Reservoir characterization and pore system heterogeneity analysis of the Eocene Mangahewa Formation: Insights from Kapuni Field, Taranaki Basin

Understanding the controls of depositional and diagenetic processes on pore system evolution is essential for predicting fluid flow behavior in sedimentary reservoirs. The sandstones of the Mangahewa Formation were deposited during a marine transgressive phase in the Taranaki Basin, New Zealand, during the Late Eocene. Despite their potential as hydrocarbon reservoirs, the pore system evolution, as well as the extent of reservoir heterogeneity within Mangahewa transgressive facies, remains poorly constrained. This study applies an integrated sedimentological, petrographic, and petrophysical approach to investigate these processes. The transgressive sandstones comprise stratified lithofacies that grade upward into intensely bioturbated sandstones interbedded with siltstones and mudstones. The sandstones are arkosic to subarkosic arenites affected by compaction, feldspar dissolution, and selective carbonate and clay cementation. Reservoir heterogeneity is controlled by the combined influence of depositional facies, sandstone composition, and diagenetic modification, which govern pore system architecture and connectivity. Petrophysical analysis reveals a wide range of porosity (1%–24.9%) and permeability (0.01–10,000 mD), indicating pronounced heterogeneity. Statistical heterogeneity indicators and hydraulic flow unit (HFU) classification identify six HFUs with contrasting porosity–permeability relationships and flow capacities. Stratified sandstones form the main flow conduits, whereas bioturbated and fine-grained transgressive deposits act as flow baffles and barriers due to restricted pore-throat connectivity. These results highlight the critical role of depositional architecture and diagenetic evolution in controlling reservoir quality and fluid flow behavior in transgressive sandstone systems, thereby improving the prediction of high-quality reservoir intervals in the Upper Eocene Mangahewa Formation and analogous settings.