Jeffara Basin Research Articles

Geochemical and mineralogical characterizations of Silurian “Hot” shales: Implications for shale gas/oil reservoir potential in Jeffara basin-southeastern Tunisia, North Africa

The Silurian organic-rich hot shale deposits represent the origin of 80–90% Paleozoic sourced hydrocarbons on the entire Saharan Platform. As such, it is an area of significant interest for exploring its potential as an unconventional reservoir and gaining new insights into its properties. In this study, the geochemical and mineralogical analysis of eighty-two borehole samples from three studied wells (T, B, and M) located in Jeffara Basin, in southeastern Tunisa, distributed over a depth between 1200 m and 2200 m, show that the hot shale layer has significant potentials of an unconventional shale gas reservoir. According to the rock-eval and GCMS results, the total organic carbon (TOC) of the hot shale deposits is good to great, ranging between 0.54 wt% and 23.86 wt%, and the average hydrogen index (HI) is around ∼200 mg HC/g of TOC, which indicates that the hot shales were originally rich in type II/III organic matter (OM). The thermal maturity of the preserved OM is determined by the kerogen type with values varying from 0.4% to 1%, and Tmax values reaching 460 °C in T, B, and M wells which indicate that the hot shales preserved early mature to mature OM placed within oil/gas generation window. The calculated geochemical parameters reveal an average hydrocarbon generation potential (HGP) for the three studied wells of 20.37 kg HC/ton rock. In addition, the transformation ratios (TR) in T, B, and M wells are, respectively, 21.12%, 30% and 35.25%. However, only B and M wells show a cross-over layer, with oil saturation index (OSI) exceeding 100%, with significant potential as a hydrocarbon saturated source rock, making it a considerable target for unconventional oil exploration. Relevant results of Rock-Eval, biomarkers and XRD exhibit coherent features that reflect reducing marine conditions during the lower Silurian period. Moreover, the mineralogical composition was investigated to estimate the importance of frackable mineral fractions such as quartz (∼15%) and carbonates (calcite-dolomite). Mineral brittleness index (MBI) was calculated and compared to a well-known unconventional reservoir in the Neal shale member of the Floyd shale group in the Black Warrior Basin (US), which displays moderate to low MBI. The hot shales in the study area have a considerable MBI, which could be easily enhanced by hydraulic fracturing to liberate oil/gas from the potentially saturated unconventional shale reservoir.

Multiscale modeling of reservoir systems using geostatistical methods

Identifying factors and quantifying the processes, influencing the extent of reservoir rocks and the distribution of their heterogeneities are fundamental prerequisites to improve the characterization of reservoir connectivity and its dynamic functioning. In this paper, we propose a multiscale approach based on combined geological–geophysical investigations, geostatistics, and 3D geological modeling to build a structural model of faulted-reservoir systems, at regional and local scales within the Jeffara basin, southeast Tunisia.

Fracture distribution along open folds in southern Tunisia: implications for naturally fractured reservoirs

Fracture networks play a critical role in fluid flow within reservoirs, and it is therefore important to understand the interactions and influences of these networks. Our study focuses on the Southern Chotts–Jeffara Basin, which hosts reservoirs within Triassic, Permian and Ordovician units containing significant hydrocarbon accumulations. Recent developments on the structural understanding of the basin have proved that a regional shortening phase occurred between the Permian and Jurassic, forming open folds and a distributed fracture network. Analysis of late Paleozoic and Mesozoic outcrops within the basin has identified several sets of fractures (with dip directions and dip angles of 150/80 and 212/86) and compressional structural features that support this shortening hypothesis. We have integrated fracture data from surface analogues and subsurface analysis of advanced seismic attributes and well data through structural linking to form a 2D hybrid fracture model of the reservoirs in the region. Through analytical aperture modelling and numerical simulation, we found that the fractures orientated 212° in combination with large-scale fractures contribute significantly to the fluid-flow orientation and potential reservoir permeability. Our presented fracture workflow and framework provide an insight into network characterization within naturally fractured reservoirs of Tunisia, and how certain structures form fluid pathways that influence flow and production. Supplementary material: Data and figures detailing fracture characterisation and modelling along open folds in southern Tunisia are available at https://doi.org/10.6084/m9.figshare.c.6904499

Integrated geophysical and geological analysis of the salt-related structures within the Jeffara basin, Southeastern Tunisia

Integrated geophysical and geological analysis of the salt-related structures within the Jeffara basin, Southeastern Tunisia

Salt tectonics and its effect on the structural and sedimentary evolution of the Jeffara Basin, Southern Tunisia

The Jeffara Basin, as part of the North African passive margin, provides a specific type of salt-wall diapirism in Tunisia. A comprehensive study, integrating seismic, gravity, drill wells and surface geological data is performed to model the induced salt geometry and delineate its kinematics. Salt bodies, originated from the Triassic Norian series, are characterized by chaotic seismic response, higher seismic velocity and gravity mass deficit. The architecture of the marine Jeffara Basin results from the complex evolution initiated by Tethyan Ocean rifting.The reconstructed palinspastic sections together with analogic halokinetic models show the successive stages of salt movement and the development of peripheral rim synclines and associated minibasins. Arid climate Triassic developed prolific upper intertidal to supratidal coastal sabkhas bordering the Tethyan Ocean enabled the deposition of thick Triassic salt within the marine Jeffara Basin. During the Early Hettangian extensional phase, associated with the opening of the Tethys Ocean, initiated a series of normal faults. Onlap reflection terminations of Late Hettangian horizons indicate the early Triassic salt rising activity. Large depocenters occurred during Santonian to Eocene; their lateral migration indicates the main active period of salt. Salt pierced the entire sedimentary cover, reached advanced salt diapir stage and set up salt walls during the Cenozoic period. Salt movement mechanism is dictated by the combined effect of tectonism and salt rheology. Salt dynamics controlled the sedimentation and the organization of depositional environments of sediments.Structural interpretation and modeling of the available seismic sections are of critical importance to get a better understating of the evolution of salt structures within the study area. Salt pillows are located at the central part of the basin. To the northeastern part, where thick salts accumulate exhibits the most mature and extended salt-walls. These geometries have an important impact on petroleum exploration.

Polyphasic evolution of the Jeffara basin in southern Tunisia, influence of halokinesis on the passive margin structuration in the Mesozoic and the Cenozoic

During the Mesozoic and Cenozoic rifting, the Pelagian Sea recorded the consequences of the African and European plate’s rapprochement. The interpretation of surface and subsurface data that is the 2D seismic reflection and petroleum well data show new ideas on the geodynamic evolution and halokinesis of the Jeffara basin during the Mesozoic and Cenozoic period. Seismic lines interpretations of the subsurface mainly reveal normal syn-sedimentary NW-SE faulting and where the Jeffara fault seems to be the major play. This syn-sedimentary faulting induced horst and graben structures materialized by major sedimentary sequences thicknesses as well as depths variations on the seismic profiles from the Jeffara fault zone overall towards the East of Jeffara basin. After the Hercynian event of the Permian - Carboniferous age, a general extension took place, which gave rise to the Tethyan opening. This extension has favored the individualization of the Jeffara basin in the South East of Tunisia, characterized by a structuring in Horst and Graben with a Permian carbonate subsidence. During the Triassic - Middle Jurassic period, the Jeffara basin is marked by a pronounced subsidence of essentially evaporate sedimentation accompanied by the birth of normal syn-sedimentary NW-SE faults following an NE-SW extension. This subsidence continuing during the Upper Jurassic period, the accentuation of which is towards the NE of the study zone at the Jerba and El Bibane sub-basin with a dominance of bioclastic limestone and dolomites sedimentation in the same extensive NE-SW direction, during this period, the Jeffara basin was characterized by a beginning of salt activity indicated by the appearance of salt nuclei at the base of the preexisting NW-SE normal faults. During the Lower Cretaceous, we are witnessing an individualization of salt complexes in the SE of the study area at Rass Ajil sub-basin, where this reactive diapirism has produced high zones and erosions in the crest above the salt bodies. During the mid Cretaceous period, the Zebbag formation, hatched by the Gattar carbonate bar, recorded a subsidence inversion phenomenon between the three sub-basins Jerba, El Bibane and Rass Ajil sub-basin, and showed the change of movement of the African plate relative to the European plate related to the opening of the North Atlantic and the beginning of the drift towards the North of Africa. This phenomenon is concretized towards the Upper Cretaceous, where we witness a strong subsidence towards the southeastern part of the study area at Rass Ajil sub-basin with sandstone, marl and clay sedimentation under a regional extensive regime and the individualization of high zones at Jerba sub-basin. The salt movements present an active aspect by piercing their cover and inducing rim synclines in the surroundings. The Cenozoic period is characterized by a strong subsidence of sandstone, clays and carbonates along the Jeffara basin, the salt activity shows a passive aspect at the beginning of this period which slows down and eventually stopped at the late Miocene period, thus indicating the probable exhaustion of the source of the salt material.

A new borehole temperature adjustment in the Jeffara Basin (southeast Tunisia): Inferred source rock maturation and hydrocarbon generation via one-dimensional modeling

In Tunisia, most borehole temperatures used to constrain the thermal histories of sedimentary basins were previously corrected using various methods, set to best-fit domains other than the Tunisian basins. This study aimed to propose a new method of borehole temperature correction suitable for the Jeffara Basin, southeastern Tunisia. A total of 92 temperature values including bottomhole temperatures (BHTs) and drill stem test (DST) temperatures were collected from 11 boreholes in the area. The correction method consists of plotting BHT versus DST. The derived equation generates corrected temperatures close to formation temperatures. An error temperature curve following the equation was calculated (x = depth in meters and y = temperature error in °C). The error varies from to ( to ) at depths below −274 m (−898 ft). A temperature error () is proposed according to depth, and to the time elapsed since mud circulation ceased intervals. To test the method consistency, the Silurian Tannezuft hot shales Formation thermal history is simulated using the BasinMod 1-D™ software program. Two runs were tested based on the new method: (1) noncorrected BHT and (2) corrected BHT. The first run indicates that the Silurian source rock is at an early stage of the oil window. The second run shows that the source rock is mature and even at a late mature stage toward the north. Oil expulsion has occurred even at an oil saturation expulsion threshold of 5%. The results explain gas condensate production in the study area.

3D-STRUCTURES WITH INTEGRATION GEOLOGICAL AND GEOPHYSICAL DATA: IMPLICATION FOR RESEARCH WATER RESOURCES IN TUNISIA: CASE OF BOTH SE JEFFARA AND UTIQUE BASIN, TUNISIA

Tunisia is enriched by many natural resources (natural water and geothermal water) that are located along the South to the North. This paper aims mainly to evaluate the resources water potential. The available remote sensing images for two typical examples (SE Jeffara basin and Utique basin) are analyzed and many seismic profiles are interpreted to delineate the surface and subsurface structures.
 The 3-D structures of Mio-Plio-Quaternary and Upper Cretaceous aquifers in South-East of Jeffara basin show an important reservoirs of water resources. The Upper Cretaceous aquifer "Complexe Terminal" is one of the most typical examples of exploitation features in hydrogeological research. It is formed by fractured limestones.
 The 3D-structure of Utique basin shows an important variation of thickness of Mio-Plio-Quaternary and Triassic series. The Mio-Plio-Quaternary series is formed by sand, clays and gypsum. The Triassic series is considered typical example for research the hydrogeological and hydrothermal water in Tunisia, especially in the North. In the subsurface, the triassic aquifer of Utique regions is formed by carbonates series. The hot spring in Utique basin is directly related to the Triassic dolostones which are extensively fractured [1].
 A geophysical survey was undertaken for groundwater in Tunisia. The interpretation of seismic reflection sections and petroleum wells allowed to precise the geometry in subsurface structure.

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.

Gravity and seismic reflection imaging of a deep aquifer in an arid region: Case history from the Jeffara basin, southeastern Tunisia

A detailed mapping of the lateral and the vertical extension of the Upper Cretaceous units which contains aquifer bearing lithologies and may be part of the Complexe Terminal aquifer in the Jeffara basin of southeastern Tunisia using land and satellite Bouguer gravity anomaly data, eight seismic reflection profiles and two deep wells was performed. Borehole data indicated that the Upper Cretaceous unit is thickest within the Jeffara basin and thins considerably in the Dahar High. Bouguer gravity data in general confirms this result but lacks the resolution to accurately determine the thickness of the Upper Cretaceous units. Eight seismic reflection profiles constrained by borehole data indicated that the Upper Cretaceous aquifer may be present at depths greater than 500m and is up to 400m thick. The seismic reflection analysis showed that the Upper Cretaceous units contain two distinct zones. Within the northeastern portions of the Jeffara basin, the Upper Cretaceous units are influenced by horsts, grabens and tilted blocks where the Upper Cretaceous units are deeper and thicker units occur within the grabens. A second zone exits toward the basin’s southwestern section where the Upper Cretaceous units are shallower and gradually thin against the Dahar High. Structural contour maps of the top and the base of the Upper Cretaceous aquifer constructed from the seismic reflection profiles reveal the presence of fractured depressions in the northeastern part of the Jeffara basin that are favourable to the hydrogeological exploitation.

Hydro-geochemical behaviour of two coastal aquifers under severe climatic and human constraints: comparative study between Essaouira basin in Morocco and Jeffara basin in Tunisia

Since surface waters are mostly irregular and rare, groundwater in arid and semi-arid regions are submitted to high human constraints enhanced by severe climatic conditions. Multiple isotope analyses and chemical tracing of groundwater from the Essaouira basin (South-Western Morocco) and from the Jeffara basin (South-Eastern Tunisia) reveal that salinisation processes are mainly related to natural conditions such as dissolution of evaporate rocks (gypsum and halite minerals) along the recharging outcrops. However, since these basins are coastal, seawater intrusion is one of multiple salinity sources that could affect the quality of groundwater if intense exploitation goes on. The comparison of hydro-geochemical and isotopic data available for the two basins provide a framework for a comprehensive diagnostic in which different states of risks could be defined according to climatic and human constraints. We argued that these heavily exploited coastal aquifers have exacerbated vulnerability given relatively low current recharge and salinisation processes.

Deep structure and crustal configuration of the Jeffara basin (Southern Tunisia) based on regional gravity, seismic reflection and borehole data: How to explain a gravity maximum within a large sedimentary basin?

Abstract The Jeffara basin of southern Tunisia contains a thick sequence of mainly Triassic and Permian sediments that is characterized by a gravity maximum. To explain the positive gravity signature over the Jeffara sedimentary basin and to obtain a more quantitative representation of the subsurface structure, a regional 2.5D gravity model constrained by seismic reflection and borehole data was constructed along a NE–SW trending profile. The depth to the crust/mantle implies that the Jeffara basin is associated with crustal thinning. The gravity model also implies that subsidence is controlled by a basement stepped down by relatively low-displacement faulting. This sedimentary subsidence, as described by a listric-faulting model, is probably caused by a thinned crust.

Missing response data: To impute or not a impute?: J.M. Boswick, K.L. Lee, R.M. Califf, E.J. Topol, and the TAMI study Group Duke University, Durham, North Carolina(67)

The “red beds” of the Triassic succession outcropping at Tejra-Medenine (southern Tunisia, Saharan Platform) have yielded rich fossil assemblages of both freshwater and brackish-marine invertebrates and vertebrates. The new discovered fauna indicates an Anisian-Lower Ladinian age for the Tejra section. Its lowermost part is considered as equivalent of Ouled Chebbi Formation, while the medium and upper parts are considered as equivalent of the Kirchaou Formation. Both sedimentological characteristics and fossil assemblages indicate the increasing marine influences within the middle part of the section and the migration of brackish and freshwater fauna into the lacustrine/playa environment at the top. The marine fauna-rich interval of the Tejra section correlates well with the well-known Myophoria-rich carbonate stratigraphic marker confirming the Middle Triassic (Ladinian) major transgression well recorded eastward in the Tunisian Jeffara basin and in Libya. The use of this Ladinian stratigraphic event in addition to the Carnian carbonate transgressive events of the Jeffara escarpment outcrops was of great help for regional lithostratigraphic correlations between the Triassic outcropping series and those currently buried in Ghadames and Berkine basins. The age of the sandstones of “Trias Argilo-Gréseux Inférieur” (TAGI) which forms the main oil and gas reservoir in the Saharan domain is attributed to the Anisian-Carnian and considered as coeval of Ouled Chebbi and Kirchaou Formations of the Dahar escarpment. An updated synthetic stratigraphic chart is proposed for the Triassic of Saharan Platform domain on the basis of the compilation of the new obtained results and the subsurface data taken from published literature.