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New Geological and Geophysical Perspectives on the Subsurface Structure of the Northern Tunisian Margin – Impacts on Petroleum Exploration Strategy

Abstract

Petroleum exploration in the northern Tunisian margin has long been encouraged by both the widespread occurrence of oil seeps onshore and the gas/oil discoveries in the nearby geological provinces (e.g., Sicily and southern Italy). The exploratory works, however, have been compromised by the relative lack and low quality of onshore and offshore subsurface data and the complex structural geology of northern Tunisia, which is part of the southern sector of the Maghrebide-Apennine fold-thrust belt. Striking points of intense debate and uncertainty are: (1) whether mechanical or stratigraphic contacts exist between the main stratigraphic units, (2) if deformation was so intense that produced vertical duplication of the major units, (3) the role of the African (metamorphosed) Paleozoic basement in controlling the overlying structures, (4) the geographical patterns of source-rock maturation, and (5) offshore extrapolation of onshore geology. To shed more insights into these questions, we present recently acquired onshore and offshore geological and geophysical data in northern Tunisia and propose subsurface models that suggest deep carbonate targets should be within reach of onshore and/or offshore drilling.

Detailed onshore structural and stratigraphic observations confirm the existence of two main tectono-stratigraphic units that are separated by a major thrust front. These units are (1) the ∼2500–3000 m-thick, turbiditic facies of the Numidian Flysch Formation (Fm.) (Oligocene to lower Miocene clastic target), and (2) the underlying Tellian units (i.e., Jurassic to Oligocene marine carbonates and mudstones, including the carbonate targets of the Upper Cretaceous Abiod Fm. and the lower Eocene Bou Dabbous Fm.). Our data indicate that, in surface exposures, the major thrust front is frequently outlined by Palaeogene mudstones and also by Triassic evaporites, which are proposed as major detachment levels at depth and whose main activity probably occurred during the middle Miocene as a result of the opening of the Western Mediterranean. Consequence to this deformational phase, the Numidian Flysch Fm. exhibits NE-SW, relatively extensive thrusts and back-thrusts (up to ∼150 km long), together with faulted synclines and anticlines (up to 0.5–2 km wide). Importantly, though, sedimentological and biostratigraphic data indicate that there is no major vertical duplication of the Numidian Flysch Fm. onshore and suggest its thrusts and back-thrusts may be rooted on the major thrust front just above the Tellian units. In other words, our model supports the Numidian Flysch Fm. is an allochthonous unit in northern Tunisia.

Below, the Tellian units are interpreted to have relatively intense deformational degree compared to the Numidian Flysch Fm., as they experienced at least Mesozoic rifting (with associated movement of Triassic evaporites) followed by compressional stages during the late Eocene and the middle Miocene. Onshore seismic quality is low but titled blocks with intensely faulted anticlines and synclines are interpreted in the Tellian units, resulting from these deformational events. In surface exposures, both Numidian Flysch Fm. and Tellian units are affected by NW-SE strike-slip faults, whose activity could be intensified during the middle Miocene thrusting and continued during Pliocene deformational events.

What is novel in our study is that, using new offshore gravity/magnetic and 2D seismic surveys, we tried to constrain offshore the position of the main NE-SW onshore thrusts. Regionally extensive offshore gravity anomalies (i.e., basement highs and lows) are detected and follow the same NE-SW orientation seen in the main structures onshore. Main thrusts in offshore seismic sections seem to be above basement gravity anomalies. Thus, offshore thrusts may be related to basement topography. Furthermore, the new offshore data are being used to refine: (1) the depth to basement, (2) the position of the orogenic front (i.e., limit of the European-derived Kabylie basement with the African basement), and (3) the occurrence of igneous intrusions within the sedimentary cover.

Finally, if our onshore estimate of ∼2500–3000 m-thick Numidian Flysch Fm. is confirmed offshore, an implication is that the underlying carbonate targets of the Tellian units (i.e., Abiod Fm. and Bou Dabbous Fm.), which are proven fractured reservoirs in nearby provinces, are within reach of (onshore and/or offshore) drilling. Other petroleum elements (e.g., hydrocarbon expulsion, migration pathways, and timing with trap formation) have to be considered. Yet, the former implication adds exploratory value to the northern Tunisia fold-thrust belt.