In this study, the diagenetic processes and paragenetic sequences of Oligocene–Miocene deposits in a subsurface section of the Marun Oil Field were investigated. The lower boundary of the Asmari Formation is in contact with the Pabdeh Formation, and the upper boundary is with the Gachsaran Formation. Petrographic studies led to the identification of several diagenetic processes such as micritization, neomorphism (both aggrading and degrading types), cementation (Equant, fibrous isopachous rim, poikilotopic, blocky, drusy, pervasive, and dolomitic cements), compaction (mechanical and chemical), dissolution and development of porosity (fabric-selective and non-fabric-selective), and replacement (iron-staining, dolomitization, and replacement by terrigenous materials). Based on petrographic evidence, the paragenetic sequence of the Asmari Formation deposits in this oil field was interpreted within four diagenetic environments: marine, meteoric (freshwater), burial, and uplift zones. Three diagenetic stages—early (eogenesis), middle (mesogenesis), and late (telogenesis)—were identified for the studied deposits. Micritization and cementation, particularly in mud-supported facies, have led to the occlusion of intergranular spaces and reduced primary porosity. Overburden-induced compaction further decreased porosity and limited reservoir quality. Dissolution enhanced secondary porosity mainly in grain-supported facies, thereby improving reservoir quality. Dolomitization showed a dual effect: in some intervals, the development of intercrystalline porosity increased reservoir quality, while in others; dolomitic cementation reduced permeability and sealed pore spaces. Consequently, the reservoir quality of the Asmari Formation is heterogeneous and dependent on both the facies type and the intensity of diagenetic processes. The best reservoir potential is observed in grain-supported facies and zones affected by selective dissolution and dolomitization.