Depositional Environments and Diagenesis of Upper Cretaceous (Coniacian to Maastrichtian) Strata of Azadegan Field in Zagros Basin, Southwest Iran

Forooz Keyvani and Amir Golian
NIOC, Teran, Iran

Sedimentology, diagenesis, and reservoir quality of Upper Cretaceous (Coniacian to Maastrichtian) strata were investigated in five wells of Azadegan oil field in the Abadan Plain province in Zagros Basin. Ilam, Gurpi, and Tarbur Formations are the main subjects of consideration in this study. The overall characteristics of these formations were controlled by the relative sea-level fluctuation. The limestones of the Ilam formation (Coniacian - Santonian) were deposited during the highstand and possibly early lowestand . A major transgression resulted in deposition of dark gray marl of the Gurpi formation (Campanian-Maastrichtian). The Tarbur formation (Maastrichtian) formed patch reefs along the shelf margin during highstand. A disconformity separates Maastrichtian and Danian (Early Paleocene) in this area. The study revealed a variety of lithofacies indicative of carbonate sand shoals, middle ramp, patch reef, outer ramp, proximal deep marine and distal deep marine depositional environments. Major diagenetic alterations occurred during shllowing upward or highstand in Ilam and Tarbur formations . Meteoric processes resulted in generation of porosity and permeability, and dolomitization, forming good reservoir quality strata in the Ilam and Tarbur Formations.