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Hydrocarbon Prospectivity in Salt-induced Structures-Sahel area- Tunisia


Sahel area is well covered by seismic and well data; however, most of the drilled wells are dry. Petroleum exploration mainly focused on faulted and four way dip closed structures. Commonly, timing of source rock maturation and petroleum migration inside seals is a crucial condition for hydrocarbon entrapment. Black shales within the Fahdène (Albian), Bahloul (Turonian) and lowermost horizons of the Boudabbous (Ypresian) Formations are proven source rocks within north-eastern Tunisia. Hydrocarbon evaluations point to T.O.C averages which lie in the range 1.3 – 2%. Seismic interpretation shows a domain which is affected by directed N-S, E-W and NE-SW major paleofaults which have been reactivated since the Mesozoic. They have delimited distensive structures in form of grabens and half grabens in response to the dominant transtensive tectonic phase related to the divergence of African and Eurasian tectonic plates during the Mesozoic. Since the Santonian age, the domain has registred the beginning of the convergence of African and Eurasian plates. This has induced prominent transpressive tectonic stages during the Upper Cretaceous-Paleogene, that have evolved inversion of pre-extensional structures in the form of fault-related folds, inverted grabens and salt-cored folds. These structures constitute important petroleum targets as they display both structural and stratigraphical traps. Source rock maturation was promoted by a significant subsidence and in place a higher geothermal gradient. However, engagement of Triassic evaporites through folded structures can critically affect the thermal maturation and the hydrocarbon migration pathways. Buried salt has high heat conductivity, can store it and later acts as a pathway to channel heat to the surface. Hence, the oil-generating window is larger and persists longer in folded structures, formed above thick evaporite sequences. Moreover, Cenozoic compressive stages have induced tectonic uplift and consequently the cooling of in-depth series varying the thermal maturation and destructing potential traps. Basin modelling studies have proved that oil expulsion took place at Early Tortonian times that coincides with the chronology of the latter compressive stage. Therefore, petroleum investigations should focus on traps which have escaped from the latest compressive stage of the Tortonian age, consisting on sealed faults delimiting half grabens and stratigraphical targets as unconformities, onlaps and pinchouts.