Sequence stratigraphic controls on reservoir quality: Insights from the Middle Miocene Birsa Formation, Gulf of Hammamet, Southern Mediterranean

The Middle Miocene Birsa Formation in the Gulf of Hammamet, northeastern Tunisia, records the evolution of a tectonically influenced deltaic system controlled by variations in accommodation space, sediment supply and relative sea-level changes. This study presents an integrated sedimentological, petrophysical and sequence stratigraphic characterization of the Birsa Formation based on well-log analysis from a concession in the Gulf of Hammamet. The investigated interval consists of interbedded sandstones and shales organized into eight reservoir subzones that exhibit significant lateral and vertical heterogeneity. Gamma-Ray, resistivity density and neutron log responses were used to identify depositional facies ranging from distributary channels and mouth-bar deposits to distal delta-front and prodelta shales reflecting successive lowstand, transgressive and highstand depositional phases. Structural analysis indicates that tectonic inversion reactivated pre-existing extensional faults forming elevated fault-bounded blocks that acted as favorable hydrocarbon traps whereas structurally lower areas are predominantly water-bearing. Petrophysical evaluation reveals that reservoir quality is primarily controlled by effective porosity, clay volume and water saturation. Among the identified depositional sequences, the most favorable reservoir intervals occur within the lowstand systems tract (LST) where sand bodies exhibit higher effective porosity, lower clay content and improved hydrocarbon saturation compared to transgressive and highstand deposits. This case study illustrates the value of integrating sequence stratigraphic interpretation with petrophysical and structural analyses to improve the understanding of reservoir distribution and quality variations within inverted graben settings. The results provide new insights into the combined influence of depositional architecture and tectonic evolution on hydrocarbon accumulation in the Birsa Formation and offer a robust framework for improving reservoir characterization in the Gulf of Hammamet.