Seifeddine Gaidi

The Abiod carbonate reservoir of the Campanian‐Early Maastrichtian age represents a major oil reservoir In the Pelagian block of eastern Tunisia, forming complex fractured networks that are poorly understood. Given the total absence of geological outcrops of Campanian‐Maastrichtian age in the study area, 2D seismic reflection, geological modelling and well log datasets were integrated to characterise the subsurface geodynamic structures, investigate the petrophysical characteristics, the relationship between fault systems and fracturing and define their implications for petroleum exploration. Analysis of seismic profiles, lithostratigraphic logs, borehole data and geological modelling has shown the prevalence of three major fault networks: northwest–southeast (NW–SE), E–W and northeast‐southwest (NE–SW). These faults have controlled the thickness of the limestones of the Abiod reservoir, favouring highly subsiding NW–SE trending zones where the thickness has exceeded 500 m. Based on responses of the Gamma Ray (GR) and Sonic curves, this formation is made up of four different lithological units (U1–U4). The wireline log analyses concluded that the gross thickness of the Abiod reservoir includes good reservoir property intervals with average effective porosity ranging from 8.6% to 20.6% and average shale volume ranging from 2% to 17%. Furthermore, analysis of the fracturing affecting the Abiod reservoir, based on oil well FMS image logs, has shown the presence of open and partially open fractures that are key to hydrocarbon accumulations. The rose diagrams of fracture directions at the wells and the rose diagram of major fault directions of the map are in perfect agreement with the clear predominance of the NW–SE directions. Beyond the main uplift faults, the structural features that impact the studied area's stratigraphy are divided into three main categories. This study showed that the brittle tectonics play a role in the enhancement of the petrophysical characteristics of the studied limestone reservoir and controlled the charge access of the hydrocarbons into the reservoir. The majority of the fractures affecting the Abiod reservoir are parallel to the large fault and have the same direction of movement, while the others are distributed in a conjugate family close to perpendicular to the large fault and with complementary directions of movement. The combination of geological factors and logging parameters indicates that the tectonic legacy and structural inversions play an important role in the structuring of the cover and the evolution of the petroleum system, which have made this chalk one of the most important oil reservoirs in eastern Tunisia. Finally, the developed 3D structural model suggests favourable locations for high‐potential areas in the studied region and provides insights for the reservoir development.

Cenozoic extension in the Western Mediterranean has been related to the dynamics of back‐arc domains. Although, in most of its orogenic belts extension propagated into the fore‐arc nappe domains. Here we revisit the structure, metamorphism and radiometric ages of the Tunisian Tell, where HP/LT rocks (350°C at 0.8 GPa), were exhumed by the sequential activity of extensional detachments after heating and decompression (410°C–440°C at 0.6–0.3 GPa) in a plate convergent setting. Normal faults thinning the Tunisian Tell detached at two different crustal levels. The shallower one cuts down into the Atlas Mesozoic sequence, involving Tellian Triassic evaporites in the hanging‐wall forming halokinetic structures in the Mejerda basin late Miocene. The deeper‐detachment bounds metamorphic domes formed by marbles and metapsammites from the Atlas domain. Illite crystallinity on Triassic rocks shows epizonal to anchizonal values, at deep and intermediate structural depths of the Tell‐Atlas nappe belt, respectively. New U‐Pb 49.78 ± 1.28 Ma rutile ages from Tellian metabasites, together with existing phlogopite 23–17 Ma K‐Ar ages in Atlas marbles from the footwall of the deepest detachment, indicate a polymetamorphic evolution. The Tell rocks underthrusted the Kabylian flysch in the early Eocene. Further, early Miocene shortening thrusted the metabasites over lower‐grade sediments, producing HP/LT metamorphism and ductile stretching at the base of the Atlas belt. The exhumation of midcrustal roots of Western Mediterranean nappe belts after tectonic shortening is a common feature related to tearing at the edges of the subduction systems and inboard delamination of their subcontinental lithospheric mantle.

Cenozoic extension in the Western Mediterranean is related to the dynamics of back-arc domains. However, extension propagated into the external Foreland Thrust Belts (FTB) of the region. Here we revisit the structure, metamorphism and radiometric ages of the Tunisian Tell FTB, where HP/LT blastomylonitic rocks (300-370ºC at 0.9-1.0 GPa), were exhumed by the sequential activity of extensional detachments. Normal faults thinning the Tunisian Tell FTB detached at two different crustal levels. The shallower one cuts down into the Atlas Mesozoic sequence, involving allochthonous Triassic evaporites at the base of the hanging-wall that form halokynetic structures intruding the Mejerda basin Late Miocene infilling. Meanwhile, the deeper-detachment level bounds metamorphic domes formed by marbles and metapsamnites. Illite crystallinity of Triassic rocks in the region shows epizonal to anchizonal values, at deep and intermediate structural depths of the Tell-Atlas FTB, respectively. New U-Pb 49.78 ± 1.28 Ma rutile ages together with existing K-Ar ages in marbles at the footwall of the deepest detachment, indicate a polymetamorphic evolution. The Tell Triassic rocks underwent Cretaceous extensional metamorphism, followed by crustal thickening and rutile growth in the Early Eocene. Further, Early Miocene thickening thrusted the metadolerites over lower-grade sediments, producing HP/LT metamorphism overprinting the base of the FTB. The exhumation of midcrustal roots of western Mediterranean FTBs after the tectonic shortening phase is a common feature of other FTB´s, like the Betics and Rif, which underwent a late-stage tearing at the edges of the subduction system together with delamination of their subcontinental lithospheric mantle.

This work analyses the tectonic evolution of Northern Tunisia from the Late Miocene to Present. Two orthogonal extensional systems with ENE- and SE-directed transport produced the extensional collapse of the Tell and Atlas Foreland Thrust Belts (FTBs) in northern Tunisia during the Late Miocene to Pliocene in a context of NW-SE plate convergence between Africa and Eurasia. These systems produced the extensional denudation of the Tunisian Atlas and Tell foreland thrust belts, which we related to deep mantle tectonic mechanisms, known as a common feature in other FTB´s in the western Mediterranean, i.e. Betics, Rif, Calabria and Apennines. Low-angle normal faults have extended and reworked the Tunisian Tell external foreland thrust belt, exhuming midcrustal lower-greenschist metapelites and marbles with Triassic protholiths, and forming Late Miocene basins. This extension was followed by later Pliocene to Present tectonic inversion, developing the active shortening structures in Northern Tunisia. The main shortening structure is formed by different reverse and strike-slip fault segments, linked forming the 130 km long Alia-Thibar fault zone. Restored Plio-Quaternary deformation observed on reflection seismic lines indicates deformation rates around 0.6-0.8 mm/yr in the studied segments and larger amounts of shortening to the West of Northern Tunisia (16%) than to the East (7%), which suggests that tectonic inversion started earlier to the West and later propagated eastwards, reaching Northeastern Tunisia in the Late Pliocene. Due to the young age of this tectonic inversion, the present relief of Northern Tunisia is characteristic of a young thrust and fold belt, with dominating axial valleys along synforms and an incipient transverse drainage development propagating from West to East.

Exploration of territories not previously analyzed by landslide experts provides interesting findings. The Chgega landslide, in northern Tunisia, represents a paradigmatic mass movement. It can be classified as a complex landslide, or more specifically as vast rock spreading that evolved into a block slide. It involves a great block of limestone—about 900 m long and 400 m wide—sliding over ductile clays and marls. The viscoplastic creep of the clays drives the landslide and creates, in its crown, a graben ~800 m long and ~120 m wide that breaks the summit of Chgega Mountain. Using Interferometric Synthetic Aperture Radar (InSAR) technologies, we demonstrate that this complex landslide is currently active and moreover shows progressive movement without clear episodic accelerations. The velocity of the limestone block is just above 2 mm/yr. The occurrence of gravity-induced joints indicates that the movement has an orientation towards 333° of azimuth on average, conditioned by the landscape around Chgega. These results were obtained through the analysis of a 3D model and a high-resolution orthoimage created from photographs acquired by an Uncrewed Aerial Vehicle (UAV). We may conclude that the landslide movement is determined by normal faults with directions N060°E and N140–150°E. This characterization of the Chgega landslide can serve as the basis for future studies about the origin of this slope movement. Furthermore, the data provided here may support the recognition of Chgega as a singular geological point that deserves to be declared a geosite.