Asociación entre autodescripciones y actividades del taller: aspectos complejos

Libya Sirt Basin

In the Sirt Basin, Campanian-Maastrichtian organic shales occur in the graben areas and include the Sirte Shale Formation. The deposition of the Sirte Shale Formation mainly took place in the protected environment of sediment-starved rift grabens. This protected environment prevented the oxidation of the organic matter content through restricted circulation and prevented the dilution of the organic content due to low sediment influx, which led to the development and preservation of source rock quality shales (Schröter, 1996). The Sirte Shale represents the deepest facies consisting of a transgressive succession (Barbieri, 1994) and reflects deeper marine, slow deposition in grabens under anaerobic conditions (Tawadros and Tawadros, 2001). The Sirte Shale Formation is overlain by the upper Maastrichtian Kalash Formation and underlain by Lower Campanian Tagrifet Limestone (Barr and Weeger, 1972). There is a gradual transition in the lower part of the Sirte Shale Formation that passes laterally into the dark-brown, argillaceous micritic limestone with planktonic foraminifer assemblages, rare benthic fauna and the pyrite Tagrifet Formation. Similar to the Sirte Shale Formation, the Tagrifet Formation is characterized by a low oxygen, low energy conditions, with a supposed water depth at approximately 40 metres (El-Alami et al., 1989). The lithology of the Sirte Shale Formation is characterized by laminated, non-bioturbated, grey to dark grey to brown, calcareous silty shale in the lower part of the section. This represents the deepest facies that contains transgressive sequences which reflect deeper marine sediments and which showed slow deposition in trough areas under anaerobic conditions (Lüning, 2003). According to benthonic foraminiferal analysis, the lowest Sirte Shale Formation is believed to represent a middle neritic setting, as in Figure 1.2 (Lüning, 2003), while the middle and upper parts of the Sirte Shale Formation mark a further deepening and an increase in oxygen-deficiency (Lüning, 2003). At the same time, where oxygen was available within the water column, carbonate sedimentation took place leading to deposition over platform areas.

The thickness of the Sirte Shale Formation varies and ranges from a few hundred metres to 900 metres in the graben areas. The Sirte Shale sedimentary section is

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characterized by high gamma-ray values associated with maximum organic richness in the section type situated in well O2-59 on the edge of the Al Beda Platform (Hallett, 2002).

Tunisia and Algeria

Generally, the Algeria and Tunisia structures encompass the Atlas Fold and Thrust belt to the north and the Saharan Platform to the south, separated by the South Atlas Fault.

Over the Saharan Platform, Cretaceous deposition is relatively thin and weak affected by tectonic movements, with the exception of local strike-slip movement (Bishop, 1988; Busson, 1998). In general, the sediments in the Atlas belt are highly influenced by sedimentary extensional and compressional phases (Bishop, 1988; Busson, 1998).

Eastern Algeria and Tunisia were covered by a large shelfal sea in Campanian-Maastrichtian times; see Figure 1.2. Most of the sediments mainly consist of marls and carbonates, but in Algeria and Tunisia the Campanian-Maastrichtian organic-rich strata have not been recorded, due to a low water depth that did not allow the deposition of the black shale formation (Askri, 1995). The Cenomanian-Turonian Bahloul Formation represents the main organic-rich zone in the Cretaceous/Tertiary boundary, and also The Eocene Bou Dabbos Formation (e.g. Gaaya and Ghenima, 1998).

Egypt

Structurally, the subsequent late Triassic/early Jurassic break-up of the Turkish-Apulian terrane from Egypt, where the northern part of Egypt was subjected to extensional stress, resulted in the formation of half-grabens (e.g. Moustafa and Khalil, 1990; Hirsch et al., 1995; Guiraud and Bosworth, 1999). In Egypt, the half-grabens became inverted during the Turonian-Coniacian periods onwards (e.g. Luening et al., 1998). This intra-plate deformation was important in the beginning of the collision of Afroarabia and Eurasia along the Cyprus subduction zone and the Bilits-Zagros suture zone. During Oligocene times from about 30 Ma, the rifting and separation of Arabia from Africa in the southern Red Sea started, and in the early Miocene at about 20 Ma

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in the northern Red Sea and Suez Gulf rifting and separation also commenced (e.g.

Baldridge et al., 1991).

The Campanian-Maastrichtian Brown Limestone is organically rich in the Gulf of Suez area (Alsharhan, 2003). In addition, the Campanian-Maastrichtian interval is organically enriched in other places, such as the Duwi and Sudr formations in the Red Sea at the surface exposures in the Safaga and Quseir areas. In Egypt, in some places, the Campanian-Maastrichtian stratum was removed by Tertiary uplift and erosion (Dolson et al., 2001). The Brown Limestone, consisting of limestones with interbeds of highly calcareous laminated shales and marls, represents marine outer sublittoral facies. In the Gulf of Suez region, the Brown Limestone has thicknesses ranging from 25-70 metres. It is also characterized by abundant phosphorite and glauconite. This formation was deposited during a strong Campanian sea level rise (Haq et al., 1987) because of widespread flooding of the Northeast African and Northwest Arabian shelf (Figure 1.2).

The Duwi Formation consists of marl interbedded with phosphate beds. It is conformably overlain by the Dakhla Formation (Said, 1962). The lower Dakhla is similar to the Duwi Formation, characterized as organic-rich but lacking the development of the phosphorite beds, which may be due to raised water depth (Robison, 1995; Alsharhan, 2003). Robison (1995) reported that the Brown Limestone, Duwi and Dakhla Formations are organically enriched in the Red Sea region, in the southern Western Desert, and in the Gulf of Suez. Their total organic carbon values reach up to 8 wt%, and locally to 20 wt%. The Brown Limestone contain the highest average TOC of all potential source rocks in the Gulf of Suez, with Type II oil-prone kerogen, and also Type I is present (Alsharhan, 2003). The, organically rich laminated shale of the Brown Limestone was deposited under dysoxic to anoxic marine conditions (Robison, 1995). It has a low abundance of pyrite, suggesting that little iron was available, resulting in the formation of sulphur-enriched organic matter (Robison, 1995). The Brown Limestone Formation is thought to be the main candidate source

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rock in the Gulf of Suez (Robison, 1995; Alsharhan, 2003). The black shale of the Campanian-Maastrichtian Brown Limestone Formation is mature in large parts of the Gulf of Suez and represents the oil kitchen source rock (Robison, 1995).