The research and analytical work performed here led us to identify 12 suitable areas represented by deep saline confined and semi-confined aquifers (Table 4 and Fig. 1a). This analysis of the storage potential of the sedimentary basins in southwest Algeria for GSC allows us to highlight the attractive potential of the Ahnet–Gourara Basin (Fig. 1a). In fact, its final score of 0.92 % (Table 3) is greater than those of the other basins, and it is a priority basin for further investigations to identify specific sites and basins for CO2 injection.
The purpose of site characterization is to determine whether a site is suitable and safe for sequestration and to compile the data necessary for the permit application. The process includes geologic, geophysical, and engineering evaluations. Characterization is performed to obtain the geological and hydrological data needed to design the infrastructure, develop reservoir models, and design the monitoring program. In this phase of site development, a determination is made regarding whether the reservoir has adequate porosity, permeability, and continuity for long-term injection. In addition, the ability of overlying units to confine the injected CO2 and prevent vertical movement is also determined, including an evaluation of the presence of non-sealing faults and other potential pathways for migration. This analysis primarily focused on the geology (stratigraphy and structure) and geophysical data (seismic reflection and logging) for the characterization of potential reservoir-caprock systems.
Suitable areas (Ahnet and Gourara basins) are characterized by thick sediment, porous and permeable formations with saline water saturations that may exceed 97 %, and caprock with very low porosity that can act as an impermeable waterproof covering; the various characteristics of the reservoirs are listed in Table 4. Figure 4b shows the variation of water saturation in the Ahnet–Gourara basins.
Based on the work of Bachu (2003), twelve potential areas were considered to be moderately hot basins with geothermals not exceeding 32 °C/km, lying at depths of 1100 and 2200 m with temperatures and pressures sufficient to ensure the supercritical state and buoyancy of the injected CO2. Most of these saline aquifers are favorable for storing CO2 because of the thick Cambro–Ordovician layers of quartzitic sandstone with matrices of laminated clay intercalation and dispersed clay. The caprock has a minimum thickness of 200 m and is composed primarily of Silurian clay, which completely seals the potential reservoir. This initial assessment of the GSC capacity in Algerian deep saline aquifers encountered difficulties related to the storage efficiency factor, Esalin, and the potential storage capacity was calculated assuming that 0.54 or 5.4 % of the total pore volume could be filled or saturated (NETL 2010). Our calculations provide a very conservative estimation of the effective capacity of the determined areas’ pro-sequestration of CO2 of approximately 1 GT or 5 Gt (Esalin = 1 and 5 %, respectively; Table 4).
Below, we present the main characteristics of the most promising area for CCS in Algeria, the Ahnet Basin. We noted a significant number of structures of varying sizes via the controlled mapping of the surface outcrops. Analyzing the seismic maps reveals a degree of intense structuring in this area, which contains an interesting mix of prospects and structurally complex types. The Ahnet Basin differs from other areas in the Saharan platform by its degree of intense structuring linked to the evolutionary history of the West African craton junction, which is thought to have been stable for approximately 2 billion years. The East African craton is considered mobile and cratonic because of the Pan-African orogeny (approximately 550–600 million years ago). The Ahnet Basin is related to a joint area of these two cratons. Their collision created brittle tectonic activity of the substratum level and is probably Paleozoic in age (Fabre et al. 1996). This old tectonic activity occurred during the following phases:
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Taconic (late Ordovician);
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Caledonian (Late Silurian beginning Devonian (Siegenien);
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Hercynian, the most important phase (late Permian); and
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Austria, an essentially post-Hercynian compression phase (Upper Cretaceous).
The current structure was primarily formed during the Hercynian orogeny, which completely modeled this basin (e.g., faults, gouge zones, anticlinal structures, and intense erosion). The Austrian phase wrinkling caused replays in which slip formed drive folds along preferential axes.
Moreover, this basin was also strongly influenced by tectonics linked to Hoggar and characterized by the presence of structural trends in the sub-meridian direction attached to the extension; the deformations north of the base are typical in the Hoggar.
Below, we present the main characteristics of two of the most promising areas for the application of CCS in Algeria. These two areas are located in southwest Algeria and in the onshore region and have been named “Tidikelt North” and “Akabli 02”, respectively (Fig. 1a).
Tidikelt North
Tidikelt North is located in the Ahnet Basin, specifically in the Ouallen Sub-Basin, which is bound to the east and west by large collisions and is filled by a thick sedimentary series of more than 7000 m, including lower outcrops to the east and west that represent one of the main structural elements in the field. Drilling and seismic studies established in the region show purple surmounted by a series of Cambro–Ordovician or infra-tasilienne unconformities located at a depth of 1200 m with a thickness of 700 m and an oblique stratification (Fig. 6).
The potential reservoir formations suggest an eolian deposit consisting of sands and silty sands from the Precambrian. These deposits have been interpreted as the bottom of the sub-basin. The reservoir is locally more than 500 m thick with an effective thickness exceeding 250 m, as recorded by several drill holes. Its sand layers are often saturated with salt water, as indicated by the spontaneous potential and resistivity logs and an analysis of the formation water that revealed salt saturation of 120–180 g/L. All wells drilled in this region have logging and the petrophysics of the logging recordings or the core, and a porosity of 18 % was assessed via a combination of different logs (namely, the sonic, neutron, and density logs), other photoelectric factors, and cores.
From seismo-stratigraphic and seismic perspectives, these deposits are represented by a considerable amplitude and subparallel and continuous reflections and overcome a Cambro–Ordovician unconformity or infra-tasilienne unconformity. Additionally, a thick, 1200-m sedimentary series of shales of Ordovician and, specifically, El Gassi Clay play the role of caprock in addition to the Silurian shales, which represent the regional coverage for the Cambro–Ordovician reservoirs. To the west of the reservoir series based on infra-Cambrian formations of clay and green finely laminated silts clays often containing dropped pebbles (dropstones), the depositional environment corresponds to a sea or lake environment (Fig. 7).
With an area of approximately 1150 km2, this structure is one of the most promising sites for CO2 sequestration. Additionally, because of its density of approximately 920 kg CO2/m3 (Fig. 3b), this site could store over 476 Mt of CO2.
Akabli 02
Akabli 02 is located in the region of Akabli in northwest Ahnet. The current geometry of the region is marked by various superimposed major structural axes resulting from a complex polyphase history. The N–S and NW–SE directions approximately follow the collisional frontline between the rigid Precambrian craton in West Africa and the bulk of East Africa, which has experienced terrane accretion and additional distortion. The NW–SE direction controls the sedimentation from the Silurian with the opening of the basin to the north and marks the structuring of the region because of the reversal of the paroxysmal movements from the Hercynian to the late Carboniferous. Finally, the E–W and NE–SW directions experienced transgressive movements in the Hercynian.
The area subsided during the Mesozoic, and the tectonic impact of the Cretaceous and Tertiary was relatively limited at the basin scale. Structural traps, which primarily formed during the Carboniferous, are usually associated with large reverse faults with depressions of up to several hundred meters. They have high amplitudes but moderates size (several kilometers of expansion along the structure’s axis).
Akabli 02 is a high-amplitude anticline with a WSW-ENE axis approximately 380 km2 at its structural closure at the roof of the Ordovician (1165 m below sea level). This structure is located in a relay zone between two reverse faults trending sinistral N70° and showing decoupling.
This structure is affected by numerous faults related to the tectonic style and primarily the decoupling zone. It is pinched between two thrust faults with strong subsidence and, likely, a slight sinistral strike slip motion (Fig. 8). Its establishment is associated with a principal stress oriented at N120°, which corresponds to the direction of the Hercynian compression observed in the region.
Regional data show that the Cambrian deposits are regular and very uniform over large distances. Cambrian sands were deposited in a continental environment as infilling and were amalgamated into braids. They are the main structure in the reservoir.
More resent episodes of an estuarine environment are also revealed in the core drilling intervals and are reflected and increased relative to the thinner clay sediment. This depositional environment explains the lack of contrast and outstanding figures in the logs, which make them difficult to correlate.
The homogeneity of the reservoir’s mode of filling is reflected by the absence of reliable subdivision within the Cambrian and significant extension to the fluid flow barrier. However, the top of the Cambrian unit is identifiable based on the responses of the porosity, particularly the density and sonic logs. The Cambrian unit is the main reservoir. Regional data show that this reservoir was deposited relatively uniformly over large areas with large thicknesses. Thus, this reservoir is modeled with a thickness of 250 m, a uniform structure, and a porosity of 15 %. Other formations found during drilling were taken as secondary reservoirs and were evaluated by means of logs, which demonstrated the existence of salt water.
Ordovician Units IV & II are primarily sandstone with past micro conglomeratic clays. This reservoir is the main gas reservoir in the Ahnet Basin. The porosity exceeds 15 % in the southern edge of the region and is approximately 6–8 % in the central part.
The Tournaisian unit contains fine sandstone with glauconite and bioclastic in the form of 50-m marine bars. The reservoir’s characteristics are generally good. The porosity exceeds 15 %, and the permeability exceeds 100 mD; however, it is not considered a favorable reservoir because of its depth (less than 800 m).
The Strunien unit often corresponds to sandstones in communication with the Tournaisian sandstones and is essentially an aquifer. The average porosity exceeds 20 % at the top and decreases with depth. The permeability rarely exceeds 100 Md. However, this reservoir is not considered favorable because of its depth.
The Gedinnian unit corresponds to sandstone bar-type deposits of bioturbated sandstone and clay. The porosity ranges between 8 and 16 %, and its permeability rarely exceeds 1 mD. The thickness is variable and sometimes exceeds 95 m. Logs indicate a column of 82 m with porosity as high as 14 %.
The Givetian unit consists of bioclastic calcareous sediments alternating with clay and sandstone. The thickness is variable and sometimes exceeds 50 m. Logs indicate a column of 48 m with porosity as high as 25 %.
Coverage of these reservoirs is provided by Silurian clays, which are regionally well developed and provide a good coverage of the Cambro–Ordovician reservoirs. The base of these clays is highly radioactive with abnormally high pressures, increasing their coverage effectiveness. The Tournai reservoir sandstones are covered by Tournaisian and Namurian clays, and the Middle and Upper Devonian clays provide a caprock for the Gedinnian and Siegenien sandstones (Fig. 9).
The different relationships of the porosity/permeability and Cortege clay are shown in Fig. 10. With an area of almost 380 km2, this structure is one of the most promising sites for CO2 sequestration with a density of approximately 960 kg CO2/m3 (Fig. 3b) and could store over 345 Mt.