Abstarct: The Oligocene-Miocene Asmari Formation is one of the important reservoir formations in the Zagros Basin. In this study, following samp ling from drilling cores of one of the Iran’s hydrocarbon fields, and preparing thin sections, the microfacies and depositional environments of this formation were investigated by petrographic studies. The optical microscopic studies of the Asmari Formation showed that this formation comprises 9 different microfacies, which were deposited in tidal, lagoonal, shoals, and open marine environments within a carbonate ramp in various parts of the inner, middle, and outer ramp. More detailed petrographic studies using a field emission scanning electron microscope equipped with an EDS detector revealed that after deposition of the Asmari Formation, various diagenetic alterations have influenced this formation. These processes include bioturbation, neomorphism, dissolution, fracturing, dolomitization, micritization, compaction, and cementation. Among these alterations, dissolution, fracturing, and dolomitization had the most significant impact on enhancing reservoir quality, while compaction and cementation had the greatest effect on reducing reservoir quality. Anhydrite, calcite, and secondary dolomite are the most important diagenetic minerals in the Asmari Formation. Microthermometric studies of aqueous fluid inclusions revealed that anhydrite cement in the dolosparite facies formed at an average temperature of 93°C and from pore water with an average salinity of 18 wt% NaCl, during the mesogenetic stage, while in the dolomicrite facies, this mineral precipitated at a temperature of 62°C with an average salinity of 27 wt% NaCl, during the eogenetic stage. Calcite cement in the Asmari Formation formed within a temperature range of 38 to 120°C and from pore water with salinity range of 15 to 34 wt% NaCl, indicating its formation during various diagenetic stages, from near-surface to deep burial conditions. Secondary dolomites also formed at an average temperature of 79°C, from pore water with an average salinity of 25 wt% NaCl during the mesogenetic stage. Hydrocarbon fluid inclusions studies under ultraviolet light indicated that this type of inclusion is absent in anhydrite cements of the dolomicrite facies and shallow calcite cements, suggesting reservoir filling with hydrocarbons occurred after the formation of these minerals. In contrast, anhydrite cement in the dolosparite facies, secondary dolomites, and deep burial calcite cements contain oil fluid inclusions, indicating the presence of hydrocarbon during the formation of these minerals and showing that hydrocarbon migration into the reservoir happened before the formation of the mentioned minerals. In such studies, examining diagenetic alterations, their relative timing, and formation mechanisms significantly assists in understanding, describing, and evaluating hydrocarbon reservoirs.