Paleozoic sediments

Despite the claim by Bartholome (1966b) that the minerals are predominantly Tertiary in age, they seem to persist in older non-metamorphic sediments of Western Europe. Paleozoic sediments are rare there, being almost always... 

The West Saharan synclinorium subsided strongly during the Paleozoic, and over it Paleozoic sedimentation took place in a vast basin with a thickness of 7-8 km, whereas the Central and the East Saharan Synclinoria during the same time were less subsiding areas with marine deposits only 2-4 km thick (Fig. 2.5). 

Paleozoic Sediments of the East Saharan Synclinorium (Ghadames and lllizi Basins)... 

Lower Paleozoic sediments in the Ahnet, Mouydir, Reggane, Tindouf and North Timimoune Basins are characterized by a high level of diagenesis (R = i.2-i.6%). In deeply buried areas in the Tindouf and Reggane Basins, R reaches 2-3% in the Cambro-Ordovician sediments. On the other hand, the Sbaa sub-basin (South Timimoune Basin) is distinguished by relatively moderate thermal gradients and R is somewhat o.9-i.o% in the Silurian and Devonian sediments. 

Fig. 4.1. Transformation of mixed-layer days of illite-smectite types (unordered) into ordered phase of vermiculite-chlorite type vs. transition of Fe into Fe. Paleozoic sediments of the Illizi Basin...

Fig. 4.16a. Block diagram of Paleozoic sediments, Ahnet-Mouydir Basin (boreholes stratigraphy and lithology are based on Sonatrach s drilling data)...

On the whole, although the Silurian source rocks are found at present in the gas window , they have never been strongly warmed up. This apparent contradiction may plausibly be explained by the uplift and erosion of the Paleozoic sediments during the Hercynian orogeny. In many instances, the present-day subsidence depth of source rocks is smaller than it was before the Hercynian uplift. In the Illizi Basin, in particular in its southern and western regions, the deepest subsidence took place prior to the Hercynian uplift. A slight uplift could not cause a substantial temperature decrease ... 

Fig. 5.3. Organic isotope and geochemical data, Triassic Province, a Carbon isotope composition of oil and bitumenoid fractions for Paleozoic sediments, Triassic Province b Carbon isotope composition of oil from Saharian fields c Geochemical log for Takhoukht geological section based on shale pyrolysis data d Classification diagram HI-OI for Silurian and Devonian shale sediments, Triassic Province e Distribution of residual oil potentials vs. organic carbon content in Silurian and Devonian oil source rocks, Triassic Province...

Sumary of the Generation History. Two stages - Paleozoic and Mesozoic - are clearly distinguished over the course of tectonic evolution, with the predominance of subsidence processes. They are separated by the Hercynian orogeny. The area of deepest subsidence, characterized by an increase thickness of Paleozoic sediments, is localized in the south of the Oued el-Mya depression. 

An analysis of the profiles of present-day temperatures and paleotemperatures in the Paleozoic sediments has indicated favorable conditions for the hydrocarbon generation mostly in the south and south-west of the region. As to the hydrocarbon generation in the central and northern areas, it might have taken place mainly in the Mesozoic. 

Thus, by the end of the Carboniferous the average depth of subsidence was about 3.0 km for the Ordovician, 2.8 km for the argillaceous Silurian, 2.5 km for the Early and 2.2-1.5 km for the Middle and Late Devonian (see Fig. 5.5). The Hercynian orogeny entailed a very significant uplift of the western and northern parts of the depression, which resulted in the formation, in the central part of the area, of a depression which was progressively subsiding in the south-east direction to form a saddle in the SED-i area. The Paleozoic sediments were highly eroded in the northern and western areas where the Ordovician, Silurian and Early Devonian deposits were exposed directly beneath the Hercynian unconformity. 

The western Saharan syncline was subjected to rapid subsidence during the Paleozoic and early Mesozoic. This resulted in the formation of a large basin (7-8 km deep) of Paleozoic sediments. The central and eastern Saharan synclines in the Paleozoic were areas of slower subsidence (in individual periods of the Paleozoic), and marine sediments are comparatively thin (from 2 to 4 km) (Fig. 6.13). During the early Mesozoic, the eastern Saharan syncline, including the Trias, Oued el-Mya, and Ghadames Basins (Figs. 6.12 6.13a,b), subsided rapidly. This resulted in the accumulation of up to 4 km of marine and, in places, continental Triassic, Jurassic, and Cretaceous sediments. 

The proportion of the illitic layers in the I/M increases with depth and the unordered phases are transformed into ordered phases of the allevardite type (Fig. 8.1). This transition is easily recognized in diffractograms of samples saturated with ethylene-glycol by the disappearance of the 17-A peak and the appearance of a peak at 13-14 A indicative of a structural ordering in short chains (Fig. 8.2). Such a phase transformation has been initiated at various levels in the Oued el-Mya Basin at depths of 2 300-2 500 m. The transformation of I/M of the allevardite type into those of the kalkbergite type with >80% illitic layers in the lattice takes place at a depth of 4.3-4.S km in the Paleozoic sediments. 

The mineralogy of the shales, i.e. appearance of certain structural polytypes, crystalline peculiarities, or ordering in mixed-layer clays, as well as vitrinite reflectance Ra in several parts of the Oued el-Mya basin (Allal and Hassi Messaoud-Agreb domes) and even in the Ghadames Basin confirm that the paleotemperatures were higher than the present ones which were determined in boreholes in Paleozoic sediments of the domes mentioned. 

Crystalline features of the clay minerals can probably be used as geothermal indicators. The correlation of the temperatures calculated from the reaction of I/M and the appearance of chlorite Ilb in the Paleozoic sediments of the southern Oued el-Mya Basin with the (uncorrected) values measured in the boreholes revealed that the enclosing sediments in the past had experienced temperatures some 30-40 C higher than those obtained from diagraphic data. 

---