The Asmari Formation, as the principal carbonate reservoir of the Zagros Basin, exhibits significant lateral facies heterogeneity on a regional scale. This study aims to identify the depositional model governing the marginal parts of this basin through microfacies analysis and palaeoenvironmental interpretation of the Tange-Chug surface section in the Eshger Anticline (northeastern Izeh Zone). Based on the study of 280 thin sections prepared from the Pabdeh and Asmari successions, 20 microfacies belonging to outer ramp, middle ramp (distal and proximal), shoal, and inner ramp (open lagoon to intertidal zone) environments were identified. The gradual vertical transition of these facies, coupled with the complete absence of gravity-flow deposits, continuous barrier reefs, and coated grains (ooids), confirms the establishment of a very low-angle homoclinal carbonate ramp in this area during the Oligo-Miocene. This model is fully consistent with findings from adjacent studies in the Izeh Zone, which indicate the dominance of heterozoan assemblages and oligo-mesotrophic conditions. Furthermore, the overall congruence of this pattern with ramp models proposed for more central parts of the basin (e.g., the Dezful Embayment) suggests a shared primary control of eustatic sea-level fluctuations on the basin geometry. However, significant facies differences—such as the development of lower-energy shoal facies and the presence of anhydrite layers in the upper parts of the sequence documented in this study compared to southeastern fields and the Interior Fars—highlight the role of local controlling factors like palaeogeographic position, basement topography, and potential clastic influx in lateral facies variability and heterogeneity.