Porosity Type and Distribution in the Shu’aiba Reservoir, Southern Iranian Persian Gulf

The Late Aptian-Early Albian unconformity, widespread in the Persian Gulf, marks the boundary between a seal facies, Nahr Umr shale, and carbonate units belonging to the Upper Thamama Group. The latter includes the Shu’aiba Formation, a major hydrocarbon bearing reservoir in several Middle East giant fields. At regional scale the gross facies distribution can be referred to a carbonate ramp. Depositional textures controlled the predominant secondary pore type, while the overall reservoir properties seem related to diagenesis. In order to establish the role played by depositional environment and diagenesis process on the Shu’aiba reservoirs, data from cored wells Alpha 1, Alpha 2, Rakish 6, IM-R4 and IM-O2 (Central Gulf, Iranian waters) have been studied. The early diagenetic evolution of the Shu’aiba Formation was dominated by fresh water influx as a consequence of the emersion during Late Aptian-Early Albian time, as indicate by stable isotope of Carbon and Oxygen and trace elements analysis. This influx determined secondary porosity and cementation. The late diagenesis is represented by minor dolomitization along fractures and by a probable porosity enhancement linked to H2S as a product of TSR, which is proved by the presence of thiaadamantanes.