Middle to late Miocene carbonates from Central Luconia, offshore Sarawak, Malaysia, contain significant hydrocarbon reserves. However, the complex pore system of the carbonate reservoir poses drilling and production challenges, such as water coning. Moreover, capturing and storing CO2 in depleted carbonate buildups requires the pore type architecture to be well understood. The aim of this study was to investigate pore types in a stratigraphic context and to propose a 3D conceptual model of the pore type distribution. The case study discussed here is the E11 Field. E11 is considered the type location for Central Luconia carbonates because of its unique, almost complete core coverage. The data used for this study included a 3D seismic volume, core descriptions, together with petrographic and petrophysical data. The workflow used involved partitioning the buildup into specific lithofacies, pore, and cement types within stratigraphic sequences and depositional environments. Results show that the E11 Field represents a coral and foraminifera-dominated isolated carbonate platform. Fifteen lithofacies and ten microfacies were identified. Paragenetic alterations include five stages of calcite cement, three stages of dolomite cement, one stage of dedolomite, and a minor stage of pyrite mineralization. Diagenetic changes took place in various environments ranging from early marine phreatic, to mixed meteoric-marine, to meteoric realms. Minor burial diagenesis led to the formation of late-stage cements. Early diagenetic alterations closely resemble the primary facies arrangement in distinct environments of deposition and stratigraphic sequences. Interestingly, these sequences mimic in places distinct changes of the seismic geomorphology of buildups. In particular, the middle to upper Miocene boundary (TF2/TF3) coincides approximately with a major reduction in buildup diameter. This backstep corresponds to a meter-thick, low-porosity flooding interval observed in the core of the E11 buildup. Tight (low-porous) layers in the E11 buildup mark the upper and lower boundaries of stratigraphic sequences and are partially traceable on seismic reflection data across the buildup. A lithological correlation across the E11 field showed that wells located near the inner, lagoonal part of the buildup are more prone to dolomitization and attract higher thicknesses of low-porosity flooding interval.
The combination of depositional sequences, diagenetic phases, and seismic geomorphology allowed the buildup to be divided into six stratigraphic sequences, each approximately 50–70 m thick. These sequences can be compared to neighboring buildups and to regional stratigraphic sections using biostratigraphic and chemo-stratigraphic data. Larger benthic foraminifera; i.e., Miogypsina and Austrotrillina, are restricted to the middle Miocene stage “TF1” and “TF2” (where TF is a stage of the Tertiary Period), (19–11.1 Ma), whereas Amphistegina and Cycloclypeus are more indicative of the late Miocene stage TF3 (11.1–7.1 Ma). The biostratigraphic boundary TF2/TF3 was correlated with its strontium isotope signature. This allowed the age of the middle to late Miocene boundary to be estimated.
These observations from the E11 buildup were synthesized in a conceptual depositional and diagenetic model. The description of E11 may serve as an analog for carbonate buildups elsewhere in Southeast Asia (Vietnam, Indonesia, and Philippines) and aid in the proposed CO2 storage project.
