The National Agency for the Valorization of Hydrocarbon Resources "ALNAFT"

SAHARAN DOMAIN

The Saharan domain is located in the south of Alpine Algeria and belongs to the North African Craton. It includes a Precambrian base on which rests in unconformity a powerful sedimentary cover, structured in the Paleozoic in several basins separated by high zones.

Its generation results from a succession of compressive and extensive phases from the Proterozoic to the Tertiary:

- A 690-650 Ma : during the Cryogenian, an accretion phase takes place with the formation of Gondwana. The collision of the West African and Saharan cratons leads to the formation of the Pan-African chain. Large North-South stalling accidents will then take place to accommodate this deformation, within the Hoggar shield

- A 650 – 470 Ma : From the Ediacaran, the withdrawals of the slabs associated with the West African and Saharan craton, will lead to the collapse of the Pan-African chain and the opening of the various basins of the Saharan platform. Thus will successively open from West to East, the Tindouf / Reggane / Sbaa basins, then the Ahnet / Timimoun, Oued Mya / Mouydir and finally Berkine / Illizi basins.

- A 420 – 320 Ma : From the Upper Silurian (Pridoli), the Saharan platform records various compressive phases which are set up around the periphery of the zone. They result from the gradual closure of the Iapetus and then Rheic oceans. Thus, the subduction of the Iapetus Ocean leads to the formation of the Caledonian chain, which will result in inversions along the normal faults within the Saharan basins.

From the Carboniferous to the Permian, the subduction of the Rheïc Ocean will conduct to the formation of the Hercynian chain, with the collision between Gondwana and Laurentia. This collision will result in the formation of the Ugarta chain and inversion along normal faults and stall faults. Thus the entire platform is in compression. This phase will be marked by a major erosion which will seal the deformations (Hercynian discordance).

The basins of the Saharan domain were generated using to different mechanisms:

- The Tindouf / Reggane / Sbaa basins, associated with the West African craton, correspond to an extension of the upper crust and develop along normal faults. These are pericratonic basins that are located along the West African shield

- The Ahnet / Timimoun, Oued Mya / Mouydir and Berkine / Illizi basins result from transtensive movements along the north-south faults inherited from the pan-African chain. Thus these pull-apart type basins are bordered by sub-vertical faults. They are generally narrow and locally deep, and separated by structural tops.

Thus, these basins will present different geometries and tectonic histories over time and with the activation or not of the various fault segments bordering them.

Map showing the major structural elements of the different Saharan basins (Alnaft study, carried out by Beicip-Franlab).

 

Tindouf, Reggane and Sbâa basins, associated with the West African Craton:

To the west, Tindouf and Reggane are two pericratonic basins that exhibit a geometry shaped by the movements of the 2,500 Ma (Archean) Reguibat shield during the various deformation phases.

Bordered to the north and west by the suture of the West African Craton, they are separated by the Bou Bernous saddle where the preserved series are not very thick.

To the north, the Tindouf basin is separated from the Timimoun basin by the Ugarta chain oriented N140 ° and composed of two main links enclosing a perched basin (Sbâa basin), where all the Paleozoic series are preserved.

The Ugarta extends southwards through the base outcrop of Bled El Mas and then through the Azzel Matti which separate the Reggane basin from the Ahnet.

Tindouf and Reggane mainly feature Paleozoic infill and a dandruff or absent Mesozoic series.

These are asymmetrical basins with a south (Tindouf) and west (Reggane) flank rising gently towards the basement outcrops and a north (Tindouf) and east (Reggane) border with very thick formations (base of the Cambro-Ordovician to more 5000 m deep) and deformed during the Hercynian phase with creation of narrow folds aligned along the edges of the basins (EW north of Tindouf and N160 ° east of Reggane).

Finally, it should be noted that the formations of these two basins are invaded by intrusive rocks (dolerites, sills) whose presence is generally linked to the opening of the North Atlantic at Lias.

Schematic section crossing the basins of the Saharan platform (Alnaft study, carried out by Beicip-Franlab).

 

Ahnet, Mouydir and Illizi basins north of Hoggar

These basins have the common characteristics of intracratonic basins marked by the heterogeneities inherited from the base: located north of the Hoggar, they imagine the latter being divided into large north-south panels that play in piano keys, and whose age varies between the Paleo-Proterozoic (2000 Ma) and the Neo-Proterozoic (600 Ma). This heterogeneous heritage of the base results from the Pan-African collision of two continents identified as the West African craton and the East Saharan craton.

The Paleozoic formations go back structurally towards the south where they outcrop on the edge of the Hoggar (Tassilis) and the basins are laterally limited by the extensions of the large faults cutting the base with which are associated high axes with a main NS orientation.

These basins deepen towards the north where they are extended under the Mesozoic cover by more subsisting basins which cut the platform into a complex whole, inherited from the structure of the substratum and having replayed during all the major deformation phases during the Paleozoic and Meso-Cenozoic.

It is thus mentioned from west to east:

- The Ahnet basin bordered to the west by the positive trend of the Azzel Matti - Bled El Mas and to the east by the Arak Foum Belrem accident extending to the north by the positive axis of the Idjerane, then by the Voute d'Allal: this basin continues north through the Timimoun basin, the main orientations of which gradually change from NS to N160 ° reflecting the influence of the associated chains from the Ugarta to the West. at the limit of the West African Craton,

- The Mouydir basin, between the positive trends of Arak Foum Belrem Idjerane in the West and Amguid El Biod in the East. To the north, the Mouydir extends through the Oued Mya basin where the structural orientations gradually turn from NS to N30 °,

- The Illizi basin, located between the NS axes of Amguid El Biod in the west and the Tihemboka in the east. It extends northward, beyond the east-west mole of Ahara by the Berkine Basin also extending into Libya (Ghadames Basin) in which the main orientations are N30 °.

The repetition of the structural configuration of these basins underscores the importance of the Pan-African heritage. The presence across all of these basins of Paleozoic series varying from continental facies in the South to more marine facies in the North reflects an almost permanence of the proximal to distal polarity during the entire Paleozoic.

Timimoun, Oued Mya and Berkine basins

These basins are well delimited in their spatial extension by well-marked anticlinal axes in all directions except Berkine to the east bounded by the border. Towards the south, the connections with the basins backing onto the Hoggar can be made through a high zone such as the Djoua saddle between Ahnet and Timimoun and the Ahara Mole between Illizi and Berkine. They deepen quickly and can run up to 5,000m series in Timimoun and 6,000m in Berkine.

We will also mention the presence of the Mesozoic covering on these three basins, skin on the Timimoun basin, with the exception of its northern end at the level of the Sillon de Benoud. The thicknesses of the Mesozoic increase sharply in the Oued Mya basin (3000 m) and in the Berkine basin (3500 m).

Finally, at the base of the Mesozoic, Triassic sandstone reservoirs develop under a salt cover of Triassic to Liasic age which constitute the elements of the petroleum system of the Hassi R'Mel deposits NE of Timimoun, of Haoud Berkaoui in the heart of the Oued Mya and the main deposits of the Berkine basin (trend Rhourde Nouss - Ourhoud - El Borma).

Chronostratigraphic chart on the scale of the Algerian Saharan basins (Alnaft study, realized by Beicip-Franlab).

 

1. THE BERKINE BASIN

The history of the Berkine and Illizi basins is intimately linked until the Devonian and the final establishment of the Ahara Mole which separates the two basins. There is a sedimentary continuity observed in the Silurian between these two zones.

The Illizi basin, to the south, contains so-called "proximal" deposits and the Berkine basin, more subsisting, to the north, of so-called "distal" deposits. In fact, sediment inputs came mainly from the south and from the Hoggar massif with a low sedimentation rate estimated at 10 m / MA, typical of intracratonic basins.

Schematic section illustrating the architecture of the Illizi-Berkine basins (Alnaft study, carried out by Beicip-Franlab).

 

Geological and structural framework

The Phanerozoic history of the Berkine basin is marked by a very distinct multi-phase tectonics;

The major structural event is the phase of Hercynian orogeny: the end of the Hercynian regime is marked by a period of regional emersion, accentuated by an uprising in the North, a visible consequence of which is the heights of Djeffara-Dahar. The Berkine basin is the most affected by Hercynian tectonics. It is followed by a period of rifting linked to the emergence of the passive margin of the North African Tethys then an extension phase during the Lias and the Triassic marked by a gradual shift towards the north of the underlying Paleozoic platform. 

 

Stratigraphic aspects

The Paleozoic of the Berkine basin is characterized by a succession of transgressive and regressive second-order cycles whose mega-sequences are composed of extensive sandstone deposits mainly of fluvial and fluvial-deltaic origin interbedded with intervals of marine clay.

These cycles are delimited by regional discordances (Infra-Tassilian Discordance, Intra-Arenig Discordance, Hirnantian Glacial Erosion Surface, Caledonian Discordance, Frasnian Base). They all include a transgression peak associated with the regional development of offshore clays, locally anoxic and acting as cover and / or Source Rocks.

Continental Triassic deposits cover the Paleozoic wedges and constitute the best reservoir in the Basin. They are surmounted by an excellent evaporite cover of Triassic and Liasic age.

Chronostratigraphic charter of the Illizi-Berkine basins (Alnaft study, carried out by Beicip-Franlab).

 

Seismic transect with North-South stratigraphic covering crossing the Berkine basin.

 

2.  ILLIZI BASIN

Geological and structural framework:

The Illizi Basin is a stable platform basin, spanning 100,000 km; its sedimentary cover is on average 3000 m thick, mainly made up of Paleozoic deposits.

Numerous tectonic events affected the Illizi basin from the Precambrian to the Tertiary. The first order tectonic phases at the origin of the main faults and unconformities are::

- Pan-African compressive events in the Precambrian: at the origin of deep North-South faults rooted in the base and their conjugate faults

- Hercynian movements, from the Carboniferous to the Permian: they mainly resulted in significant erosion and tilting of the basin; few sets of faults or structural deformations can be dated from this period, unlike what is observed in the SW basins.

- Trias-Lias, the opening of the Thetys and then of the North Atlantic reactivated previous normal faults to the south and generated new accidents further north in the Rhourde Nouss region and in the Berkine basin.

- In the Cretaceous, the displacement of the rotation pole of the African plate generated a particularly acute EO tightening on the Amguid El Biod mole and not very sensitive in the heart of the Illizi basin.

- In the Tertiary, alpine compressive phases reactivated old faults and reshaped structures; the significant uplift of the Hoggar tilted the basin towards the north giving it its current configuration causing all the series to outcrop at the level of the Tassilis.

 

Stratigraphic aspects

Resting on a crystallo-metamorphic base of Precambrian age (which would be of the same nature as that of the Hoggar), the sedimentary series constituting the Illizi basin are mainly of Paleozoic age, buried in the center of the basin, and flush to the south. -East and on the southern margin of the basin at the level of Tassilis; The thickness of these shows variations from the South (1,000 m to 1,500 m) to the North (1,500 m to 2,000 m). Conversely, the Mesozoic deposits are eroded in the southern half and outcrop in the center of the basin forming a succession of east-west facing cliffs, where the latter lie unconformable on the Paleozoic land and their thickness approaches 1 000 m.. 

Tertiary deposits develop mainly in the northwestern part of the basin and at the level of the Hamada du Tinrhert; the Quaternary, for its part, is represented by dunes marking the southern limit of the Grand Erg Oriental.

North-South seismic transect crossing the Illizi basin (Alnaft study, carried out by Beicip-Franlab).

 

3. Amguid Messaoud

With a total area of 157,793 km², the Amguid-Messaoud pier is distinguished by various tectonic elements delimiting a basin where the stratigraphic column is more or less complete.

Chronostratigraphic Charter of Amguid Messaoud.

The Amguid-Messaoud zone consists of 3 morpho-structural elements, namely;

The Hassi Messaoud ride, occupying the central part; the Amguid spur, located further south; and the vault of Touggourt, ensuring the junction with the furrow of Melrhir

The Hassi Messaoud region is located in the central part of the Algerian Sahara, known for its oil-producing wells mainly in the Cambrian reservoirs. Several deposits have been identified, namely El Agreb, Zotti, El Gassi, Rhourde El Baguel, Mesdar and the super-giant field of Hassi Messaoud.

The Cambrian deposits which are represented by sandstones and quartzites are the best known and constitute important reservoirs (Cambrien Ri, Ra).

The Ordovician reservoir (Hamra quartzites), eroded under the Hercynian discordance and constituting the aureole of Hassi Messaoud, is an oil play with great potential.

Chronostratigraphic Charter of Amguid Messaoud.

 

4. OUED MYA- MOUYDIR BASIN

The Oued Mya basin covers an area of 5,300 km², it is more precisely located in the central province, and is limited to the north by the upper zone of Djemaa-Touggourt, to the south by the Mouydir basin, to the east by the structural top of Amguid-El Bioud-Hassi Messaoud and to the west by the vault of Allal. The main structural elements are of direction N-S and NE-SW.

The region of Oued Mya is represented by the Triassic Saharan Basin, which constitutes an intra-cratonic sub-basin of the Saharan platform, It forms a vast NE-SW depression. In the northern part of the platform (Oued Mya), the typical sedimentary series, whose thickness can reach 6000 meters, presents Paleozoic deposits often eroded up to the Ordovician and the Cambrian. The Mesozoic, discordant with the Paleozoic, is present from the Triassic to the Cretaceous. The Cenozoic is represented by a thin detrital series from the Mio-Pliocene.

Diagram illustrating the architecture of the Mouydir-Oued Mya basin.

 

The structural context of the Oued Mya and Mouydir basins is still poorly understood. It is nonetheless a given that the basin could have been affected by 3 major structural events:

- The Hercynian deformation phase,

- The rifting and break-up of the central Atlantic in the Jurassic period, accompanied by the emission of CAMPs (dolerites). According to thermochronological data (Leprêtre, 2015), this episode is followed by a period of cooling and therefore exhumation. Erosion is more significant in the south of the Oued Mya basin.

- One or more phases of Cenozoic compression, accompanied by erosion.

 

5. AHNET TIMIMOUN BASIN

Geological and structural framework:

The Ahnet-Timimoun basin appears as an essentially Paleozoic filled depression, ranging from the Cambrian to the Carboniferous. The Meso-Cenozoic is incomplete and poorly developed. It is bounded to the north by the shallow bottom of Oued Namous, to the west by the Ugarta chain, to the south by the Tuareg shield and to the east by the Idjerane-M'zab ridge.

To the east and west, the basin is delimited by structural ridges or north-west / south-east orientation tops which are mainly of Hercynian age but which also existed as slight structural highs. throughout the Paleozoic. The Idjerane Spur forms the eastern border which separates this area from the Mouydir basin, while the top of Azzel Matti corresponds to the western border which separates the Ahnet from the Reggane basin. To the south, the Ahnet basin is attached to the great Hoggar massif.

East-West regional geological sections of the Timimoun and Ahnet basins (after J. Craig - 2006)

 

Stratigraphic aspects

The sedimentary cover is made up of a Paleozoic series which rests in major discordance on the Precambrian basement. Its thickness can exceed 4000 m. It corresponds to a thick sequence of strongly folded and faulted Cambrian to Carboniferous sediments during the Hercynian orogeny.

The Silurian marine clay deposits rest on the compact Ordovician ones. Likewise, the Frasnian age interval is represented by transgressive organic marine deposits. These two intervals (Lower Silurian and Frasnian) constitute the source rocks of these basins.

  A slightly deformed series of Mesozoic and Cenozoic sediments that lie in angular unconformity on Paleozoic terrain.

 

Scheme illustrating the architecture of the Ahnet-Timimoun basin.

 

6. BECHAR BASIN

The Bechar-Abadla basin is a very subsisting Permo-Carboniferous basin, in its central part the Carboniferous can reach 6000 meters thick.

The movements generated by the Hercynian phase disrupted the architecture of the basin; generating the rise of the anticline of Chebket Mennouna oriented East-West and which thus separated the region of Abadla in the South from that of Bechar-Kenadza in the North; thus giving rise to a Mezo-Cenozoic basin of more than 1200 m of Cretaceous sedimentations with alternating argilo-evaporite.

Atlas and even Quaternary tectonic movements created structures, detachment surfaces and straightened layers up to vertical along major faults close to the South Atlas accident.

The pre-Carboniferous terrains are structured independently of the overlying series. The Carboniferous is discordant over the Devonian.

The Lower Carboniferous, showing variations in terms of facies; it is sandstone in the southern part of the perimeter while elsewhere it is either flyshoid or carbonate in the high areas where it constitutes mud mounds and subrecifal facies. Outcrops also revealed discontinuous Namibian sandstone paleochannels.

 

East-West regional geological cut passing through the Bechar basin.

 

East-West regional geological section passing through the Bechar basin.

 

7.  Tindouf  bassin

The Tindouf basin extends over nearly 50,000 km²; On the geological level, it is a pericratonic type basin, is present an asymmetry of the southern flank with a very slight dip in relation to the northern flank, very straightened and strongly structured, in relation to a series of complex faults with an overall EW direction. and NE-SO.

It is located on the edge of the West African craton, and is limited to the north by the Anti-Atlas and the Ugarta chain. To the east, it is separated from the Reggane basin by the upper zone of Krettamia-Bou Bernous, and its southern limit is represented by the Réguibat ridge.

The structure of the basin bears witness to a tectonic history deeply marked by the Hercynian events. Noting that the latter is affected by numerous late intrusions, occurring in the form of sills and dykes of basic rocks (gabbros, dolerites, basalts, etc.) thus intersecting the previous formations.

Regional geological section crossing the Tindouf basin.

 

Stratigraphic aspect:

The thickness of the Paleozoic lands, where the main oil fields are located, varies from 1500 to 8000 m, and evolves favorably from East to West and from South to North. These deposits are covered in transgression by a thin Tertiary sedimentary cover (about 130 m thick).

Chronostratigraphic charter of the Tindouf basin.

 

The filling of the basin is mainly of Paleozoic age resting on an Archean gneissic base,

The Cambrian occurs in a powerful quartic series with conglomerate past, with some argillaceous intercalations, these are considerably reduced in the North.

 The Ordovician is characterized by fluvial deposits in the South and marine deposits in the North

The Silurian consists of an argillaceous graptholite formation, discordant on the underlying deposits and showing variations in thickness from the northern flank to the southern flank of the basin; passing from 100 m in the S to nearly 800 m in the North.

The Devonian series are also very thick, consisting mainly of silty clays with carbonate intercalations, as well as frequent doleritic sils.

The Carboniferous presents a predominantly marine sedimentation ending in a continental series and some doleritic intrusions.

 

8. REGGANE BASIN

Geological and structural framework:

The Reggane pericratonic basin, with an area of 140,000 km², is located on the eastern edge of the West African craton, even straddling the latter and a so-called Pan-African domain located further east. The juxtaposition of these two domains is the result of a collision (600 Ma) resulting from the closure of a Proterozoic paleo-ocean. This event is known throughout North West Africa as the Pan-African.

Schematic section showing the architecture of the Reggane basin.

 

The Reggane basin presents an asymmetrical transverse profile, with a strongly straightened, very structured, narrow northeast flank containing anticlinoriums concealing compartmentalized structures of complex geometry, while its western flank appears as a gently dipping monoclinal. It is separated from the Tindouf basin to the west by the Boubernous mole.

It is bordered on the north by the folded chains of the Ugartian arc, on the west and south by the Reguibat shield. It covers an area of 140,000 km².

Chronostratigraphic Châtre of the Reggane Basin.

 

Stratigraphic aspects

The sediment cover is made up of:

- A Paleozoic series which rests in major discordance on the base. Its thickness can exceed 3000 m. It corresponds to a thick sequence of strongly folded and faulted Cambrian to Carboniferous sediments during the Hercynian orogeny.

- Such Paleozoic deposits are predominant in the North for the Silurian to Carboniferous series, and in the South for the Cambrian to Silurian series. The Precambrian constituting the substratum of the basin has been found in rare wells.

- A slightly deformed series of Mesozoic and Cenozoic sediments which rests in angular discordance on the Paleozoic terrain.

 

9. SBAA BASIN

The Sbaa basin is located in the center-western province of the Algerian Sahara. It is essentially made up of Paleozoic clay-sandstone formations ranging from the Cambro-Ordovician to the Namurian (maximum thicknesses crossed by soundings: approximately 2000 m), topped by terrain Mesozoic soils.

Stratigraphic chart of the Sbaa basin.

 

On a SW / NE section , crossing the basins of Reggane, Sbaa and Timimoun. There are clearly fundamental structural differences between Sbaa and its neighbors. The Reggane basin is indeed a quasi-monocline structure, defined by a major fault, oriented NW / SE, active from the Cambrian to the Upper Devonian, with very significant thickenings observed up to the structural axis of Kahlouche. The Ouggarta chain therefore appears quite clearly as the reversal of a Cambrian rifiting at the time of the Hercynian orogeny.

The Sbaa basin clearly appears to be anomalous, with reduced thicknesses of all the Paleozoic series, and an absence of the presence of Lower Devonian deposits. The southwest part of the Timimoun basin (Oufrane and Gara-Ouibet wells) also includes a series of horst and grabben of medium discharge, (1 to 2 km), with a major NW / SE orientation, up to the structure of Hassi Mahdjib, horst reactivated with the Hercynian, and bordering the depression of Gourara. This is defined by a very large discharge fault, defining a pre-Cambrian series thickness much greater than what can be observed in the Ahnet and Reggane basins.

SW / NE section, crossing the Reggane, Sbaa and Timimoun basins.