Oued el-Mya Basin

In contrast to this, with the start of the Mesozoic, the region of the East Saharan synclinorium suffered intense burial under the accumulation of 4 km of marine and locally continental Triassic, Jurassic and Cretaceous sediments. Thus a vast Mesozoic sedimentary depositor (Triassic Basin, Oued el-Mya Basin, etc.) was formed which encompassed the present Ghadames Depression or the so-called Grand Erg Basin (Figs. 2.4 and 2.6). 

Fig. 2.12. Model of paleogeographic and depositional paleoenvironments for Triassic clastic sediments in north Oued el-Mya Basin... 

This type of dia- or catagenetic trap is developed in the north of the Oued el-Mya Basin in the regions of Haniet el-Beida (HEB),Kef el-Argoub (KG) and Hassi Ladjouad (HLJ), as illustrated in Fig. 2.14. 

Fig. 3.1. Generalized curves for porosity and density gradients and echelons of sandstones and siltstones with depth in the Oued el-Mya Basin (based on statistical processing of porosity and density data)...

Fig. 3.6c,d. Mechanical compaction features of Saharan res-servoirs. c Correlation curves of contact index with depth for Triassic, Devonian and Cambro-Ordovician reservoirs, Oued el-Mya Basin i sandstones with 20-30% ductile cement, 2 sandstones with 20-10% ductile cement, 3 sandstones with less than 10% ductile cement, d Comparative correlations of tight packing index for Devonian sandstones with 10-15% ductile cement from i Ghadames Basin, 2 Oued el-Mya Basin,... 

Fig. 3.7. Correlation curves of a porosity and b permeability with secondary silica for Palaeozoic reservoirs of Oued el-Mya Basin / oil-bearing reservoirs, II water-bearing reservoirs. Striving for representative results, the studied sandstones were selected so that they have a similar medium grain size (0.2-0.3 mm), and a close argillaceous cement content (10-15%) with very little or no carbonate cement...

Fig. 3.10. Curves of relative change of porosity and permeability with thickness of beds (based on average statistical data). I Sandstones from Ahnet-Mouydir-Gourara Basin (PZ), II sandstones from Illizi Basin (D + C), III sandstones from Oued el-Mya Basin (PZ), IV sandstones from Ghadames Basin (D)... 

Fig. 3.12. Correlation curves of I reservoir porosity, 2 adjacent shales density, montmorillonite/illite proportion in adjacent shales, 4 potassium content in shales, with depth for Triassic and Palaeozoic sediments, Oued el-Mya Basin...

Fig. 3.16. Infrared spectra of quartz in Triassic, Devonian, Silurian and Cambro-Ordovi-cian sandstones from Oued el-Mya Basin (MGD-r, TEG-i, OS-i, GBC-i and OCT-i boreholes). Evolution of quartz doublet 800-780 cm" for sandstones with increasing burial depth ...

Fig. 4.7. Isotope composition and porosity correlation in Triassic reservoirs (North Oued el-Mya Basin), south-east-west...

Transformation and dissolution of carbonate cement in the Paleozoic, i.e. Silurian, Devonian and Carboniferous, reservoirs in all basins are similar in nature to those in the Triassic reservoirs of the Ghadames Basin and certain parts of the Oued el-Mya Basin as these Paleozoic reservoirs alternate with numerous thick rather mature shales rich in organic matter. These beds are able to generate large volumes of H COj as weU as of organic acids which have immediate access to adjacent reservoirs. 

The calculation of the hydrological parameters necessary for establishing the above-mentioned silica balance appears to be a rather complicated problem as intergranular pressure solution leads to a dynamic reduction in rock mass and volume and frequently also to a lowering of the porosity. These parameters have been simulated for different petrophysical and geochemical conditions. The first results show that for the large amount of secondary silica observed to become exported, 0.5 x io -o.5 x lo cm of water is required for every cm of the Cambrian sandstones of the Oued el-Mya Basin. If we assume, on the other hand, that the quartz cement in the Ordovician sandstones from Ahnet Mouy-dir resulted within the rocks themselves from pressure solution then the water flux necessary was small or virtually nil and this formation approached an isochemical system. 

The major source rocks in the Oued el-Mya Basin are Silurian and Devonian shales and, to a certain extent, Ordovician shales (Figs. 5.1,5.2,5.3). The organic matter of the... 

The input parameters for the model include the present-day sedimentary cross section, estimates of the amplitude and rate of erosion, the lithological composition and petrophysical characteristics of rocks, the structure of the lithosphere (basement) and its rock parameters, paleotemperature markers (vitrinite reflectance), paleoclimate, sea paleodepths, present-day surface heat flow, depth-temperature profiles, and information on the paleotectonics and the present-day tectonic setting of the basin. The evolution of the Oued el-Mya Basin is used to demonstrate the model. Table 6.1 presents the basin s main stages of evolution, which include sedimentation, hiatus, and erosion. 

Table 6.1. Main stages of Oued el-Mya Basin evolution ...

Fig. 6.3. Numerical modeling of the thermal regime in the lithosphere of the Oued el-Mya Basin, Takhoukht region. Solid line ist the base of the lithosphere as determined by intersection of the current geotherm with solidus curve for peridotite (shown in Fig. 6.4). Long dashed lines are isotherms Moho = base of the crust comp, transition = location of pyroxene peridotite-garnet peridotite compositional transition in the mantle. The considerable rise of isotherms is related to periods of thermal activation in the lithosphere in the Permian-Tfiassic and Cretaceous, Reduction of the crust thickness occurred during basement stretching (in the Ordovician-Devonian and Cretaceous) and during Permian erosion...

Fig. 6.4. Calculated initial and present-day temperature profiles in the lithosphere of the Oued el-Mya Basin, Takhoukht region. The solidus temperature of peridotite rocks with small content of HjO (Wyllie 1979) were used to determine the base of the lithosphere in the basin modeling...

Fig. 6.5. Computed variations in heat flow during the Oued el-Mya Basin modeling, Takh-oukht region. The difference between the heat flows through the surfaces of the basement and sediments is due mainly to radioactive heat generation in sediments...

Initial heat flow, Q, is estimated from the present-day values of surface heat flow in the regions with tectonic settings similar to that expected for the time of the basin initiation. For example, Q s 100-110 mW m" is typical of continental rift areas (Baikal Lake, African rift system) however, on rift shoulders, Q can be nearly 65-85 mW (Smirnov 1980). Estimations of the initial heat flow can be adjusted by analysis of variations in tectonic subsidence of the basement (see following sections). In the case of the Oued el-Mya Basin, Q is approximately 52 mW m" (Fig. 6.5). 

Fig. 6.7. Burial, thermal, and maturation histories of the sedimentary section in the Tahoukht region of the Oued el-Mya Basin resulting from basin modeling in the variant without erosion. The first thermal activation in the variant without erosion is less intense than the thermal activation in the main variant in Fig. 6.2...

Three regions in northern Africa were considered (Fig. 6.11) the Takhoukht area of the northern part of the Oued el-Mya Basin where there were contrasting Paleozoic and Mesozoic tectonic and sedimentation histories and Hercynian erosion was at a maximum the Akfadou area of the Ghadames Basin where there was moderate Hercynian uplift and erosion and the Mereksen structure of the Illizi Basin where there was Hercynian erosion. 

For brevity, the details of the tectonic and thermal histories are given only for the northern Oued el-Mya Basin. Directly relevant thermal features for hydrocarbon generation are reported for the Ghadames and Illizi Basins. Geochemical and modeling data acquired for areas in other basins have been compiled and integrated into the regional synthesis and conclusions. 

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