Traps Analysis and Petroleum Potential in Reggane Basin, Algeria

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Traps Analysis and Petroleum Potential in Reggane Basin,
Algeria
Sofiane Djezzar, University of North Dakota.
Aldjia Boualam, University of North Dakota.
Habib Ouadi, University of North Dakota.
Ahmed Merzoug, University of North Dakota.
Abderraouf Chemmakh, University of North Dakota.
Aimen Laalam, University of North Dakota.
1-Introduction
Reggane is an intracratonic basin located on the eastern edge of the West African craton. It is
oriented NW-SE. Reggane basin is bordered to the north by Ougarta Mountains, to the east by
the Ahnet basin, to the west by BouBernous spur, and to the south, it merges with the Eglabs
massif. The juxtaposition of these two domains results from a collision (620 Ma) and closure of a
Proterozoic-age paleo-ocean. Reggane basin covers an area of 140,000 km2 and has an
asymmetrical transverse profile. Reggane basin is characterized by a polyphase tectonic history
and significant subsidence, allowing the deposit of more than 6000 m of Paleozoic sediments in
its axial zone (Fig.1).
Figure 1: Area of study

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2-Petroleum Systems
The Reggane basin is a gas province. The Cambro-Ordovician, Devonian, and Carboniferous
reservoirs represent the most promising petroleum system. Most of the traps are linked to
structures of complex geometry, leaning on reverse faults. The generation of hydrocarbons
happened in the Paleozoic and probably during the overheating of the Jurassic period. The basin
results from a superposition of several tectonic events from the Caledonian to the Hercynian. The
Reggane basin’s petroleum system is in the chart resumed below. The main reservoirs are in the
northeastern part of the Reggane basin. They comprise the Ordovician, Devonian, and
Carboniferous reservoirs. Gedinnian, Siegenian, and Emsian are primary reservoirs in the
Reggane basin. The Carboniferous and Ordovician reservoirs became second objectives after
gas discoveries in Kahlouche and Anzeglouf area (Fig.2).
Figure 2: Petroleum system chart
3-Structural Framework
Two principal structural axes characterize the Reggane basin. This basin is oriented NW-SE,
relatively asymmetric, with a southern flank plunging to the North and affected by rare faults. On
the other hand, the northern side is intensely fractured (Fig.3). Two fault networks characterize
the basin. The NW-SE constitutes the main fault set that appears along the north edge of the
basin and has the same direction as the Ougarta mountains. The N-S faults set are in the SE
part of the basin, constituting a transition zone between the Reggane basin and the Azzel-Matti
slab. This N-S fault set includes an extension of the submeridian accidents in the Hoggar shield.
Two other directions of less importance stand out: the E-W faults forming an accident relay along
the edge of the basin with the Ougarta mountains and the NE - SW faults (Fig.4).

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Figure 3: Cross-section NW-SE in Reggane Basin
Figure 4: Fault system and main fields in Reggane Basin

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4-Trap Analysis
The structural analyses of major traps underlined in the Reggane basin confirm that:
➢ Basement faults control all anticlinal structures of the Paleozoic cover.
➢ Positive structures vary in their origin.
➢ Structures generated from pure compression happen when the normal faults oriented
perpendicular to the Ougartian stress (N040°) are reactivated in reverse and the creation
of anticlines against faults (Fig.5).
Figure 5: Structure anticlines against fault
➢ A half-graben is exposed to compression with the rising of the bottom compartment and
the creation of anticlines against fault (Fig.6).

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Figure 6: Anticline against fault structure and normal faults
➢ A graben is subjected to pure compression to create a symmetrical Pop-up anticline
(Fig.7).
Figure 7: symmetrical Pop-up anticline
➢ Structures generated by transpression happen when the basement submeridian faults,
oriented obliquely to the Ougartian stress (N040°), are reactivated in dextro-inverse and
create sub anticlines parallel to the faults (Fig.8).

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Figure 8: dextro-inverse faults and create sub anticlines parallel to the faults.
➢ Back-overlap ascends at the connection of the ramp with the Frasnian-Famennian clays
formations, specifically at the level of the Atinim North structure located to the NW of the
Reggane basin (Fig.9). This zone occurs in most cases where the overlapping deformation
is associated with a back-overlapping commonly called the triangular zone or “triangle
zone.”
Figure 9: Overlapping deformation associated with a back-overlapping (triangular zone)

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5-Conclusion
Surface and subsurface observations show that the Hercynian event is the main phase that
structured the Reggane basin. The foreshortening could be aged post-Namurian, with a NE-SW
direction of foreshortening (N040). The Hercynian phase, especially and possibly the post-
Hercynian phase, impacts the petroleum system. The reactivation of faults can indeed lead to the
dismigration of the hydrocarbons trapped in the Hercynian anticlines. The maximum horizontal
stress (SHMax) has an NW-SE direction except for the Atinim structure, located at the extreme
NW of the Reggane basin, where the maximum horizontal stress is oriented NE-SW at the ATM-
1 well. This can be interpreted by a change in the SHMax stress regime, the existence of a fault
or volcanic intrusion. Two large sets of fractures are perceptible on these structures through the
seismic data, and the borehole imaging logs: more dominant NW-SE oriented fractures and NE-
SW oriented fractures probably resulting from Hercynian orogenic movements. Fractures having
the same direction as the maximum horizontal stress are generally conductive (open) and
therefore contribute to the improvement of the petrophysical parameters of the Lower Devonian
reservoirs. At the same time, those which are perpendicular to SHmax are generally resistive
(closed) and thus constitute permeability barriers.
Theory / Method / Workflow
The geochemical studies show that the basin has a vast petroleum potential. However, new wells’
exploration and drilling are still low compared to other basins in the Algerian Saharan platform.
3D seismic, seismic attributes, and borehole imagery will bring new insights into this basin.
Results, Observations, Conclusions
Surface and subsurface observations show that the Hercynian event is the main phase that
structured the Reggane basin. The foreshortening could be aged post-Namurian, with a NE-SW
direction of foreshortening (N040). The Hercynian phase especially and possibly the post-
Hercynian phase has an impact on the petroleum system. The reactivation of faults can indeed
lead to the dismigration of the hydrocarbons trapped in the Hercynian anticlines. NW-SE fractures
are as well visible on borehole imagery, curvature analysis as in 2D, 3D seismic, they are
generally open, it would be preferable to implant the future wells near the NW-SE faults which
can generate fracture corridors favoring the maximum productivity of the Lower Devonian
reservoirs by matrix and natural fractures
Novel/Additive Information
This work brings new insights for the successful exploration and decreases the exploration risks
in this Reggane basin.
Acknowledgments
The authors want to thank Sonatrach Exploration for allowing us to present these results

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