The Impact of Depositional Facies and Diagenesis on Reservoir Characteristics of the Abu-Madi Giant Gas Field Sandstones (Upper Miocene), the Nile Delta Basin, Egypt

Marcelo Ketzer¹, Alaa Salem², Sadoon Morad³, Raafat Rizk⁴, and Ihsan Al-Aasm^5
1 PUCRS-Pontifícia Universidade Católica do Rio Grande do Sul, Porto Alegre, Brazil
² Durham University, United Kingdom
³ Uppsala University, Sweden
⁴ Petrobel oil Company, Cairo, Egypt
⁵ University of Windsor, Windsor, ON

Reservoirs of the Abu Madi Giant Gas Field consist of fluvial and estuarine sandstones, deposited in a wide and deep valley, incised in Messinian marine mudstones and limestones. The lower part of the valley is filled with braided and meandering fluvial deposits, which consist of fine- to coarse-grained sandstones, and intercalated floodplain fines. The upper part of the valley contains estuarine deposits, which consist of fine-grained sandstones with mud drapes, and bioturbated siltstones and mudstones with brackish-water fauna. The overall valley fill succession suggests a progressive relative base level rise, with fluvial deposits at the base passing to estuarine deposits at the top. Diagenetic processes have induced porosity and permeability destruction, preservation, or generation. Processes that have destroyed the reservoir quality of the sandstones include mechanical and chemical compaction, as well as cementation by calcite and quartz. Reservoir-quality preservation was due to the formation of chlorite fringes, which inhibited cementation by quartz overgrowths, whereas the generation of porosity was caused by the formation of secondary intragranular pores owing to the dissolution of feldspars and rock fragments. Although there are no major differences in the spatial and temporal distribution of diagenetic alterations in the fluvial and estuarine sandstones, the impact of these alterations on reservoir quality is different. Poorer reservoir quality of the estuarine sandstones is attributed to somewhat finer grain size and to the presence of chlorite fringes, and more extensive mechanical and chemical compaction (enhanced by more abundant micas).