Depositional environment controls on petroleum potential of the Eocene in the North of Tunisia

Outcrop data as well as petrographical, tectonic and geochemical data were used to reconstruct the depositional environments of Ypresian–Lower Lutetian-aged carbonate systems bearing Nummulites. They are shelf deposits which pertain to the Metlaoui Group from the northwest and central-east Tunisia, constitute exploration targets for hydrocarbons and form the reservoirs of some important oil-fields at Ashtart and Sidi El Iteyem. Cyclostratigraphy (sedimentary figures, syndepositional faulting, sequence stratigraphy and framework) and laboratory investigations (petrography, geochemical evaluation) were made of these chalky series, to develop a sequence model for the Metlaoui group and clarify the main relationships between global sea-water level changes, lithofacies repartition and source-rock potential.The rock units that include three main third-order sequences, were deposited in a variety of shelf to platform environments with distinct associations of phosphorite/clay/evaporite and carbonate facies. Associated with Nummulitid facies of two main outer shelves, are Globigerinid facies, Discocyclinid subfacies, pack–grainstones enriched in glauconite and phosphate, and Bivalve and Algae-bearing packstones and grainstones. Due to tectonic activity, channels were often cut into the shelf and materials eroded from paleohighs often bypassed the shelf and were deposited in the deep basins. Basin deposits include two main cross-sections: (1) Globigerinid-rich limestones and cherts of the Boudabbous formation and (2) a lateral equivalent with interbedded limestones, clayey limestones, marls and microbreccias which exhibit abundant pyroclastic fragments that testify to an intense Ypresian volcanism.The deposits display substantial heterogeneity resulting from depositional, diagenetic and structural processes. Porosity development in Nummulitid-bearing strata is principally a result (1) of meteoric diagenesis due to uplift and emersion (tests dissolution) and locally dolomitization at the end of short-term cycles (Milankovich cycles) and (2) frequent stylolitization and microfracturation due to late compaction concurrent with transpressional tectonics. Elevated amounts of organic matter were determined in these series, and lithofacies which present high petroleum indices would provide sourcing for seals where the rocks have been deeply buried, notably beneath the Oligocene–Miocene thick series.