Diagenesis and Reservoir-Quality Evolution of Incised-Valley Sandstones: Evidence from the Abu Madi Gas Reservoirs (Upper Miocene), the Nile Delta Basin, Egypt

Elucidation of the distribution of diagenetic alterations in incised-valley sandstones within a sequence stratigraphic framework allows a better understanding of their reservoir-quality evolution during burial. Gas reservoirs of the upper Miocene Abu Madi Formation (present-day depth 3350 m, temperature 116°), the Nile Delta Basin, Egypt, consist of lowstand systems tract (LST) fluvial and transgressive systems tract (TST) estuarine sandstones deposited in an incised valley. These sandstones have a wide range of porosity (2 to 29%) and permeability (0.01 to 6071 mD), which reflect both depositional facies and diagenetic controls. Diagenetic events that influenced the reservoir-quality evolution include mainly the formation of extensive grain-rimming Fe-Mg chlorite, mechanical compaction of ductile grains, pressure dissolution of quartz, and quartz cementation. The chlorite rims, distributions of which are not clearly related to sequence stratigraphy, have preserved reservoir quality by inhibiting quartz cementation but have increased microporosity values in the reservoirs. Calcite cementation (18O = −13.5‰ to −6.0‰, δ13C = −14.3‰ to −1.0‰, and 87Sr/86Sr = 0.707124 to 0.708181), has not influenced the overall reservoir-quality evolution of the sandstones because of its localized occurrence. Other diagenetic events that have little influence on reservoir quality include the more frequent formation of kaolin in the LST fluvial sandstones and formation of pyrite in the TST estuarine sandstones. This study demonstrates the possibility of constraining and evaluating the impact of diagenetic alterations on reservoir-quality evolution in incised-valley deposits and their sequence stratigraphic distribution, and thus has an important impact on hydrocarbon exploration in such settings.