Hakim Gabtni

Neogene sediment deformations and tectonic features of northeastern Tunisia: evidence for paleoseismicity@@@تغيرات رواسب العهد النيوجيني وميزات التكتونية في شمال شرق البلاد التونسية: علاقتها بالأحداث الزلزالية القديمة

The Neogene stratigraphic series is characterized by predominant clayey facies alternated by other sand layers. The outcrop and subsurface studies show varied and complex styles of deformations and lead to relate the structures to paleoseismic events. The seismicity of eastern onshore and offshore Tunisian margin follows the master fault corridors oriented globally N–S, E–W, and NW–SE that correspond to the bordering faults of grabens and syncline corridors and associated faulted drag fold structures oriented NE–SW. Epicenters of magnitudes between 3 and 5 are located along these border fault corridors. The Neogene strata record brittle structures, including numerous and deep faults and fractures with straight and high-angle dipping planes. The structuring of NE–SW en echelon folds and synclines inside and outside NW–SE and E–W right lateral and N–S and NE–SW left lateral tectonic corridors indicates the strike-slip type of bordering faults and their seismogenic nature. Wrench fault movements that induce mud and salt diapirs, mud volcanoes, and intrusive ascensions are related to seismic shocks. Seismic waves caused by activity along one, or most likely, several tectonic structures would have propagated throughout the Quaternary cover producing seismites. The similarity of deposits, structuring, and seismites between the Tunis-Bizerte to the North and Hammamet-Mahdia to the South accredits the hypothesis that the seismic episodes might have affected sedimentation patterns along the Sahalian large geographic area. The paleoseismic events in northeastern Tunisia might be related to tectonic fault reactivations through time. This hypothesis is consistent with the geomorphologic context of the study area, characterized by several morphostructural lineaments with strong control on the sediment distribution, as well as uplifted and subsiding terrains. The estimated magnitude of the seismic events and the great regional tectonically affected areas demonstrate that the northeastern Tunisia experienced stress through the last geological episodes of its evolution. This Neogene kinematic reconstruction highlights the neotectonic system inducing the actual seismicity on this margin. Therefore, there is a straight relationship between deepseated faults and seismicity.الخلاصةتتميز ستراتيغرافيا العهد النيوجيني (Neogene) بطبقات طينية سميكة تتخللها طبقات أخرى من الرمل. وتظهر الدراسات الميدانية و تحت سطح الأرض (جيوفيزيائية) أنماط متنوعة ومعقدة من التشوهات، تؤدي إلى ربطها بأحداث زلزالية قديمة. تبدو الزلازل في شرق البلاد قد اتبعت ممرات الصدوع الرئيسية والمتجهة عموما شمال-جنوب، شرق-غرب و شمال غرب-جنوب شرق توافقا مع الصدوع المتاخمة للهياكل المنخفضة و المقعرة (grabens) وما يرتبط بها من طيات ذات اتجاه شمال شرق-جنوب غرب. ويتراوح مركز الزلزال بين 3 و 5 درجات على مقياس ريشترRichter))، على امتداد ممرات الصدوع. تظهر طبقات النيوجين هياكل هشة تحتوي على صدوع عديدة وعميقة ذات زاويا مرتفعة. كما أن هيكلة الطيات و المنخفضات المتجهة شمال شرق-جنوب غرب داخل وخارج الممرات التكتونية ذات التحرك الجانبي الأيمن والمتجهة شمال غرب-جنوب شرق و شرق-غرب والأخرى ذات التحرك الجانبي اليساري والمتجهة شمال-جنوب و شمال شرق-جنوب غرب، تشير إلى نوعية الصدوع و طبيعة السيسموجينية (seismogenic) المرافقة لها. إن التحركات الجانبية للصدوع والتي رافقتها هياكل نتيجة تحركات و صعود الطين (mud diapirs) والملح (salt diapirs) والبراكين، قد ارتبطت بالصدمات الزلزالية. و يبدو أن الموجات الناجمة عن نشاط واحد أو العديد من الهياكل التكتونية قد انتشرت عبر طبقات العصر الرباعي منتجة بذالك العديد من السايسميت (seismites). إن تشابه الترسبات والهياكل و السايسميت بين جهتي تونس-بنزرت من الشمال والحمامات-المهدية من الجنوب، يدعم فرضية أن الحلقات الزلزالية قد أثرت على أنماط الترسيب على مساحة جغرافية واسعة من الساحل التونسي. تبدو الأحداث الزلزالية القديمة (paleoseismic events) في شمال شرق البلاد التونسية ذات صلة بعودة النشاط التكتونى للصدوع (tectonic fault reactivations) عبر الزمن. و تتلاءم هذه الفرضية مع الإطار الجيومورفولوجي (geomorphologic) لمنطقة الدراسة و الذي يتميز بعدة عناصر مورفو بنيوية (morphostructural) تتحكم في توزيع الرواسب والتضاريس المرتفعة و المنخفضة. إن حجم الأحداث الزلزالية وأهمية التكتونية الإقليمية تثبت أن شمال شرق البلاد التونسية قد تعرض لضغوط خلال الحلقات الجيولوجية الأخيرة من تطوره. وتبين نماذج التغير الديناميكي للعهد النيوجيني أهمية التكتونية الحديثة (neotectonic) في تحفيز الأحداث الزلزالية في الوقت الحالي. لذالك توجد علاقة مباشرة بين توزيع الصد وع العميقة و نوعيتها وتحركاتها عبر الزمن، و الأحداث الزلزالية المسجلة.

Geodynamics of the Southern Tethyan Margin in Tunisia and Maghrebian domain: new constraints from integrated geophysical study

The geodynamic evolution of the Southern Tethyan Margin of Tunisia is investigated using geophysical studies. Analysis of gravity and seismic reflection data in the Maghrebian domain and Southern Tunisia reveals the geodynamic role played by the North Saharan Flexure (NSF) in the evolution of the Southern Tethyan Margin. The Saharan Atlas Mountains (Atlasic Basin) and the African Craton (Telemzan High) are separated by the NSF which is a regional-scale feature that may represent a significant basement discontinuity that has controlled the Paleozoic, Mesozoic, and Cenozoic evolution of the Tunisian and Maghrebian Tethyan Basin.

Structural and geodynamic study in central Tunisia using field and geophysical data: new structural interpretation of the N–S axis and associated Atlassic structures

Abstract A structural and geodynamic evolution of central Tunisia is constructed from field studies and geophysical data within the Tunisian Atlassic domain. Bouguer gravity and horizontal gradient magnitude (HGM) maps indicate that computed anomalies are related to subsurface structures and are in accordance with the general structural trends of central Tunisia. Near Cherichira fault, the strike is ENE–WSW, whereas it is N–S along the N–S axis, a major discontinuity between the Pelagian shelf to the east and Central Atlas Mountains to the west. E–W-oriented HGM profiles on the north side of the Cherichira fault show moderate HGM variations, which correspond to the NE–SW fault system of the Ousselet–Bou Dabbous. This trend is also expressed by the second-order vertical derivative map of this area. However, on the south side of the fault, a dominant peak is indicated rather than the anomalies characteristic of the north side of the fault. This anomaly is associated with the N–S axis discontinuity. Seismic reflection data are also used to elucidate the structural configuration and style north and south of the Cherichira fault. The seismic lines demonstrate that faulting and associate halokinesis in Ousselet–Bou Dabbous area are not comparable to the paleogeographic fault system of the N–S axis southward. To the south of Cherichira fault, the seismic lines show massive Triassic evaporates piercing along the major paleogeographic faults associated with thickness variations of Mesozoic and Cenozoic stratigraphic units and a reverse slip of western structural elements. It seems that the N–S axis corresponds to a steeply dipping major zone of weakness in the Tunisian Atlassic Mountains and indicates evidence of several stages of activity, including the rise of Triassic evaporates rise up which outcrop at several localities such as Rheouis and Mezzouna. The loading and upward rise of Triassic evaporites along this weak zone are a key factor in the thinning of the Mesozoic–Cenozoic sedimentary cover along this paleogeographic discontinuity. These new data indicate the northeast extrapolation of fold and grabens of the Central Atlas of Tunisia to the northeast of Kairouan, whereas the northern extent of the N–S axis is limited to south of the Cherichira fault. The Cherichira fault is NE–SW oriented and parallels the Ballouta–Zaghouan fault system, which reflects the regional strike of the Tellian orogeny and the associated nappes to the north. The geometry and kinematics of these Tellian features are linked to the tectonic evolution of the northern African margin, including Mesozoic extension separating the African and European plates to the inversion of these structures since the Late Cretaceous. The N–S axis is correlated in its direction and tectonic framework to the strike-slip fault system of the African craton, which is present to the south in the Algerian Saharan domain of Hoggar.

Basement structure of southern Tunisia as determined from the analysis of gravity data: implications for petroleum exploration

Gravity data were analysed in the Ghadames Basin and surrounding regions in southern Tunisia in order to determine the basement structure of the region and its relationship to petroleum exploration in relatively unexplored basins. The analysis included the construction of regional Bouguer gravity anomaly and horizontal gravity gradient maps. These maps indicate that the Ghadames Basin is not a simple sag basin but consists of a series of sub-basins and uplifts. The northern boundary of the basin which we call the Telemzan–Ghadames transition zone is marked by a NE-trending high amplitude gravity gradient anomaly which decreases in amplitude toward the east and breaks into a series of north–south- and east–west-trending anomalies implying a more structurally complex region. When the known petroleum fields are overlain on to the gravity gradient anomaly maps, the fields mostly occur along or next to linear alignments of horizontal gravity gradient maxima. We interpret the correlation of the petroleum fields and horizontal gravity gradient maxima to indicate that the basement was involved in forming the petroleum traps. This study illustrates that a regional gravity analysis can be useful in determining where additional exploration can be applied in relatively unexplored basins.

Integrated gravity and seismic investigations over the Jebel Es Souda-Hmaeima structure: implication for basement configuration of the eastern frontal fold-and-thrust belt of Tunisian Atlasic Mountains

A gravity and seismic analysis was conducted over and around Jebel Es Souda-Hmaeima, located on the eastern border of the Tunisian Mountains between the Atlasic block to the west and the Pelagian Block to the east, as part of a study to investigate the subsurface structures. These data, together with outcrop geology, well data, and measurements of physical properties of rock samples, were integrated with a new interpretation of the tectonic model of Jebel Es Souda-Hmaeima anticline. This structure represents a backfolded anticline associated with a steep east-vergent thrust above a blind thrust fault along the base of Triassic formations. The proposed model emphasizes the role of transpressional deformation along deep-seated basement faults and has implications for petroleum generation, migration, and entrapment in central Tunisia.تم تطبيق قيس شاذات بوجيه وقيس الذبذبات السيسميكية لدراسة تركيبة جبل لسودة – حميمة ، الواقع بالجبال الشرقية التونسية بين المنطقة الأطلسية والمنطقة البلاجية الساحلية.مكننا تحليل شاذات بوجيه والمقاطع السيسميكية ، إضافة إلى المعطيات الخاصة بالجيولوجيا السطحية والآبار العميقة والخصائص البتروفيزيائية للأحجار ، من تحديد تفسير جديد للأنموذج التكتونيكي لجبل لسودة –حميمة.هذا الأنموذج الجديد يبرز أهمية التركيبة المتصلة بتحركات الفوالق وبالخصائص البتروفيزيائية للأحجار الترياسية كما يحدد مدى تأثيراتها على تكوين ، تنقل وخزن الموارد البترولية.

Fault pattern delineation and structural interpretation of the Gafsa trough (onshore central Tunisia) using gravity data

The purpose of this investigation was to identify subsurface lineaments in Gafsa trough (onshore central Tunisia) after gravity data analysis. The Bouguer and residual gravity maps show a gravity values decrease from west to east associated with subsidence variation and confirmed by a regional seismic reflection profile. The deep structural map of the study area is elaborated after the application of two methods: (1) the automatic lineament tracing after horizontal gravity gradient and (2) 3D Euler method. The dominant trends show approximately NW–SE, E–W, and NE–SW directions. Some of these trends are well correlated with the major faults systems. We can qualify the deep structuration model as a mosaic of quadratic blocks bounded by significant deep flower fault corridors. The elaborated structural map of the study area constitutes also a useful document for rationalizing the future petroleum exploration in the Gafsa trough.

Petroleum assessment of Berkine–Ghadames Basin, southern Tunisia

Hydrocarbon exploration in the Berkine–Ghadames Basin in southern Tunisia has generally followed global economic trends. In recent years, improvements in seismic data acquisition combined with experience gained in log interpretation in low-resistivity reservoirs have resulted in oil and gas discoveries in the Upper Silurian Acacus Formation, enriching the hydrocarbon potential of the Berkine–Ghadames Basin in southern Tunisia. Presently, the Tunisian daily oil production is approximately 43,000 bbl, about half of which comes from the fields located in southern Tunisia and is produced from the Acacus Formation. The Berkine–Ghadames Basin is an intracratonic basin formed during the Pan-African Orogeny. It covers an area of approximately 350,000 km2 (135,135 mi2) and extends into Algeria, Libya, and Tunisia. The sedimentary section within the basin ranges from Cambrian to present and is approximately 7000 m (23,000 ft) thick in the depocenter. The basin has experienced several tectonic events, which have modified its architecture and affected the petroleum systems and hydrocarbon pathways. In this study, the main elements of the petroleum geology systems are described. With the geochemical modeling results, the petroleum potential, hydrocarbon generation, expulsion time, and quantity of hydrocarbon are assessed. The main petroleum systems are also defined. They are represented by the Silurian Tannezuft hot shale source rock with Ordovician Djeffara and Silurian Acacus reservoirs, by Silurian Tannezuft hot shale with Kirchaou reservoirs, and by the Devonian Aouinette Ouinine Formation Member III source rock with Kirchaou reservoirs. The hydrocarbon traps in the area are mainly structural types. The study describes and emphasizes the hydrocarbon migration pathway mechanism from source rocks to traps. To predict and derisk future drilling locations, fairway maps are generated for the three plays: Ordovician, Silurian, and Triassic. Although the paper focuses on southern Tunisia, an attempt is made to introduce Libyan and Algerian knowledge to evaluate the basin in a regional context.

The role of E–W basement faults in the Mesozoic geodynamic evolution of the Gafsa and Chotts basins, south-central Tunisia

The Gafsa and Chotts intracratonic basins in south-central Tunisia are transitional zones between the Atlasic domain to the north and the Saharan platform to the south. The principal aim of this paper is to unravel the geodynamic evolution of these basins following an integrated approach including seismic, well log and gravity data. These data are used to highlight the tectonic control on the deposition of Jurassic and Lower Cretaceous series and to discuss the role of the main faults that controlled the basin architecture and Cretaceous–Tertiary inversion. The horizontal gravity gradient map of the study area highlights the pattern of discontinuities within the two basins and reveals the presence of deep E–W basement faults. Primary attention is given to the role played by the E–W faults system and that of the NW–SE Gafsa fault which was previously considered active since the Jurassic. Facies and thickness analyses based on new seismic interpretation and well data suggest that the E–W-oriented faults controlled the subsidence distribution especially during the Jurassic. The NW–SE faults seem to be key structures that controlled the basins paleogeography during Late Cretaceous–Cenozoic time. The upper Triassic evaporite bodies, which locally outline the main NW–SE Gafsa fault, are regarded as intrusive salt bodies rather than early diapiric extrusions as previously interpreted since they are rare and occurred only along main strike-slip faults. In addition, seismic lines show that Triassic rocks are deep and do not exhibit true diapiric features.

Joint evaluation of gravity, electrical and magnetotelluric methods for geothermal potential of fractured Aptian reefal carbonates in the Hmaima–El Gara area (Oued Serrat basin, Central–Western Tunisia)

Hmaima–El Gara area is located in Central–Western Tunisia and is known as an important geothermal province. In this study, we attempt to delineate the subsurface structures of the area using integrated interpretation of gravity, electrical and magnetotelluric data. The Hmaima thermal aquifer, associated with fractured Aptian reefal limestones, is characterized by high gravity and high resistivity. Horizontal gradient and Euler deconvolution method has been applied to the gravity data and provided fast information about both the depth and trends of the shallower subsurface structures in the area. As several of the mapped lineaments correlate with published geological fault trends, the other lineaments may be indicators of new insights for hydrothermal exploitation in the Hmaima–El Gara area (economical potential favorite zones).

Aeromagnetic study of buried basement structures and lineaments of Sahel region (Eastern Tunisia, North Africa)

This study presents original results regarding the use of aeromagnetic to explore deep subsurface structuring in southern part of Tunisian Sahel petroleum province (Eastern Tunisia, North Africa). Several filters and techniques were applied to the total magnetic intensity (TMI) grid. First, an adequate reduce to the pole (RTP) grid was generated. The RTP map shows 17 positive and negative anomalies associated with short- and long-wavelength amplitude anomalies. Positive anomalies correspond to high magnetic basement structures estimated from seismic lines in the area. Circular anomalies are also distinguished locally and can be explained by Cretaceous magmatic rocks recognized in numerous petroleum wells. Second, specific qualitative and quantitative filters (e.g., residual-regional separation, horizontal tilt angle (TDX), tilt angle (TILT), total horizontal derivative of tilt angle (THDTILT)) were applied to elucidate the form and the extent of buried magnetic anomalies and lineaments. The resulting deep structural map revealed the presence of NW-SE, N-S, and E-W regional magnetic basement structures and lineaments and a regional tectonic node surrounding Henchir Keskes-Agareb-Mahres-Hachichina area. Two magnetic inversion models calculated across the study area highlight west to east crustal thinning trend and permit depth to basement estimation. These results are valuable for future conventional and unconventional petroleum exploration in this underexplored southern part of Sahel plain.