Structural Styles and Prospectivity in the Precambrian and Palaeozoic Hydrocarbon Systems of North Africa




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НазваниеStructural Styles and Prospectivity in the Precambrian and Palaeozoic Hydrocarbon Systems of North Africa
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Figure Captions

Figure 1: Location and distribution of Palaeozoic sedimentary basins in North Africa.

Figure 2: Hercynian subcrop map and age and nature of the main structural elements across North Africa. The outline of the major Palaeozoic basins is well reflected in the pattern of subcropping stratigraphy below the Hercynian Unconformity. The locations of the cross-sections shown in Figure 3 are approximate.

Figure 3: Regional geoseismic cross-sections through North Africa illustrating the geometry of the main Palaeozoic basins and the distribution of the main structural provinces. See Figure 2 for the approximate locations of the sections. The locations of key wells used in the construction of the sections are shown.

Figure 4: Correlation chart showing the general relationship between the key lithostratigraphic divisions of the Palaeozoic succession in different regions of North Africa (modified and updated from Shelmani, 2000).

Figure 5: Summary tectono-stratigraphic correlation chart for the major sedimentary basins in North Africa showing the generalised lithology, tectonic activity and the main phases in the tectonic evolution of the basins (in part after Coward, 1998).

Figure 6: Palaeogeographic reconstruction illustrating the tectonic evolution of East and West Gondwana during the Pan-African orogeny (after Jacobs and Thomas, 2004).

Figure 7: Palaeogeographic reconstruction of Gondwana at the end of the Pan-African orogeny (c. 550-530 Ma) showing the distribution of stable cratonic blocks and the surrounding Pan-African “mobile-belts” (after Gondwana Eight, Hobart, Tasmania, 1991).

Figure 8: Structure of the Pan-African Trans-Saharan belt. (a) Distribution of the main north-south trending tectonic units of the Trans-Saharan belt in the Hoggar Massif, southern Algeria and surrounding areas (after Fabre et al., 1988). (b) Schematic geodynamic subduction model of the Trans-Saharan belt at about 600 Ma (after Caby and Monie, 2003).

Figure 9: Tectonic setting of North Africa during the Infracambrian showing the distribution of Infracambrian rifts and associated salt basins throughout northern Gondwana and their possible relationship with major strike-slip fault systems (in part after Husseini and Husseini, 1990; Talbot and Alavi, 1996; Sharland, 2001 and Kusky and Matsah, 2003).

Figure 10: Seismic section showing a remnant of a large-scale Infracambrian? pull-apart basin at depth below the main Palaeozoic sequences in the Kufra Basin, southeast Libya (published with permission of the National Oil Corporation of Libya).

Figure 11: Generalised geological map of the Anti-Atlas mountains of southern Morocco (after Choubet, 1963) and their location relative to the Pan-African Trans-Saharan belt (in part after Azizi Samir et al., 1990).

Figure 12: Infracambrian lithostratigraphy in the Anti-Atlas mountains of southern Morocco and the Ougarta mountains of western Algeria (after Bouima and Mexghache, 1994).

Figure 13: Schematic cross-section through an Infracambrian half-graben in the Pan-Afrian belt, Ahnet Basin, Algeria (after Ait Kaci and Moussine-Pouchkine, 1987).

Figure 14: Schematic cross-section through an Infracambrian half-graben in the Anti-Atlas mountains of southern Morocco.

Figure 15: Generalised palaeogeography of Gondwana during the Early Palaeozoic.

Figure 16: Palaeogeographic reconstruction of Gondwana at c. 440 Ma showing the maximum extent of the Late Ordovician polar ice sheet (in part after Vaslett, 1990).

Figure 17: Chronostratigraphic subdivision and correlation of the Ordovician rocks of North Africa and Arabia.

Figure 18: Geological cross-section the Ordovician formations of North Africa illustrating the erosion associated with the Taconic Unconformity (after Echikh, 1998). See Figure 19 for the location of the section.

Figure 19: Distribution and dominant lithology of the Upper Ordovician glacigenic rocks across North Africa.

Figure 20: Geoseismic cross-section through the NC174-F (Elephant) Field in the Murzuq Basin, Libya showing the possible deposition of the Upper Ordovician glacigenic rocks of the Melez Chograne and Memouniat formations in palaeo-valleys incised into the underlying pre-Taconic Ordovician sequence and the onlap of the basal Siluarian “hot shales” onto the residual post-glacial palaeo-highs. The section is “flattened” on the top Tanezzuft formation seismic marker. Dominant lithologies are indicated by gamma-ray log profile for each of the key wells.

Figure 21: Chronostratigraphy of biostratigraphically dated sectionsj containing Upper Ordovician glacigenic rocks in North Africa, Saudi Arabia, Central Spain, Portugal and Argentina.

Figure 22: Schematic chronostratigraphic development for the Upper Ordovician glagicenic rocks in North Africa (after Sutcliffe et al., 2000).

Figure 23: Distribution and facies of the Silurian Tanezzuft Formation and its equivalents in North Africa (in part after Lüning et al., 2003b).

Figure 24: Chronostratigraphic subdivisions of the Early Silurian and the timing of “hot-shale” deposition in North Africa and Arabia (after Lüning et al., 2003). The different facies developments on Early Silurian palaeohighs and in Early Silurian palaeolows are shown.

Figure 25: Characteristics of the basal Silurian “hot-shales”. (a) Gamma-ray log from well E1-NC174 in the Murzuq Basin, Libya showing the gamma-ray “peak” associated with the highly radioactive organic-rick shales and (b) an expanded section of the same log showing the highly condensed nature of the section covering the interval from the persculptus to the triangulatus or cyphus graptolite biozones. (c) Section of core from the “hot-shale” interval in well E1-NC174 showing the black organic-rich shales and flat-based, convex-top pyrite nodules.

Figure 26: Distribution of the basal Silurian “hot-shale” source rock in blocks NC115 and NC174, northern Murzuq Basin, Libya based on seismic and well data (after Lüning et al., 2000; 2005).

Figure 27: Seismic sections illustrating fault-controlled variations in thickness of the Silurian strata in the Ahnet Basin, Algeria associated with an important phase of extension/transtension in the basin. The syn-depositional faults were subsequently reactivated during the Hercynian Orogeny forming prominent “flower structures”.

Figure 28: Main uplifts and fault zones active during the Palaeozoic in the Murzuq Basin, southwest Libya (after Adamson, 1999).

Figure 29: Schematic cross-section through the northern Tihemboka Arch on the western flank of the Murzuq Basin, southwest Libya, illustrating its uplift, erosional and depositional history during the Devonian. Uplift is interpreted to have occurred during the late Silurian/early Devonian and mid Devonian, interrupted by a phase of subsidence in the early Devonian (after Adamson, 1999). See Figure 28 for location of section.

Figure 30: Seismic sections through selected structures in the Murzuq Basin, southwest Libya showing evidence of abrupt thickening of Cambro-Ordovician and Late Devonian to Carboniferous sequences across steeply-dipping faults (A, B) and localised evidence of Silurian-Devonian compression or transpression (C). The style of the structures suggests that strike-slip movement were important in the development of the main hydrocarbon traps in the basin.

Figure 31: Plate tectonic reconstruction of the north Gondwana continental margin and Palaeo-tethys during the Middle Devonian (after Stampfi and Borel, 2000). The opening of Palaeo-tethys separated the Hun Superterrane from Gondwana.

Figure 32:

Palaeogeographic setting of North Africa during the Late Carboniferous-Early Permian (“Hercynian”) Orogeny.

Figure 33: Geodynamic model of Late Carboniferous-Early Permian (“Hercynian”) compression in North Africa as a result of plate collision between Laurasia and Gondwana (after Doblas et al., 1998).

Figure 34: Seismic section illustrating major NW-directed thrusting in the Hercynian “high-intensity” deformation zone, offshore Casablanca, Morocco. This area was located within the collision zone between Laurasia and Gondwana (data courtesy of ONHYM).

Figure 35: Seismic section through the F (Elephant) Field fault block in the northern Murzuq Basin, southwest Libya, showing the presence of a “Hercynian” angular unconformity (Top Mrar Formation) in the footwall of the fault. The Hercynian event in the Murzuq Basin is generally marked by a disconformity and is often characterised by a change from marine to terrestrial deposition. Angular unconformities are rare (and subtle when present).

Figure 36: Hercynian deformation in the Sbaa, Reggane, Oued Mya and Mouydir basins of Western and Central Algeria.

Figure 37: Low-angle Hercynian unconformities and disconformities in the Cyrenaica Platform in northeast Libya and the Kufra Basin in southeast Libya.

Figure 38: Geological cross-section through the Ghadames Basin from western Algeria through southern Tunisia to eastern Libya showing the Hercynian Unconformity cutting strongly across the underlying Palaeozoic succession in the northeast of the basin (after Echikh, 1998).

Figure 39: Palaeogeographic setting of North Africa during the late Triassic/early Jurassic (after Stampfli and Borel, 2000).

Figure 40: Intensity of Mesozoic extensional deformation across North Africa. The Triassic-Jurassic rift system was related to opening of the Atlantic and separation of terranes from the North African margin (e.g. Turkish-Apulian terrane). Early Cretaceous rifting is thought to be associated with contemporaneous extension across North and Central Afria due to an extensional continental stress field triggered by the opening of the Atlantic Ocean.

Figure 41: Distribution thickness and facies of the Middle Triassic (TAGI/Ras Hamia) Sandstone in the Ghadames (Berking) and northern Oued Mya basins of eastern Algeria and western Libya.

Figure 42: Geological cross section through the Essaouira Basin showing the typical structural configuration of the Late Triassic half-graben in western Morocco associated with the opening of the Central Atlantic (after Broughton and Trepanier, 1993).

Figure 43: Depth map on the top of the Triassic TAGI Sandstone in the Ghadames Basin of eastern Algeria showing the main structural elements and the location of the “Berkine Trend” oil and gas fields.

Figure 44: Regional seismic section across the northern part of the Kufra Basin, southeast Libya. Graben features affecting the entire Palaeozoic succession in the Kufra Basin are likely to have formed during the early Cretaceous when rifting occurred in the Sirte Basin to the north.

Figure 45: Early Cretaceous rifting in the Sirte Basin also strongly affected the Palaeozoic succession. Much of the Palaeozoic strata has been eroded on graben shoulders, with better preservation along the graben axes. Identifying areas of partial erosion is especially important with respect to the lower Silurian Tanezzuft Formation which locally maybe have favourable petroleum source rock characteristics and may have sourced some of the petroleum occurrences in the Sirte Basin. After Thusu (1996).

Figure 46: Tectonic evolution and regional stress directions in Central Algeria showing alternating pulses of extension and compression during the Mesozoic and Cenozoic (after Boudjema and Tremolieres, 1987).

Figure 47: Block diagrams illustrating the geological evolution of the High Atlas, including Triassic-Jurassic rifting, Cenozoic sagging and Cenozoic inversion (after Stets & Wurster, 1981).

Figure 48: Cross-section through the Gassi El Adem Field in the Ghadames (Berkine) Basin of eastern Algeria showing the development of a mid-Cretaceous (Aptian) anticline probably associated with sinistral transpression along the north-south trending Trans-Saharan Fracture Zone.

Figure 49: Mid Cretaceous (Aptian) inversion of Late Triassic-Early Jurassic graben in the Oued Mya and Mouydir Basins, central Algeria (after Boudjema and Tremolieres, 1987).

Figure 50: Palaeogeographic setting of North Africa during the main (“Laramide”) phase of “Alpine” compression in the Late Cretaceous (after Stampfli and Borel, 2002).

Figure 51: Intensity of Alpine compressional deformation across North Africa.

Figure 52: Seismic section showing Late Cretaceous-Tertiary (“Alpine”) inversion of Late Triassic-Early Jurassic rifts in the Missour Basin and the Atlas Mountains, Morocco (after Beauchamp et al., 1996).

Figure 53: Late Cretaceous-Early Tertiary folding of the Jabel Akhdar anticline in northern Cyrenaica. Contemporaneous compression also occurred in neighbouring Egypt and along the Eastern Mediterranean Levant coast (incl. Sinai, Negev, Syria). The intraplate stresses originated from plate collisional processes in the Afroarabian – Eurasian suture zone (data kindly provided by NOC).

Figure 54: Modelling based on AFTA data from a well in the Murzuq Basin. Apatite fission track analysis indicates a significant tectonic uplift of the Qarqaf Arch and northern Murzuq Basin during the Late Mesozoic and Cenozoic Alpine phase of deformation.

Figure 55: Seismic section across part of the Cyrenaica Platform in northeast Libya showing extensional faults affecting the Palaeozoic and younger succession. The extension seems to be mainly Tertiary in age as strata of Palaeocene and possibly younger age are clearly affected (uninterpreted seismic data after Sola and Ozolcek, 1990).

Figure 56: Summary of the structural evolution of the “Bouri” Area, offshore Libya (after Bertello and Zua, 2003).

Figure 57: Trap styles, timing of deformation and the age of associated reservoir horizons for proven Palaeozoic hydrocarbon systems in North Africa. The main trap styles include various forms of anticlines and fault blocks generated during the main Silurian-Devonian, “Hercynian” and “Alpine” compressional and Mesozoic extensional phases.

Figure 58: Average timing of maturation of the basal Silurian “hot-shale” source rock in the North African Palaeozoic Basins.

Figure 59: Volumetric distribution of the hydrocarbon resources of North Africa between the various Palaeozoic and Mesozoic reservoirs.

Figure 60: Summary of the basal Silurian/Late Devonian sourced Palaeozoic Hydrocarbon System and the timing of trap formation in North Africa. The dominant age of structural trap formation in the proven Palaeozoic (Sirte Basin: Mesozoic) hydrocarbon systems in North Africa is shown. “Hercynian” compression was dominant in trap formation in western and central Algeria, whereas further east other tectonic phases such as the pre-“Hercynian” transpression and transtension, Mesozoic extension and “alpine” inversion become more dominant. The locations of the cross-sections shown in Figure 58 are approximate.





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