A new play concept is presented for the prospectivity of the Miocene of the Gulf of Hammamet (Central Mediterranean Pelagian Basin, offshore Tunisia), built upon three new elements: the role of the Langhian unconformity, the specific type of migration model, and the mechanism of tectonic inversion, based on an extensive subsurface mapping of the area, on vintage 2d and 3d and newly acquired and processed 3d seismic data using available well logs.
The sandstone reservoirs are part of the Oum Douil Group (Serravallian-Tortonian; e.g. Saouaf, Birsa, Zelfa, Somoaa, Beglia Fm’s), and are deposited in the Miocene foreland Basin which developed on the African continental margin, on top of the Langhian unconformity which shows substantial erosion of the Cretaceous-Paleocene sequences, and angular relationships as evident in seismic. We interpret this as related to a foreland bulge connected to the Early Miocene approaching of the Maghrebian (Atlas) mountain front from the North which culminated in the Late Burdigalian shortening pulse and Langhian transgression, a front that moved to the South during the Mio-Pliocene up to the Present position (Sicily-Tunisian Atlas).
Our analyses showed that important thickness variations occur in the Miocene deposits; Most of the discovered fields are related to older structural highs showing relatively thin reservoirs. Major burial occurred during the Pliocene deposition in the Jirba trough. All traps (fault bounded structures with sealing faults that in many cases reach the seafloor) are related to the Late Plio–Pleistocene tectonic pulse which inverted the thicker portion of the Miocene sediments which have become structurally higher and constitute the new prospects we identified (e.g. the Houta Prospect).
Source rocks are the Fahdene shales (Albian), deposited upon the Mesozoic rifted continental margin, and subcropping below the Langhian unconformity. Important reservoirs and regional seals exist between Fadhene Fm and Birsa Fm, making direct vertical hydrocarbon charge unlikely. Moreover, several fields show an effective regional vertical seal (Paleocene El Haria shale) between Birsa Fm and in the underlying producing Abiod limestones Fm (Upper Cretaceous). We postulate that hydrocarbons migrated through sub-cropping windows in the Langhian unconformity within the Ain Grab bioclastic limestones (deposited on top of it) as carrier beds, and that further migration occurred through juxtapositions on existing fault surfaces. This concept, new for the Miocene play in the Pelagian Basin, explains in a satisfactory way the hydrocarbon occurrences in the discovered fields.
The new play concept, which has been overlooked previously because the main exploration objectives were the deeper Mesozoic and Paleogene sequences in the structural highs, is a typical example of a New Play in an Old basin, which opens-up new hydrocarbon exploration opportunities in the Tunisian offshore area.
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