Hydrocarbon Potential of Algeria: A New Tectonic Model
Tectonically, Algeria comprises two domains. In the north is the Northern Algerian or Maghrebian Belt, part of the Alpine Belt running from Gibraltar to Calabria, which is a result of Cretaceous–Miocene Alpine events. The Saharan Platform, unaffected by these major Alpine upheavals, encompasses southern Algeria. They are separated by the major Middle–Upper Eocene South Atlasic Fault.
Studies from parts of the Maghrebian Belt, including the Internal Zone and parts of the South Atlas range, have revealed the importance of major tectonic transpressive faulting and movement. This has caused the Belt to be split into three main segments or blocks (Western, Central and Eastern, see Figure 1) by two major fault zones, the dextral strike-slip Ténès–Bousaada fault and the sinistral strike-slip Tigzirt–Djebel Onk fault, which are separated by a large block that has moved south-eastward. This has resulted in a gap in the South Atlas mountain range, and has caused the External Zone of the Maghrebian Belt to outcrop at the Mediterranean coast to the east.
Since the previous understanding of the geology of the Maghrebian Belt was based on the mid-20th century theory of geosynclinal belt-building, with a dominant west–east continuity, this new interpretation requires a reconsideration of the whole strategy of hydrocarbon exploration and the style of major plays in North Algeria. To further this analysis, we look at the important features of the Maghrebian Belt: the three blocks and the two major faults zones separating them.
Two Major Fault Zones of the Maghrebian Belt
The eastern fault zone runs north-west to south-east, from Tigzirt on the Mediterranean coast to Djebel Onk at the southern limit of South Atlas range. It was first identified within the Metamorphic Core Complex (MCC) of the Internal Zone, where it was defined as the Souama shear zone, which bounds the metamorphic formations of the Grande Kabylie area (see Figure 2). Its major effect was to move the MCC up, down-throwing the Eastern Block of the Maghrebian Belt, acting as a sinistral strike-slip fault. The scale of the displacement is a few kilometres vertically but several tens of kilometres in the horizontal direction. The Oligocene–Miocene molassic formation (flysch) lies unconformably on the metamorphic rocks at the eastern edge of the MCC in Grande Kabylie and marks the base of the Cretaceous turbidites (see Figure 2a).
The Tigzirt–Djebel Onk Fault Zone is also seen in the Djurdjura mountains, cutting the Calcareous Range where it outcrops at Col de Chellata. This important Alpine feature then disappears, reappearing over 100 km further east, south of Collo. The previous interpretations based on the geosynclinal concept assumed the Calcareous Range underlaid this area at depth, but its absence is simply the result of the north-westerly displacement resulting from the sinistral strike-slip movement.
The Calcareous Range (see cover photo) is a flower structure resulting from the Eocene west–east transcurrent tectonic movement along the Djurdjura mountains (Saadallah et al., 1996). The axis of the flower structure plunges about 70° westwards, so all the formations involved, from Palaeozoic to early Eocene, disappeared under the post-tectonic Eocene–Miocene molasse cover in the area east of Kouriet. These factors suggest that the Calcareous Range at Col de Chellata was thrown up rather than down, as well as to the north-west.
The External Zone’s ‘Tellien de Dellys’, Cretaceous marls with the specific signature of ‘boules du Sénonien’, is only found within the Eastern Block of the Maghrebian Belt (the brown area around Tigzirt-sur-Mer, Figure 2). This again suggests strike-slip motion in the fault zone, bringing the External Zone northwards to outcrop at the Mediterranean coast.
Further south-east, several northwest to south-east-striking faults can be seen in Figure 1, shifting the southern boundary of the Saharan Atlas range far to the south-east.
The major Ténès–Bousaada fault zone also trends northwest to south-east and forms the western boundary of the Central Block, which it displaces several kilometres to the south-east. It is a dextral strike-slip fault, and results in Internal Zone sediments outcropping at the Mediterranean coast, as several en echelon segments of the fault zone shifted segments of the Internal Zone south-east from west to east, i.e. from Ténès, Algiers, Lakhdaria and finally to the eastern edge of the MCC in Grande Kabylie, at the Tizi Ghenif Fault zone (Figure 2). The Western Block was probably overprinted by the major thrust of the Tellian Nappes, the South Front of the Tellian in the area of Ain Oussera, north of Ouled Nails Mountains (Figure 1). The Ténès–Bousaada fault also affected the Saharan Atlas, downthrowing it and pushing it south-eastwards horizontally.
Movement of the Central Block of the Maghrebian Belt
The Central Block of the Maghrebian Belt is used to define the Internal, External and Foreland Zones including the MCC, the Calcareous Range, the Tellian Nappes and the Atlas Domain. Most of the northern part of the Block was intensively deformed by distension during the opening of the Mediterranean Sea during the Miocene–Pliocene.
The Central Block pushed the Saharan Platform southeastward and, as it moved, it appears to have tugged the northern edge upward and the southern edge downward (Figure 3). In addition, in the north the eastern edge was pulled upwards while the western side plunged downward, so the MCC and Calcareous Range rocks all dip westwards.
The section of the Maghrebian Belt west of the Ténès–Bousaada fault zone is characterised by the absence of the Internal Zone, the existence of a large External Zone and the presence of the Foreland or Atlas Domain, a wide flat platform forming the sub-horizontal High Plains. The apparently horizontal non-deformed High Plains display some ‘pop-up’ compressive structures revealing basement formations within horst and flower structures, as seen at the mountains of Ghar Rouban, Tifrit and Nador.
The Saharan Atlas Range
South of the Plains lies a broad section of the Saharan Atlas range with obvious en echelon structural features. The Atlas Domain is composed of Mesozoic formations, predominately Jurassic carbonates and Cretaceous sands, overlying Palaeozoic basement. Triassic evaporites (gypsiferous shales with salt) played a major tectonic role as a décollement horizon. The Saharan Atlas in northern Algeria is composed of four major mountain ranges: the Ksours, Amour, Ouled Nails and Aures mountains. The first three are separated by minor dextral north-west to southeast strike-slip faults, while the Aures range is isolated from the rest of the Saharan Atlas by the major dextral northwest to south-east shift of the Ténès–Bousaada fault zone.
The Saharan Atlas is known for narrow ‘anticlines’, separated by wide ‘synclines’ with flat bases. In fact, these ‘anticlines’ are actually antiformal structures generated by strike-slip shear faults, often with a reverse component, while the ‘synclines’ are large undeformed sigmoidal lenses, bounded by shear zones, in the classic pattern often found in large sheared areas. Most of the features indicate the dextral character of the transpressive shearing tectonics. The structure of the Saharan Atlas is therefore the result of transpressive dextral tectonics as the Algerian Alpine zone sutured to the Saharan Platform along the major shear faulting zone, the South Atlasic Fault. The High Plains remained predominantly outside the shear zone.
This transpressive north-east to south-west dextrally-striking tectonic event is dated Middle–Upper Eocene (39–33 Ma), with the movement terminated during the Upper Bartonian (34–33 Ma) by north-west to south-east striking dextral faults, probably at the same time as the Central Block of the Maghrebian Belt shifted south-eastward.
Overall, the Saharan Atlas dips north-east, so the Jurassic formations usually outcrop in the south-western part of the Saharan Atlas, while the Cretaceous and Cenozoic sediments are found along the north-eastern edge (Figure 4). This is contrary to the westerly dip of the Internal Zone, as explained previously.
The Eastern Block of the Maghrebian Belt has to be reviewed carefully in the light of these new analyses, but this is beyond the remit of this article.
Impact on Hydrocarbon Exploration in Algeria
This new tectonic interpretation highlights major impacts on hydrocarbon exploration. The effect of the suture of Alpine northern Algeria to the Saharan Platform, followed by the southerly movement of the Central Block during the Eocene, raises questions related to the migration of hydrocarbons northwards from Saharan formations after the Eocene. The fact that the southern part of the Central Maghrebian Belt has been pushed south-eastwards should allow for potential migration from the Platform, and probably also along the multiple northwest to south-east trending faults, which are sealed by the molasses and other deposits resulting from post-Eocene events. What potential paths could they use, and into which structures?
When considering the effect of this new analysis on the hydrocarbon potential of the Saharan Atlas area, two major features need to be kept in mind: firstly, the overall northeastward dip of the Saharan Atlas; and secondly, which formations should be targeted? The sub-Triassic may have potential, with Triassic evaporites forming a good seal for any potential underlying reservoirs. For these targets, it is thought that the western edge of the Saharan Atlas could be the most prospective area, while its eastern edge could be a better location to look at the Cretaceous and Mesozoic formations.
The Next Step
From the little information I have had the opportunity to access, it is clear that the tectonic setting of Northern Algeria has to be reconsidered through the interpretation of large amounts of data. There are about 20 wells, only in the Foreland Atlas domain, plus 15,000 km of 2D and 3D seismic, many geologic maps and several cross-sections and reports, with roughly eight source rocks, from Devonian to Cretaceous, and four potential reservoirs ranging from Devonian to Jurassic. These all need to be studied carefully, with the collaboration and permission of Sonatrach.
These analyses and interpretations need to be reinforced by further studies incorporating subsurface data from oil companies. The model also needs to consider the eastern section of the Maghrebian Belt, into Tunisia. This work is in progress, including the compilation of published data, but is not yet ready to be published.
We know that ‘oil is first found in the brain of the geoscientist’ – but the proof still lies in the drilling results, so the next step should be an exploration programme based on interpretation of the existing data held by interested oil companies. This programme could include vertical drilling on the western edge of the Saharan Atlas targeting the sub-Triassic formations, and in the eastern Saharan Atlas to investigate the Cretaceous–Cenozoic formations – avoiding those false anticlines! Also interesting would be a deviated (horizontal) west-east well intersecting the Ténès–Bousaada fault zone in the Central Block, as these faults may act as drainage features for the hydrocarbons.
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