Carbonate reservoirs quality assessment using sequence stratigraphy study and diagenetic analysis: A case of the oxic Cenomanian-Turonian deposits in Dahar Cliff, Southern Tunisia

Abstract

The present study relies on field work, including detailed sedimentological and sequence stratigraphic analyses of the oxic Cenomanian-Turonian carbonates in Southern Tunisia. Based on petrographic study, six microfacies were recognized (MF1-MF6). These microfacies are grouped into three facies associations referred to peritidal flat, shallow subtidal and deep subtidal environments, suggesting an inner ramp type-platform. The sequence stratigraphic analysis led to interpret these sequences as a transgressive systems tract (TST) which, locally, contains abundant microfauna of the Whiteinella archaeocretacea biozone. The TST interval is capped by a centimetric highly bioturbated limestone bed corresponding to the maximum flooding surface (MFS). The logged sections culminate with the highstand systems tract (HST) referred to the Gattar Member. Sea-level fluctuations have played a key role in shaping the sequence stratigraphic patterns and driving the associated diagenetic processes. These features were developed in meteoric, marine, mixed meteoric-marine realms including eodiagenetic and mesodiagenetic states. The impact of diagenesis on reservoir properties is twofold, with both enhancing and reducing effects. Porosity is increased by the dissolution of aragonitic rudistic shells, resulting in moldic and vuggy pores, while dolomitization creates intercrystalline porosity. Fractures improve permeability networks by creating channels for fluid movement. However, cementation, micritization and compaction gave detrimental effects on reservoir properties. The improvement of reservoir quality noted along HSTs is primarily linked to diagenetic processes associated with a drop-in sea level. In the Tataouine region, intense dolomitization occurred in the TSTs, resulting in increased porosity and a more promising reservoir potential.