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

The north domain

The north of Algeria is part of the alpine orogen of North Africa also called chain of the Maghrebids. This orogen lies between the oceanic basin of the western Mediterranean, which opened in the Oligocene, and the South Atlas flexure which separates it from the Saharan platform.

Alpine Algeria constitutes a complex region structurally linked to the collision between Africa and Eurasia which began during the Lower Cretaceous and which continues until now, this region includes several structural belts initiated and / or reactivated during the Cretaceous. lower, Eocene and Miocene.

Regional map illustrating the major geological groups of northern Algeria.

Regional map illustrating the major geological groups of northern Algeria.

 

Alpine Algeria is made up of the following structural-sedimentary groups, from North to South:

- The Tellian Atlas is the domain of water tables, with basins of the intra-mountainous type (eg Chélif basin), the sedimentary series of which extends from the Jurassic to the Miocene. Several deposits of variable interest are known there: Ain Zeft, Tliouanet, Oued Guettirini. The main petroleum targets are the Middle Cretaceous, the Miocene and the allochthonous Eocene;

- The high plateaus, Alpine foreland, with reduced sediment cover, where local distension processes have allowed the formation of intra-mountain basins like those of Telagh and Tiaret. The main oil target is the Lias;

- The Saharan Atlas was born out of a long sub-furrow pinched between the highlands and the Saharan Platform. In the Mesozoic, this furrow was filled by a powerful sedimentary series (7000 to 9000 m). During the Tertiary, a compressive tectonics reactivates the previous extensive structures in faults and reverse structures leading to the formation of this mountain range. The main petroleum objective is the Jurassic.

These sets include several basins, including:

- The Chélif basin, which is an intra-mountainous basin with a complex structure and a Meso-Cenozoic infill, comprising both allochthonous and indigenous lands,

- The South East Constantine basin (SEC), structured in the Tertiary, with Cretaceous filling (5000 m), generated and accumulated hydrocarbons mainly in the Cretaceous (Djebel Onk, Rass Toumb, Guerguet El Kihal North).

- The Hodna is a fore-pit basin whose filling sequence begins with continental deposits of Eocene and Oligocene age and continues with a marine Miocene. The main oil target is the Eocene.

Structural model of an E-W seismic transect crossing the Hodna basin, showing the Tellian domain crossed by water tables.

Structural model of an E-W seismic transect crossing the Hodna basin, showing the Tellian domain crossed by water tables.

 

The tellian domain and the water table domain:

The Tellian domain is a complex zone made up of ancient massifs (Paleozoic and older series) generally metamorphosed and allochthonous sedimentary series, established in the Lower Miocene, and on which post-Orogenic Neogene basins such as Cheliff and Hodna arose installed.

The Cheliff basin:

The Chélif basin which is part of the Tellian domain or alpine chain of North Africa where we can distinguish the neogene post-aquifers of Chéliff and native lands outcropping in the massifs of Oran, Arzew and south of Tiaret which are of Jurassic-Cretaceous age and allochthonous or sheeted units of Cretaceous to Oligo-Miocene age.

is characterized on the surface by folds and reverse accidents. The direction of the fold axes is NE-SW to ENE-OSO

The folds are the result of transpressive phases which are to be put in relation with inverse accidents, their orientations follow those of the anticlines; they are folds of ramps. In the subsurface, the Cheliff basin constitutes a neogene synclinal area affected by subvertical accidents.

The litho-stratigraphic column is characterized by the existence of a Mesozoic substratum on which rests an important Cenozoic group with predominantly detrital which can reach a thickness greater than 8000m.

 

The sedimentary series extends from the Burdigalian to the Pleistocene. The lithological sections of the ground and the logs of the boreholes show series which correspond to three sedimentary cycles:

- A discordant Lower and Middle Miocene cycle on a substratum characterized by a change in thickness and facies.

- An upper Miocene cycle, discordant and transgressive, capped by an episode of tectonic calm (Messinian).

- A Plio-Quaternary cycle.

 

The exploration of the Cheliff basin was motivated by the observations of the first showings of hydrocarbons on the surface in the region of Ain Zeft and Tliouanet; where the potential has been confirmed by modest discoveries made in the basin.

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Lithostratigraphic column of the Cheliff basin

Tectono-stratigraphic chart and elements of the petroleum system of the Cheliff basin (Alnaft study carried out by Total).

 

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Structural frame of the Cheliff basin.

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Seismic transect crossing the Cheliff basin (Alnaft study carried out by Total).

The Hodna basin:

The Hodna is a Neogene Age basin whose sequence of filling begins with continental deposits of Oligocene age and continues with a marine Miocene.

Structurally, the Hodna basin is complex having experienced several episodes of intense deformation, consequences of the Africa-Euroasian convergence; this is manifested by a stack of allochthonous layers on the Miocene age sites.

The diagrams and sections below summarize the magnitude of these episodes and provide a structural overview of the current one.

Exploration in the Hodna basin began in the 1940s, when the Oued Gueterini oil deposit was revealed, which represents a small oil producing field in the Neogene; with some 3000 m3 of light oil per year, as well as the presence of heavy oil.    

 

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Structural model of a seismic transect crossing the Hodna basin. (Alnaft study, realized by Schlumberger).

 

THE HIGH PLATEAU DOMAIN:

The Hauts Plateaux (Sens Large) are a fairly stable domain, limited to the south by the Saharan Atlas and to the north by the Tellien Domain. The northern limit is generally marked by a series of accidents involving dumping of anticline folds to the south. The latter is covered by chotts and continental lands of Pliocene and Quaternary age.

The current structural pattern of the Hauts Plateaux corresponds to a succession of mainly Mesozoic shoals and depressions, controlled by major accidents with a general north-east-south-west orientation. This area has always played in a stable zone with very little subsistence.

These are the furrows of Telagh and that of Tiaret which present an oil interest in this region.

Together, it represents basins of the intra-mountainous type where the oil potential has been proven in particular by a recent discovery of gas in the Lower Cretaceous series in the Tiaret region; The Telagh trench has generated interest with exploratory activity since the 1960s, motivated by positive results on a number of wells drilled with hydrocarbon showings encountered while drilling.

 

The Constantine basin:

Located in the eastern part of the Algerian mining domain. It is limited to the north by a very extensive allochthonous domain, consisting essentially of Mezo-Cenozoic lands.

In the South, it is a relatively undistorted area, characterized by Cretaceous plicative structures, elongated sigmoid in a NE-SW orientation.

It corresponds to a subsiding sedimentary basin whose history begins at the end of the Triassic and ends in the Tertiary, characterized by:

- An intense Triassic diapirism which takes place along the large NE-SW accidents, as well as at the intersection of the latter with the transverse accidents of NW-SE direction;

- An alignment of trends in the NE-SW direction, comprising narrow anticlinal folds separated by vast synclinal plains filled with Miocene-Pliocene and Quaternary terrain;

- Intense fracturing, represented by longitudinal faults trending NE-SW and transverse trending NW-SE.

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Structural scheme of North East Algeria (DES, 2012).

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Interpreted seismic transect and corresponding gravity profile (processed by ARKEX), in the Constantinois region

 

THE SAHARAN ATLAS

It represents a young relief, elongated in the NE-SW direction, and is characterized by large folds, with abrupt periclinal endings. The anticlines, long and narrow, are separated by wide, flat-bottomed synclines.

In general, the folds are a succession of detachments on ramps and levels following a double vergence device from the tectonic axis of the chain. The limit of the relief is marked by the South Atlas Accident

Melrhir's furrow

The Chotts El Melrhir furrow is located to the south of the Atlas flexure and marks the transition between two structural domains, it is characterized by a relatively stable domain, rather subsiding, little tectonized and having constituted a deposit-center for thick series of the Mio- Plio-Quaternary.

The latter is characterized by intense synsedimentary fracturing (linked to the early alpine cycle), as well as a compressive regime linked to the late alpine cycle (Late Eocene-Present); resulting in a compressive regime, generating folds, flaws and sliding accidents, linked to the replay of the South Atlas Accident.

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Chronostratigraphic chart of Chott Melrhir (Alnaft study, carried out by Beicip-Franlab).

 

The sedimentary material lies unconformably on a largely eroded Paleozoic. The latter rocks found under the Hercynian unconformity are mainly Cambrian in age, however at the northern flank of the Djemaa Touggourt bulge, the Ordovician and Silurian can also outcrop below the Hercynian unconformity.

The Triassic detrital deposits are the first Mesozoic sediments to accumulate in the region. They are followed by a large deposit of salt and anhydrite and then gradually move to shallow sea carbonate deposits and finally to open sea carbonates.

The Jurassic carbonate lithofacies probably represents the result of open sea conditions in the Tethysian rift system.

The Lower Cretaceous is predominantly composed of coarse clastic deposits advancing northward as a progressive front of shallow sea deposits.

The Upper Cretaceous is mainly represented by shallow sea carbonates and evaporitic deposits with some clastic passage.