minimal impact on gas recovery, allowing efficient recovery through gas expansion when reservoir pressure is lowered.