<p>The Oligo-Miocene Asmari Formation represents one of the most important hydrocarbon reservoirs in the Dezful Embayment of southwestern Iran. This study examines the combined influence of depositional facies, diagenetic processes, and sequence stratigraphy on reservoir quality within the Rag-e Safid Field. An integrated dataset comprising petrographic thin section analysis, well-log data, strontium isotope ratios (⁸⁷Sr/⁸⁶Sr), and core-derived porosity–permeability measurements was employed. Petrographic investigations led to identify ten facies deposited on a carbonate ramp system extending from mid-ramp to restricted lagoon and peritidal environments, with increasing siliciclastic input toward the upper parts of the succession. Six third-order depositional sequences, spanning the Rupelian to the Burdigalian, were delineated based on the integration of biostratigraphic, facies, and Sr-isotope data. The results indicate that reservoir quality is controlled by both primary depositional characteristics and subsequent diagenetic overprints. Following transgression stage, dolomitization and dissolution significantly enhanced porosity and permeability, particularly within lagoonal facies of the highstand systems tract (HST), whereas calcite and anhydrite cementation, together with compaction, strongly degraded reservoir properties. Sandstone-rich intervals of the Aquitanian age display the highest permeability values, reaching approximately 80 mD, while anhydrite-bearing facies represent the poorest reservoir units. These findings underscore the critical role of sequence boundaries and diagenetic modification in governing reservoir heterogeneity and vertical zonation, and provide refined insights into the stratigraphic and diagenetic frameworks of the Asmari Formation.</p>