<p>This study examines the microfacies characteristics and depositional setting of the reservoir interval of the Fahliyan Formation in an oil field situated in the Abadan Plain, southwestern Zagros Basin, Iran. The lower part of the Fahliyan Formation, which serves as the primary reservoir interval, has a thickness of around 186.5&#xa0;m and is primarily made up of carbonate sediments. Petrographic analyses identified fifteen distinct microfacies, which were grouped into five depositional sub-environments: outer ramp, middle ramp, shoal, lagoon, and tidal flat. The outer-ramp microfacies are characterized by the dominance of sponge spicules as the principal allochems. Middle-ramp microfacies are distinguished by the widespread occurrence of oligosteginids, serves sponge spicules, benthic foraminifers, and peloids. Shoal-related microfacies are primarily composed of oncoids, benthic foraminifers, peloids, and dasycladacean algae. The grains in these microfacies are well sorted and well rounded, with sizes ranging from arenite to rudite, reflecting deposition in high-energy shoal environments. Lagoonal microfacies are characterized by the presence of peloids, benthic foraminifers, dasycladacean algae, and subordinate oncoids and are dominated by fine-grained sediments with poor to moderate sorting and roundness, suggesting low- to moderate-energy depositional conditions. Dolomudstone microfacies are well developed within the tidal-flat sub-environment. Integration of the microfacies data with standard carbonate depositional models indicates that the Fahliyan Formation in the study area was deposited on a homoclinal carbonate ramp.</p>