Cenozoic tectono-sedimentary evolution of the onshore-offshore Tunisian Tell: Implications for oil-gas research

A review of the paleogeographic and tectonic reconstruction of the onshore and offshore Tunisian margin during the Cenozoic is discussed. Five unconformities (A to E) and associated stratigraphic gaps of various vertical extents allow subdivision of the stratigraphic record into depositional units in the following time intervals: (i) Paleocene-Oligocene, (ii) Oligocene-early Aquitanian, (iii) early Aquitanian-Burdigalian, (iv) late Burdigalian- Langhian and (v) Langhian-late Miocene. These intervals can in turn be grouped into four main sedimentary cycles (SC1–SC4) dated to the (1) Paleocene-Oligocene, (2) Oligocene–Burdigalian, (3) Burdigalian-Langhian and (4) Langhian-late Miocene. The oldest depositional unit reflects Eo-Alpine tectonics in the Maghrebian Flysch Basin (MFB); the others are related to the Neo-Alpine syn- and late orogenic tectonic deformation affecting the MFB. The uppermost unit represents post-orogenic deposition. Early Miocene synsedimentary tectonism led to (1) deposition of thick successions owing to a large sediment supply and (2) the occurrence of various tectofacies (unconformities, slumps, mega-turbidites, olistostromes, growth folds, chaotic intervals and heterogenous lithofacies), that all, together with the occurrence of lateral change of facies, clearly indicate non-cyclical sedimentation. During the middle Miocene the Tunisian Tell underwent polyphase thrust tectonism, followed by late Miocene strike-slip deformation with contemporaneous rejuvenation of halokinetics and magmatism (the La Galite Archipelago) that may be traced as far as the Algerian Tell. The margin experienced deep-seated compressional tectonism during the Paleogene, a foreland basin during the early Miocene, and nappe stacking during the middle Miocene, with the occurrence of wedge-top sub-basins. The evolution of the region makes the existence of petroleum resources within either the thrust belt, the foredeep and/or the foreland systems plausible. Oil and/or gas may have been trapped in either i) deep buried allochthonous thrust wedges that are located below the Numidian Nappes, and/or in ii) the imbricate Medjerda Valley domain of the Tell foredeep. The offshore area between northern Tunisia and the La Galite Archipelago may also hold potential for large oil/gas fields, as has been confirmed by exploration of the same overthrust belt in other areas such as in Sicily and the Southern Apennines

Cenozoic tectono-sedimentary evolution of the onshore-offshore Tunisian Tell: Implications for oil-gas research

A review of the paleogeographic and tectonic reconstruction of the onshore and offshore Tunisian margin during the Cenozoic is discussed. Five unconformities (A to E) and associated stratigraphic gaps of various vertical extents allow subdivision of the stratigraphic record into depositional units in the following time intervals: (i) Paleocene-Oligocene, (ii) Oligocene-early Aquitanian, (iii) early Aquitanian-Burdigalian, (iv) late Burdigalian-Langhian and (v) Langhian-late Miocene. These intervals can in turn be grouped into four main sedimentary cycles (SC1–SC4) dated to the (1) Paleocene-Oligocene, (2) Oligocene–Burdigalian, (3) Burdigalian-Langhian and (4) Langhian-late Miocene. The oldest depositional unit reflects Eo-Alpine tectonics in the Maghrebian Flysch Basin (MFB); the others are related to the Neo-Alpine syn- and late orogenic tectonic deformation affecting the MFB. The uppermost unit represents post-orogenic deposition. Early Miocene synsedimentary tectonism led to (1) deposition of thick successions owing to a large sediment supply and (2) the occurrence of various tectofacies (unconformities, slumps, mega-turbidites, olistostromes, growth folds, chaotic intervals and heterogenous lithofacies), that all, together with the occurrence of lateral change of facies, clearly indicate non-cyclical sedimentation. During the middle Miocene the Tunisian Tell underwent polyphase thrust tectonism, followed by late Miocene strike-slip deformation with contemporaneous rejuvenation of halokinetics and magmatism (the La Galite Archipelago) that may be traced as far as the Algerian Tell. The margin experienced deep-seated compressional tectonism during the Paleogene, a foreland basin during the early Miocene, and nappe stacking during the middle Miocene, with the occurrence of wedge-top sub-basins. The evolution of the region makes the existence of petroleum resources within either the thrust belt, the foredeep and/or the foreland systems plausible. Oil and/or gas may have been trapped in either i) deep buried allochthonous thrust wedges that are located below the Numidian Nappes, and/or in ii) the imbricate Medjerda Valley domain of the Tell foredeep. The offshore area between northern Tunisia and the La Galite Archipelago may also hold potential for large oil/gas fields, as has been confirmed by exploration of the same overthrust belt in other areas such as in Sicily and the Southern Apennines.Research supported by Research Project PID2020-114381 GB-I00, Spanish Ministry of Education and Science; Research Groups and Projects from M. Martín-Martín, Alicante University (CTMA-IGA)

Données nouvelles sur le contenu organique des dépôts phosphatés du gisement de Ras-Draâ (Tunisie) New data on the organic matter associated to phosphatic ores of the Ras-Draâ deposit (Tunisia)

International audienceL'étude de la matière organique (MO) associée aux sédiments phosphatés a été conduite sur des fractions séparées – grains phosphatés (pellets) et leurs matrices – de strates riches en phosphate du gisement de Ras-Draâ. Cette étude a réuni des analyses géochimiques élémentaires C, N, S, des analyses par pyrolyse Rock-Eval (RE) et des extractions des substances humiques (SH). Les données géochimiques fournies par l'analyse CNS et par la pyrolyse RE indiquent : (i) des teneurs en carbone organique total (COT) plus élevées dans les matrices (atteignant 4,00 %) que dans les pellets (≤ 1,62 % ) ; (ii) la présence, dans les deux fractions, d'une MO se rattachant à la lignée II, planctonique marine, plus ou moins oxydée ; (iii) une faible évolution diagénétique subie par la MO dans chacune des fractions (Tmax, globalement < 430 °C). Les données quantitatives de l'extraction et du fractionnement des SH en acides fulviques, humiques et humine, indiquent une forte abondance de composés humiques extractibles (CH) dans les pellets (CCH not, vert, similar 70 % de la somme du COT des fractions séparées COTfr) et une présence occasionnelle, en faibles quantités, de CH extractibles dans les matrices. Cette situation exclut que ces pellets se soient constitués de façon authigénique au sein de leur matrice, ce qui est pleinement conforme à la possibilité, récemment formulée, qu'il s'agisse de fèces de poissons. Abstract The study of the organic matter (OM) content of phosphatic sediments from the Ras-Draâ deposit, Tunisia, was carried on the two separated lithological fractions constituting the bulk sediments, namely phosphatic grains (pellets) and their associated matrices. The geochemical characterization of the OM present in pellets and in their matrices by CNS elemental analysis and RE pyrolysis indicates that: (i) the TOC content is higher in matrices (where it reaches 4.00%), than in pellets in the same strata where it does not exceed 1.62%; (ii) the presence of more or less oxidized marine planktonic OM in both fractions; (iii) a low degree of diagenetic evolution of the OM in both fractions (RE Tmax globally < than 430 °C). The chemical extraction of the humic substances (HS) from both fractions followed by the separation of fractions according to the IHSS procedure, systematically indicates a higher abundance of extractable humic compounds (HC) in the pellets (CHC not, vert, similar 70% of the sum of TOC in the separated fractions, TOCfr) and a variable but lower extraction yield in matrices (CHC not, vert, similar 18% TOCfr). This significant difference between both fractions excludes the possibility that pellets formed authigenically from, and within, their matrix. This is consistent with recent findings suggesting that these pellets could be fish feces

The Numidian formation and its Lateral Successions (Central-Western Mediterranean): a review

The widely debated late Oligocene-middle Miocene Numidian Fm (NF) consists of supermature quartzose sediments deposited in the Maghrebian Flysch Basin (MFB) outcropping from the Betic Cordillera to the Southern Apennine passing by the Maghrebian Chain. The NF is commonly composed of three lithostratigraphic members and is characterized by two vertical successions (Type A and Type B) corresponding to different sedimentation areas in the MFB. It is noteworthy the occurrence of widespread lateral successions of the NF (Types C, D and E) indicating in some cases an interference of the Numidian sedimentation with other different depositional systems and supplies. The Type C ‘Mixed Successions’, deposited in depocentre areas, are composed of supermature Numidian supply interfingering with immature siliciclastic materials, coming from the internal portion of the MFB. The Type D consists of supermature Numidian materials supplied from the Africa Margin (external sub-domains) deposited in sub-basins on the Africa-Adria margins, outside the typical Numidian depositional area. The Type E, which stratigraphically overlies both the South Iberian Margin (SIM) and the Mesomediterranean Microplate (MM), represents the migration of the Numidian depositional system to reach the opposite margins of the MFB. The occurrence at a regional scale of all the above-mentioned lateral successions reveals a great evolutionary complexity resulting also from further constraints, which must be considered for palaeogeographic and palaeotectonic reconstructions. Another important point deals with the diachronism of the top of the NF, observed eastward from the Betic-Rifian Arc and the Algerian-Tunisian Tell (Burdigalian p.p.) to Sicily (Langhian p.p.) and up to the Southern Apennine (at least Langhian/Serravallian boundary) which can be related with eastwards delay in the MFB closure. The palaeogeographic reconstruction of the Numidian depositional area presented in this paper, which is also included into a global kinematic model, represents a first attempt to use the software GPlates for this subject

New data on the characterization of humic substances extracted from phosphatised faecal “pellets” (Tunisia)

9-14 September 2007International audienceHumic substances (HS) were extracted from faecal “pellets” - ovoid phosphatic grains 50-200 µm in size - collected from commercially mined phosphatic sediments, in the Gafsa Basin. The phosphatic constituents were characterized by C, H, O, N, and S elemental analysis, Rock-Eval VI pyrolysis, Fourier-transformed infrared spectroscopy (FTIR) and solid state 13C CPMAS NMR

Nouveaux algorithmes efficaces de modélisation 2D et 3D : Temps des premières arrivées, angles à la source et amplitudes

Traveltimes, amplitudes and take-off angles of seismic waves are used in many applications such as migration, tomography, detection sensitivity estimation and microseism location. In the microseismicty context it is necessary to compute in near real time accurately these attributes. Here we developed a set of fast and accurate algorithms in 3D for highly contrasted velocity models.We present a new accurate method for computing first arrival traveltimes, amplitudes and take-off angles; more precisely we solve the Eikonal, transport and take-off angle equations based on a finite difference approach for 3D velocity models. We propose a new hybrid method that benefits from the advantages of several existing Eikonal solvers. Common approaches that solve directly these equations assume that we are locally propagating a plane wave. This approximation is not well adapted in the neighborhood of the source since the wavefront curvature is important. Travel times errors are generated near the source position and then propagated through the whole velocity model. This prevents from properly calculating the amplitudes and the take-off angles since they rely on the travel time gradients that are not accurate. We overcome this difficulty by introducing spherical operators. Indeed we reformulate the traveltimes, amplitudes and take-off angles with the perturbation method.We validate our new methods on various highly contrasted velocity models in 2D and 3D and show our contribution compared to other existing approaches. Our results are similar to those computed using full waveform modeling while they are obtained in a much shorter CPU time. These results open thus new perspectives for several applications such as migration, detection sensitivity estimation and focal mechanism inversion.Les temps de trajet, amplitudes et angles à la source des ondes sismiques sont utilisés dans de nombreuses applications telles que la migration, la tomographie, l'estimation de la sensibilité de détection et la localisation des microséismes. Dans le contexte de la microsismicité, il est nécessaire de calculer en quasi temps réel ces attributs avec précision. Nous avons développé ici un ensemble d'algorithmes rapides et précis en 3D pour des modèles à fort contraste de vitesse.Nous présentons une nouvelle méthode pour calculer les temps de trajet, les amplitudes et les angles à la source des ondes correspondant aux premières arrivées. Plus précisément, nous résolvons l'équation Eikonal, l'équation de transport et l'équation des angles en nous basant sur une approche par différences finies pour des modèles de vitesse en 3D. Nous proposons une nouvelle méthode hybride qui bénéficie des avantages respectifs de plusieurs approches existantes de résolution de l'équation Eikonal. En particulier, les approches classiques proposent généralement de résoudre directement les équations et font l'approximation localement d'une onde plane. Cette approximation n'est pas bien adaptée au voisinage de la source car la courbure du front d'onde est importante. Des erreurs de temps de trajet sont alors générées près de la position de la source, puis propagées à travers tout le modèle de vitesse. Ceci empêche de calculer correctement les amplitudes et les angles à la source puisqu'ils reposent sur les gradients des temps. Nous surmontons cette difficulté en introduisant les opérateurs sphériques ; plus précisément nous reformulons les temps de trajet, amplitudes et angles à la source par la méthode des perturbations.Nous validons nos nouvelles méthodes pour différents modèles à fort contraste de vitesse en 2D et 3D et montrons notre contribution par rapport aux approches existantes. Nos résultats sont similaires à ceux calculés en utilisant la modélisation de la forme d'onde totale alors qu'ils sont bien moins coûteux en temps de calcul. Ces résultats ouvrent donc de nouvelles perspectives pour de nombreuses applications telles que la migration, l'estimation de la sensibilité de détection et l'inversion des mécanismes au foyer

New efficient 2D and 3D modeling algorithms to compute travel times, take-off angles and amplitudes

Les temps de trajet, amplitudes et angles à la source des ondes sismiques sont utilisés dans de nombreuses applications telles que la migration, la tomographie, l'estimation de la sensibilité de détection et la localisation des microséismes. Dans le contexte de la microsismicité, il est nécessaire de calculer en quasi temps réel ces attributs avec précision. Nous avons développé ici un ensemble d'algorithmes rapides et précis en 3D pour des modèles à fort contraste de vitesse.Nous présentons une nouvelle méthode pour calculer les temps de trajet, les amplitudes et les angles à la source des ondes correspondant aux premières arrivées. Plus précisément, nous résolvons l'équation Eikonal, l'équation de transport et l'équation des angles en nous basant sur une approche par différences finies pour des modèles de vitesse en 3D. Nous proposons une nouvelle méthode hybride qui bénéficie des avantages respectifs de plusieurs approches existantes de résolution de l'équation Eikonal. En particulier, les approches classiques proposent généralement de résoudre directement les équations et font l'approximation localement d'une onde plane. Cette approximation n'est pas bien adaptée au voisinage de la source car la courbure du front d'onde est importante. Des erreurs de temps de trajet sont alors générées près de la position de la source, puis propagées à travers tout le modèle de vitesse. Ceci empêche de calculer correctement les amplitudes et les angles à la source puisqu'ils reposent sur les gradients des temps. Nous surmontons cette difficulté en introduisant les opérateurs sphériques ; plus précisément nous reformulons les temps de trajet, amplitudes et angles à la source par la méthode des perturbations.Nous validons nos nouvelles méthodes pour différents modèles à fort contraste de vitesse en 2D et 3D et montrons notre contribution par rapport aux approches existantes. Nos résultats sont similaires à ceux calculés en utilisant la modélisation de la forme d'onde totale alors qu'ils sont bien moins coûteux en temps de calcul. Ces résultats ouvrent donc de nouvelles perspectives pour de nombreuses applications telles que la migration, l'estimation de la sensibilité de détection et l'inversion des mécanismes au foyer.Traveltimes, amplitudes and take-off angles of seismic waves are used in many applications such as migration, tomography, detection sensitivity estimation and microseism location. In the microseismicty context it is necessary to compute in near real time accurately these attributes. Here we developed a set of fast and accurate algorithms in 3D for highly contrasted velocity models.We present a new accurate method for computing first arrival traveltimes, amplitudes and take-off angles; more precisely we solve the Eikonal, transport and take-off angle equations based on a finite difference approach for 3D velocity models. We propose a new hybrid method that benefits from the advantages of several existing Eikonal solvers. Common approaches that solve directly these equations assume that we are locally propagating a plane wave. This approximation is not well adapted in the neighborhood of the source since the wavefront curvature is important. Travel times errors are generated near the source position and then propagated through the whole velocity model. This prevents from properly calculating the amplitudes and the take-off angles since they rely on the travel time gradients that are not accurate. We overcome this difficulty by introducing spherical operators. Indeed we reformulate the traveltimes, amplitudes and take-off angles with the perturbation method.We validate our new methods on various highly contrasted velocity models in 2D and 3D and show our contribution compared to other existing approaches. Our results are similar to those computed using full waveform modeling while they are obtained in a much shorter CPU time. These results open thus new perspectives for several applications such as migration, detection sensitivity estimation and focal mechanism inversion