Abstract This study is focused on the sedimentary environments, facies distribution and sequence stratigraphy of the Santonian intervals (Ilam Formation of Bangestan Group) that host enormous hydrocarbon reserves in five giant and supergiant oilfields in the central and southern parts of the Dezful Embayment (SW Iran). This reservoir formation is investigated using detailed petrographic analysis assisted by microscopic image analyses to explain its depositional facies and sedimentary environment in the subsurface sections of this embayment. Petrographic studies led to the recognition of 18 microfacies that formed in four facies belts: inner ramp (including shoal facies and open to restricted lagoons), mid-ramp (including channels and patch reef talus facies), outer ramp and basin. To locate the approximate position of the studied wells in the conceptual depositional model, frequency analyses for facies associations are carried out. The studied intervals consist of two, thick shallowing-upward 3rd-order sequences. Facies variations of the Ilam Formation investigated throughout the studied oilfields using correlation in a sequence stratigraphic framework.