Mourad Bédir

Subsurface Mesozoic basins in the central Atlas of Tunisia: Tectonics, sequence deposit distribution, and hydrocarbon potential

The Mesozoic and Cenozoic outcrops of the central and meridional Atlas of Tunisia have been studied by structural, biostratigraphic, and sedimentologic approaches. We extend these studies to the subsurface, employing seismic and well data addressing structures, sequence stratigraphy, and hydrocarbon potential. The basin structures are related to strike-slip fault systems associated with Triassic evaporites. Mesozoic sedimentary sections in the areas of the Gafsa, Sidi Ali Ben Oun, Sidi Aich, and Majoura blocks were studied by an integrated seismic sequence stratigraphic and tectonic approach calibrated to wells and surface geologic controls. We identified tectonic corridors where Upper Triassic evaporites intruded younger Mesozoic rocks beginning in the Jurassic. The organization of the tectonic blocks is characterized by the distribution of subsiding basin zones of graben and rim synclines and resistant areas of platforms. Triassic intrusions induced by the strike-slip movements formed salt pillows and domes that accentuate the border elevations between the rim syncline depocenters and the platforms. Mesozoic sequence deposits are distributed along the flanks of the basin-platform borders according to lowstand prograding downlap systems tracts overlapped by aggrading and retrograding onlap and toplap deposits. The time-space distribution of the systems tract deposits indicates the possibility of formation of Triassic, Jurassic, and Cretaceous carbonate and sandstone prograding turbidites, reefs, structural and stratigraphic pinch-outs, and unconformities in the flanks of salt domes and basins. The geometry and distribution of reservoir and seal systems tracts permit the consideration of new Triassic, Jurassic, and Cretaceous combined stratigraphic and structural hydrocarbon traps around the basins and the platforms. This (Begin page 886) is a new tectonic model of the central Atlas of Tunisia that shows new potential objectives and traps.

Subsurface Geodynamics and Petroleum Geology of Transform Margin Basins in the Sahel of Mahdia and El Jem (Eastern Tunisia) (1)

Geophysical, structural, and seismic-stratigraphic analysis of the Mahdia sahel of Tunisia, based on seismic data and sedimentologic and microtectonic field measurements, reveals a dynamic basin formation from the Aptian to the Quaternary. Seismic-structural and isopach mapping of seismic horizons of Cretaceous and Cenozoic megasequences shows the partition of the mobile graben depocenter and carbonate and siliciclastic platform basins. These basins are limited by synsedimentary restrictions along strike-slip faults oriented east-west and north-south. The interplay of these wrench faults, in regional transtension and transpression during the Cretaceous and Cenozoic, induced the synchroneous opening and closing of the graben and platform basins, producing large accumulatio s of black shales and turbidites, and reef, oolite, and carbonate platform deposits. This was associated with the dragging, rotation, and blocking of dekakilometer-size blocks assimilated to lithospheric microplates. These movements were accompanied by volcanism and Triassic salt intrusions along the bordering active faults in the east-west-trending and north-south-trending corridors. In these basins, sequential inversion of basin subsidence and the superposition of rhombic basins and wrench depocenters created migrating depocenters and provided the conditions in rapidly subsiding areas for source rock maturation and, later, hydrocarbon migration into stratigraphic and tectonic traps. The Tunisian Sahel domain at the northern border of the African plate is characterized by tectonic mecha isms which provide a model for tectono-sedimentary basin evolution in a wrench context.

Erratum to: Neogene sediment deformations and tectonic features of northeastern Tunisia: evidence for paleoseismicity

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.

Geostatistical optimization of water reservoir characterization case of the “Jeffra de Medenine” aquifer system (SE Tunisia)

AbstractThis study attempts to characterize the organization, geometry and continuity of aquifer systems in a faulted setting, by geostatistical methods. It concerns the “Jeffara de Medenine” aquifers, in South-Eastern Tunisia. The quality of architectural reservoir modelling depends heavily on available data and on the fault network at the origin of its compartmentalization. In our case study, the available data consist mainly of boreholes: (i) usually sparse: the data distribution and density are very uneven within the study area, depending on the aquifers and the river network; (ii) they do not, usually penetrate the entire aquifer formation. Therefore, aquifers situated at a great depth remain unattainable for many drillings, leaving large areas under-informed and (iii) they are supplemented by seismic data which, although of variable quality, provide useful information for building the fault network at a large scale. To deal with this lack of data, an original geostatistical approach is applied in or...

Variogram Identification Aided by a Structural Framework for Improved Geometric Modeling of Faulted Reservoirs: Jeffara Basin, Southeastern Tunisia

This article describes a proposed work-sequence to generate accurate reservoir-architecture models, describing the geometry of bounding surfaces (i.e., fault locations and extents), of a structurally complex geologic setting in the Jeffara Basin (South East Tunisia) by means of geostatistical modeling. This uses the variogram as the main tool to measure the spatial variability of the studied geologic medium before making any estimation or simulation. However, it is not always easy to fit complex experimental variograms to theoretical models. Thus, our primary purpose was to establish a relationship between the geology and the components of the variograms to fit a mathematically consistent and geologically interpretable variogram model for improved predictions of surface geometries. We used a three-step approach based on available well data and seismic information. First, we determined the structural framework: a seismo-tectonic data analysis was carried out, and we showed that the study area is cut mainly by NW–SE-trending normal faults, which were classified according to geometric criteria (strike, throw magnitude, dip, and dip direction). We showed that these normal faults are at the origin of a large-scale trend structure (surfaces tilted toward the north-east). At a smaller scale, the normal faults create a distinct compartmentalization of the reservoirs. Then, a model of the reservoir system architecture was built by geostatistical methods. An efficient methodology was developed, to estimate the bounding faulted surfaces of the reservoir units. Emphasis was placed on (i) elaborating a methodology for variogram interpretation and modeling, whereby the importance of each variogram component is assessed in terms of probably geologic factor controlling the behavior of each structure; (ii) integrating the relevant fault characteristics, which were deduced from the previous fault classification analysis, as constraints in the kriging estimation of bounding surfaces to best reflect the geologic structure of the study area. Finally, the estimated bounding surfaces together with seismic data and variogram interpretations were used to obtain further insights into the tectonic evolution of the study area that has induced the current reservoirs configuration.