AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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High porosity Cretaceous carbonates in North Atlantic: sedimentology, diagenesis and reactive transport modeling (Ireland offshore).

Abstract

High porosity shallow water carbonates of Early Cretaceous were drilled by well 44/23-1 in the central part of the Porcupine basin (Ireland offshore). Such deposits likely formed two isolated banks, growing on top of the Porcupine Median Ridge. The analysis of sedimentary facies, depositional environment and diagenetic features of well samples provided a stratigraphic framework and a conceptual model for post-depositional modifications to explain the evolution of reservoir properties and the hydrocarbon charge. The stratigraphic section investigated by the well consists of fine grained packstones of platform interior/lagoon, moderate energy, overlain by coarse grained packstones and grainstones, deposited in a mid to high energy environment. The top boundary is a drowning unconformity. Post depositional modifications included stabilization of the fine carbonate matrix, scarce calcite cementation, and repeated events of dissolution/corrosion that produced significant secondary mouldic and vuggy porosity, particularly in the upper section. The conceptual diagenetic model proposed to explain multiple events of corrosion and calcite precipitation, involved repeated pulses of aggressive fluids, possibly enriched of CO2. Such mechanism may have significant implications in the oil emplacement, which is considered the main factor for porosity preservation under deep burial. The reliability of the conceptual diagenetic model was tested through numerical simulation along a cross-section through the bank, with particular attention to quantify the potential dissolution produced by migration of CO2-rich fluids into a carbonate bank under physical conditions comparable to the studied reservoir. The source for CO2 has been ascribed to Tertiary magmatic activity in the area. Results show that sea-water enriched with CO2, migrating upward from a deeper and high temperature setting, may produce significant dissolution in the carbonate bank layer, particularly on the edges of the bank. The observed relationships between diagenetic evolution and hydrocarbon migration provided significant constraints to update the Petroleum System Model.