| This study is based on petrographic examination (optical and scanning electron microscope) of 600 thin sections as well as isotopic compositions of carbonates (69 carbon and oxygen isotopes), and fluid inclusion microthermometry of cored Simsima R1, R2, and R3 reservoir units from 3 wells in the Shah Field from onshore Abu Dhabi, United Arab Emirates. The formation was deposited in a ramp setting on the west and east side of Aruma basin, which formed during the Late Cretaceous as a result of the obduction of Semail Ophiolite. The main objectives of this study are to (1) interpret the conditions of diagenetic alterations and of their impact on reservoir quality; (2) link diagenesis to sequence stratigraphy, depositional facies, structural deformation, and oil emplacement; and (3) compare the impact of diagenesis and reservoir quality distribution in oil and water zone. Petrographic, stable isotopic and fluid inclusion analyses have revealed the impact of diagenesis on reservoir quality of R1, R2, and R3 reservoir units within the context of depositional facies, sequence stratigraphy, structural deformation, burial history, and oil emplacement. Diagenetic processes include calcite cementation by grain rim cement, syntaxial overgrowths and inter and intragranular blocky cement, dissolution of allochems, dolomitization and dolomite cementation, calcitization of dolomite (dedolomitization) and replacement of near-surface nodular and bladed gypsum/anhydrite by poikilotopic calcite and/or coarse-crystalline euhedral pyrite, and stylolitization. Partial eogenetic calcite cementation has prevented porosity loss in grainstones during burial diagenesis. Intragranular/moldic blocky calcite cement has occluded porosity in packstones and grainstones, whereas, dissolution of allochems and dolomitization has enhanced reservoir porosity and permeability. Calcite cementation and recrystallization occurred as a result of hot saline waters (Th 76-95°C; and salinity 16-19 wt. % NaCl). Dolomitization of peritidal mud occurred in an evaporative sabkha setting, whereas dolomitization of subtidal packstones and grainstones may have been driven by seepage reflux mechanisms. |
