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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persculptus Zone. This triggered the ‘second strike’ of the Late Ordovician mass extinction when the associated glacioeustatic sea level rise produced a return to normal oceanic stratification and flooded the previously exposed continental shelf with anoxic waters. Analogy with late Quaternary glaciations and with chronometric data from coeval Upper Ordovician deposits in Australia suggests that the climate (and presumably ice-sheet volume) during the Late Ordovician was influenced by Milankovich-eccentricity cycles of approximately 100,000 years (Sutcliffe et al., 2000). The presence of two cycles of glacial advance and retreat in the Upper Ordovician successions of North Africa suggests that full glacial conditions may only have lasted about 200,000 years, a much shorter time than previously thought (Figure 22). This accounts for the poor biostratigraphic resolution that exists in the late Ordovician glacigenic sequences in North Africa, and has important implications for extinction rates and global climate change.

The different facies of the Upper Ordovician glacigenic succession have widely differing reservoir properties. Successful exploration for, and development of, these reservoirs require detailed understanding of sand body distribution, shape and vertical and lateral continuity. Ice-proximal fluvio-glacial deposits and high-density turbidites typically form the best quality reservoirs, but punctuated coarsening-up shoreface deposits in the post-glacial isostatic rebound succession also have considerable potential. Appraisal and development of these glacigenic reservoirs is further complicated by the presence of a wide range of syn-depositional, glacially-induced heterogeneities, including sub-glacial and ice marginal fold-thrust belts, tunnel valleys, soft-sediment load structures, intraformational shear surfaces, dewatering structures and micro-faults, all of which have the potential to act variously as barriers, baffles or conduits for fluid flow on both geological and production time-scales.

Melting of the Late Ordovician ice cap caused the Early Silurian sea-level to rise by more than 100m, leading to a major transgression that flooded the North African shelf as far south as the northern parts of Mali, Niger and Chad (Figure 23). Graptolitic, hemipelagic shales represent the dominating facies, while deposition of sandstone or non-deposition prevailed on regional palaeo-highs, such as most of Egypt, which formed a peninsula at the time (Lüning et al., 2000, 2003b). In Libya the total thickness of the shales (termed “Tanezzuft Fm.”) increases northwestwards from 50m in the proximal Kufra Basin, 500m in the Murzuq Basin to 700m in the distal Ghadames Basin reflecting the northwestward progradation of a sandy deltaic system (“Acacus Fm.”) during middle Llandovery to Ludlow/Pridoli.

The majority of the Silurian Tanezzuft Formation is organically lean, with the exception of the Early Llandovery (Rhuddanian) and Late Llandovery/Early Wenlock intervals when anoxic phases occurred (Lüning et al., 2003a; Figure 24). Organically rich, black shales with total organic carbon values of up to 16% were deposited during these two anoxic events. The older of the two black shale horizons is a highly condensed section covering a time interval represented by at least four graptiolite biozones (Figure 25) and is only developed in palaeo-depressions that were already flooded by Early Llandovery times. The earliest Silurian relief may have been controlled, at least in part, by the Cambro-Ordovician tectonic movements, in combination with Late Ordovician glacial and post-glacial processes. In contrast, the upper black shale unit is restricted to areas that had not yet been reached by the prograding sandy delta during the Late Llandovery/Early Wenlock.

Silurian organic-rich shales are estimated to be the origin of 80-90% of all Palaeozoic sourced hydrocarbons in North Africa. The same depositional system is also developed on the Arabian Peninsula, and there are age-equivalent black shales in Saudi Arabia, Syria, Jordan and Iraq (eg. McClure, 1988; Lüning et al., 2005; Armstrong et al., 2005).

Isopach and other data for the basal Silurian, organic-rich 'hot shales' in Libya and Algeria suggest that deposition of these important petroleum source rocks was restricted to palaeotopographic depressions (Lüning et al., 1999, 2000). In south-west Libya, the Silurian 'hot-shale' facies are observed locally to onlap the flanks of some pre-existing fault blocks, but are absent from their crests (Figure 26).

Distinctive limestone beds rich in Orthoceras are interbedded with the Ludlow-Pridoli shales in Morocco and western Algeria, in the most distal parts of the North African shelf. In more proximal shelfal positions sand influx was already too great for limestones to develop. The Orthoceras Limestone is organic-rich in some areas. Similar age-equivalent limestones also occur in some peri-Gondwana terranes, such as in Saxo-Thuringia where the unit is termed “Ockerkalk”.

3.2. Silurian subsidence (c. 444 Ma to c. 418 Ma)

Silurian times were marked by continued subsidence of the north Gondwanan passive margin reflecting the development of the proto-Tethyan ocean between Gondwana, Armorica and Avalonia. The North African area subsided considerably during Silurian times, developing as a northerly dipping passive ramp margin with dominant structural axes oriented at a high angle to the plate margin. The present-day Atlantic passive margin of north-east Brazil, where prominent Neoproterozoic vertical strike-slip shear zones are oriented at a high angle to the Atlantic margin (Davison 1997), could be a useful analogue for the Silurian geotectonic setting of North Africa. These major shear zones were reactivated during South Atlantic rifting and are delineated by small pull-apart basins, containing up to 2 km of sedimentary fill over 600 km away from the present-day shoreline. The shear zones compartmentalise the margin laterally, but they exert little influence on the structural style of the margin because they are highly oblique to it. By analogy, much of the localised, fault-related deformation observed within North Africa may have been accommodated along strike-slip faults oriented at a high angle to the cratonic margin. For example, significant thickening of Silurian strata in a north-striking flower structure in the Ahnet Basin (Figure 27) points to the presence of Silurian graben in western Algeria which were inverted during the Hercynian Orogeny.

Other structures in eastern Algeria, such as the east-west trending Ahara Arch, separating the Ghadames and Illizi basins, were also continuously active during the Silurian and affected local deposition (Elruemi, 2004).

3.3 Devonian Deposition and Late Silurian - Early Devonian (c. 418 Ma to c. 398 Ma) compression in Central North Africa

The uppermost Silurian and Devonian successions in North Africa are interrupted by several erosional unconformities which vary in importance across the region. The most significant of these unconformities are associated with base Devonian (“Caledonian”) and the mid-or intra-Eifelian events.

A major eustatic sea-level fall occurred during the latest Silurian/Early Devonian, resulting in a change to a shallow marine to continental facies in eastern and central North Africa. Coastal sand bar, tidal and fluvial deposits form important hydrocarbon reservoir horizons, e.g. in the Algerian Illizi Basin (Unit F6) and the Ghadames Basin (Tadrart Fm.) in north-west Libya. In the distal part of the North African shelf, towards Morocco, fully marine conditions still prevailed.

The Early Devonian of Morocco is well known for its rich trilobite horizons. Sea-level rose during the later part of the Early Devonian, leading to deposition of shelfal shales and sandstones in central North Africa. In Algeria, significant hydrocarbon reservoirs exist in sandstones of the Emsian (Units F4, F5). In western Algeria the base of the Emsian lies below a limestone bed termed the “Muraille de Chine” (“Chinese Wall”), because it commonly forms a characteristic, long ridge at outcrop. Deposition during the Early Devonian took place in a dominantly transgressive setting characterised by the accumulation of regionally extensive, coarse, dominantly fluvial sandstones in the Gedinnian – Siegenian (Tadrat Formation and equivalents) followed by finer grained, shallow marine to paralic sediments in the Emsian and Eifelian – Givetian (Ouan Kasa Formation and equivalents). A widespread top Emsian unconformity has been identified in Algeria (Boudjema 1987; Ford & Muller 1995), in the Libyan Ghadames and Kufra basins (Bellini & Massa 1980; El-Rweimi 1991; Abdesselam Rouighi, 1991; Echikhi, 1992). Emsian basaltic volcanism and intrusive activity occurred in the Ahnet Basin of Algeria (Belka 1997; Wendt et al. 1997) while regional thickness changes across features such as the Tihemboka Arch (Adamson, 1999; Figure 29) indicate that localised tectonic activity at this time enhanced regional unconformities across the crests of some structures.

Morocco and western Algeria were dominated by carbonate sedimentation during Middle Devonian times due to their distal position on the shelf and the consequent limited siliciclastic dilution. The facies in these areas includes prominent mud mounds, noteably in the southern Moroccan area of Erfoud and in the central Algerian Azel Matti area. Further to the east the sediments becomes more siliciclastic. Eifelian-Givetian tidal bar sandstones form the main reservoir (Unit FE) in the Alrar gas-condensate field in the eastern Illizi Basin.

The beginning of the Late Devonian was characterised by a major eustatic sea-level rise which resulted in deposition of hemipelagic shales, marls and limestones over wide areas of North Africa. The Moroccan Middle to Late Devonian typically contains rich cephalopods faunas (goniatites, clymenids).

Parts of western Libya and eastern Algeria were affected by significant periods of Middle and Late Devonian tectonic activity and erosion. The major tectonic pulse was pre-Frasnian in age (Echikh and Sola, 2000). Frasnian sediments unconformably overlie sequences ranging from Middle Devonian to Silurian in age across much of the Illizi Basin and the northwest part of the Murzuq Basin (eg. Collomb, 1962; Massa, 1988), and the associated unconformity is locally important in defining hydrocarbon trap configuration.

The “Frasne Event”, an important goniatite extinction event and a phase of anoxia, occurred during the early Frasnian and led to deposition of organic-rich shales and limestones in various places across North Africa (Lüning et al., 2003b). In the Algerian, Tunisian and the western Libyan portions of the Ghadames Basin, Frasnian black shales contain up to 16% organic carbon, are highly radioactive and represent an important hydrocarbon source rock (Daniels and Emme. 1995). Further east in the Libyan Ghadames Basin, this rich Frasnian “Argile Radioactive” source rock facies appears to thin, at least locally, and pass laterally into a mixed limestone-shale facies (the Late Devonian “Cues Limestone”) with lower total organic carbon content (2-4%) and consequently with somewhat reduced source potential (Hrouda et al., 2002; Dardour et al., 2004). The age-equivalent organic-rich unit also occurs in south Morocco and north-west Egypt. In parts of north-west Africa a second organically enriched horizon exists around the Frasnian-Famennian boundary, associated with the worldwide Kellwasser biotic crisis. The deposits in southern Morocco include black limestones.

A major fall in sea level occurred during the latest Devonian, triggering progradation of a Strunian (latest Devonian-earliest Carboniferous) delta in central North Africa. These clastics form an important hydrocarbon reservoir unit (F2) in Algeria and western Libya.

Compression in central North Africa (western Libya, eastern Algeria) around Late Silurian to early Devonian times is characterised by large-scale uplift in the Murzuq Basin, on the Tihemboka and Gargaf arches (Figure 28) and around the southern and south-western flanks of the Ghadames Basin, with erosion below a widespread regional Late Silurian unconformity extending in places as deep as Cambro-Ordovician levels (Dardour et al., 2004). Upper Silurian strata are progressively truncated from north-east to south-west below the Late Silurian unconformity across the southern flank of the Ghadames Basin (Echikh, 1998), such that the Lower Devonian Tadrart Formation rests directly and unconformably on the basal part of the Upper Silurian Acacus Formation in the Illizi Basin and across the Tihemboka Arch (Adamson, 1999, Figure 29). Gentle folding of Silurian strata below flat-lying Devonian sequences is common in the Illizi Basin (Sonatrach-Beicip 1975; Attar, 1987; Boudjema, 1987) and locally along the Al Kabir trend on the south-eastern flank of the Ghadames Basin in north-west Libya where low amplitude anticlines in Silurian strata overlain unconformably by flat-lying Devonian strata have been interpreted on seismic data (Echikh, 1992, 1998). Seismic, well and field data all indicate that Palaeozoic tectonic events also played an important role in the development of hydrocarbon traps in the Murzuq Basin in southwest Libya (Figure 30). Abrupt changes in thickness of both the Cambro-Ordovician and the Late Devonian to Carboniferous sequences occur across many of the steeply-dipping faults that define the main structural traps and there is widespread evidence of subtle compressional/transpressional movements along the same fault systems in the intervening Silurian and Devonian periods.

A very clear “Caledonian Unconformity” occurs on the eastern flank of the Murzuq Basin, where the Silurian sequence disappears and then re-appears beneath the unconformity surface from north to south along the outcrop.

The origin of Late Silurian to Early Devonian intra-plate stress in North Africa is currently unclear but is possibly associated either with a phase of rifting along the Gondwana margin (Boote et al. 1998) or with initial closure of the Iapetus Ocean (Fekirine and Abdallah, 1998). Localised fault activity may reflect compression promoted by the onset of closure of the proto-Tethyan Ocean between Gondwana and the northern continents of Laurentia and Baltica. However, until a comprehensive Early Palaeozoic plate tectonic framework is available for the northern margin of Gondwana, it is extremely difficult to constrain the origin of the stresses responsible for this phase of deformation. From the Late Palaeozoic onwards, plate margin activity was confined to a zone parallel with the northern margin of the African Plate and the present day North African coastline. Deformation along this east-north-east trending zone, which has been variously referred to as the Atlas Palaeozoic Transform, the North Gondwana Mobile Zone (Pique et al. 1993), the Lawrence – Azores Shear Zone (Arthaud & Matte 1977) and the North African Megashear System (Guiraud et al.1987), can be traced back to middle and late Devonian times and records the interplay of transtensional and transpressional tectonics. Many authors refer to the Late Silurian – Early Devonian compressional phase in North Africa as the ‘Caledonian Orogeny’. However, the term ‘Caledonian’ clearly relates to collisions involving the continents of Laurentia, Baltica, Armorica and Avalonia during the late Silurian times. Gondwana was located thousands of kilometres to the south at this time and was separated from the collisional zone by a major ocean (Stampfi and Borel, 2000). Tectonic events in North Africa during post-Infracambrian – pre-Hercynian times were independent of those in the collision zone to the north and the term ‘Caledonian Orogeny’ is inappropriate in North Africa (Jackson, 1997). Time-descriptive terms may be preferred instead.

3.4 Middle/Late Devonian transtension and transpression in NW Africa (c. 398 Ma to 359 Ma)

The collision between Gondwana and Laurasia that ultimately produced the “Hercynian Orogeny” first affected North Africa during the mid Devonian (Figure 31). Mid to late Devonian extension/transtension and possible crustal thinning along the North Gondwana Mobile Zone created a series of turbiditic pull-apart basins in western Morocco (Harris et al. 1991; Pique et al. 1993).

Middle Devonian deformation along the North African Megashear System (NAMS) in Morocco was characterised by the sinistral opening of extensional turbiditic basins under a N70°E oriented compression (Pique et al. 1993). Devonian collisional deformation along the North African Megashear System was coeval with semi-regional uplift of the Ghadames and Illizi basins and of the adjacent Tihemboka, Ahara, Gargaf and Brak-Bin Ghanimah arches in the mid-Eifelian and at the end of the mid-Devonian (Late Givetian) and with the related development of the 'Frasnian' Unconformity (Boudjema, 1987). An intra-Eifelian unconformity is widely developed in the Murzuq Basin in SW Libya and also, although more subdued, in the adjacent Libyan portion of the Ghadames Basin (Dardour et al., 2004) where it is recognised as a third-order sequence boundary separating a composite Lochkovian (Gedinnian) to lower Eifelian sequence (lower and lowermost Middle Devonian) from the overlying Middle and Upper Devonian. Local erosion of Lower and Middle Devonian strata prior to the deposition of the transgressive Frasnian radioactive shales is most apparent across the Ahara and Tihemboka arches, where the Frasnian shales locally rest unconformably on the Upper Silurian Acacus (F6) Formation, and across the Gargaf Arch. Klitzsch (1981), Karasek (1981) and Pierobon (1991) inferred that the Brak-Bin Ghanimah Arch was reactivated during the Middle to Late Devonian although they make no inferences regarding the causes of the uplift. It is possible that the Brak - Bin Ghanimah Arch could have been rejuvenated under dextral transpressional regime at this time in response to the compression. Localised Frasnian and Base Tournasian unconformities across the Tihemboka Arch may reflect renewed activity along this structure. The sediments deposited in the mid-Devonian pull-apart basins in western Morocco were affected by a Frasnian aged (367 Ma) low-grade, tectono-metamorphic 'Eovariscan' event (Pique et al. 1993) as a result of oblique shortening which led to coeval transtensional opening of Late Devonian-Dinantian basins in Central and Western Morocco.

3.5 “Pre-Hercynian” Carboniferous deformation in central North Africa (c. 359 Ma to c. 305 Ma)

The generally stable platform sedimentation that characterised much of the North African region during the Late Devonian continued into the Carboniferous with widespread deposition of fluvially-dominated deltaic sediments in most areas. Intra-Carboniferous deformation has been identified within the Murzuq Basin and the Kufra Basin in southeast and southwest Libya respectively, but not, as yet, to any great degree in the Algerian basins. Biostratigraphic data indicate that Early Tournaisian strata are absent over most of the area of the Ghadames and Illizi basins (Echikh, 1998). Seismic reflection data show that the Carboniferous succession is thinned, probably because of erosion of forced 'drape' folds above reactivated Lower Palaeozoic fault blocks in the Murzuq Basin. Tournaisian - Lower Visean sediments have also been partially eroded from the crests of local structures in the Tihemboka Arch, immediately to the west (Boudjema, 1987). Pique et al. (1993) inferred that the North African Megashear System was the site of transtensional basins formed in response to 'Eovariscan' north-west to south-east oblique shortening throughout the Carboniferous. Opening of these transtensional basins continued until the Namurian when oblique collision reverted to a more orthogonal, craton-margin parallel compression. It is proposed that the across-fault thickness changes of the Carboniferous sequences identified in the Murzuq Basin of western Libya (Figure 30) are caused by sinistral strike-slip reactivation of pre-existing north and north north-west-striking fault systems in response to north-east - south-west directed 'Eovariscan' shortening during the early stages of the collision between Gondwana and Laurasia.

Sea-level rise during the Early Carboniferous resulted in the development of a widespread shallow marine to deltaic facies across large parts of North Africa. A carbonate platform was established in the Bechar Basin in western Algeria during this time. Early Carboniferous dolomites of the Um Bogma Formation in south-west Sinai host important manganese and iron ores. Non-deposition and continental sandstone sedimentation occurred in southern and elevated areas, noteably across most of Egypt.

A prominent unconformity of probable “mid-Carboniferous” age has been identified on seismic data in the Kufra Basin in south-east Libya (the “Early Hercynian Unconformity” of Herzog et al., 2004). This strong unconformity cuts “down-section” progressively from north to south and strips the Silurian succession and locally also much of the Cambro-Ordovician succession from the south-east part of the basin.

By Late Carboniferous times, deposition of marine siliciclastics was restricted to north-west Africa and the northernmost parts of north-east Africa, e.g. Cyrenaica and the Gulf of Suez area. Paralic coals in the Westphalian of the Jerada Basin (north-east Morocco) and in the Bechar basin (Algeria) form the only sizeable Late Carboniferous coal deposits in North Africa. In the course of the latest Carboniferous Hercynian folding and thrusting most of north-west Africa was uplifted, and there was an associated change to a fully continental environment. Only Tunisia, north-west Libya and the Sinai Peninsula were still under marine influence at this time.

4. Late Carboniferous-Early Permian (“Hercynian”) Orogeny (c. 305 Ma to c. 270 Ma)

Palaeozoic sedimentation in many areas of North Africa was ended by a major phase of uplift and erosion related to Late Carboniferous to Early Permian, “Hercynian” compression associated with the closure of the proto-Tethyan Ocean along the line of the Lawrence -Azores Shear Zone (Figure 32) and the eventual collision between Africa and Laurasia (Figure 33).

The collision involved a large oblique component of movement related to the clockwise rotation of Gondwana relative to northern Europe which produced a deformation front that progressed from north (Early Carboniferous in Eastern Laurentia) to south (Late Carboniferous in the Appalachians). The compression produced uplift and thrusting in north-west Africa (Morocco, western Algeria) and folding and inversion in adjacent, foreland intraplate areas (Figure 34). Deformation was concentrated over the north-west coast of Morocco (Figure 35) and Mauritania and especially along the North African Megashear System. Arthaud & Matte (1977) proposed that the North African Megashear System formed part of a much larger dextral shear zone extending from the Appalachians to the Urals, along which a number of small terranes were transpressionally deformed during the Hercynian Orogeny (Stampfi and Borel, 2002; Von Raumer et al., 2002).

Local late-orogenic, intramontane basins developed within the Hercynian Orogen during late Carboniferous to Permian time. These basins filled with a variety of alluvial fan, fluvial and lacustrine sediments, before further deformation and uplift eventually exposed both the basin fills and the surrounding Hercynian foldbelts to extensive erosion during the Early and Mid-Triassic.

Ultimately, t

he “Hercynian” deformation caused significant uplift and erosion across much of North Africa including inversion of the Early Palaeozoic Saoura-Ougarta rift in eastern Algeria and upwarping of the Reguibat Arch (Ziegler et al. 1998). Most of the major structural arches in central North Africa were active at this time, including the E-W oriented Telemzane Arch/Dahar Dome and Charnian Uplift along the northern edge of the Ghadames Basin, the east-west to west south-west – east north-east trending Qargaf Arch separating the Ghadames and Murzuq basins in eastern Libya and the north-south to north north-east - south south-west oriented Amguid - El Biod Arch and Hassi Messaoud Ridge separating the Illizi-Ghadames and Oued Mya basins in western Algeria. Many east-west trending structures seem to have been particularly active at this time, and locally often overprint earlier Palaeozoic structural trends. This is particularly noteable, for example, in the Kufra Basin of southeast Libya, where a series of prominent east-west trending troughs developed to the south of the “Uweinat Block” during the Late Carboniferous and Permian (Klitzsch, 2004). Late Carboniferous and Early Permian arching resulted in the removal of most of the Palaeozoic sequence in the area of the later Sirte Basin and affected the Tibesti High, the Kufra Basin and extended northwestwards to the Naffusa High and into Tunisia (Jassim et al., 2002). Transpressional reactivation of the Pan-African 4°30'E shear zone in Algeria along the Amguid - El-Biod Arch was associated with sinistral transpression that produced en-echelon folds along the trace of the underlying Pan-African shear zones (Ramsay and Huber 1987). Structural kinematic analyses and seismic interpretation undertaken in the Murzuq Basin of Libya indicate a similar Hercynian transpressional origin for the structure of the giant 'Elephant' oil field (Figure 36) (Glover 1999).

The intensity of “Hercynian” deformation decreases eastwards across North Africa away from the collision zone (Figure 34) such that significant folding and erosion of anticlinal crests in the Algerian Sbaa, Ahnet, Reggane, Oued Mya and Mouydir basins (Figure 37) is replaced by subtle, low angle unconformities to disconformities in the Murzuq Basin in south-west Libya (Figure 36), on the Cyrenaica Platform in north-east Libya and locally, in parts of the Kufra Basin in south-east Libya (Figure 38). The Trans-Saharan Fault Zone forms an important strain-partition boundary, separating fold and thrust belt deformation typical of orogenic forelands to the west, from areas of gentle folding and warping to the east (Grabowski et al, 2002). The superimposition of broad Hercynian folds on early Palaeozoic structural axes contributed to the development of regional interbasinal arches including the Qarqaf, Amguid - El Biod and Talemzane Arches, all of which were eroded down to the Cambrian in places (Figure 2). In the Ghadames/Berkine Basin, the Hercynian unconformity cuts strongly across various Palaeozoic stratigraphic units (Figure 39), reaching as deep as the Cambro-Ordovician, and locally to the original Precambrian basement, along the axis of the Hassi-Messaoud Ridge. Further west in the Kufra Basin of south-west Libya the same unconformity truncates the Silurian succession across a prominent north-east–south-west trending and south-west plunging uplifted arch in the north-west of the basin.

The ‘Hercynian’ unconformity is strongly diachronous across North Africa. In Algeria and westernmost Libya, the main unconformity is Early Permian (Sakmarian) in age and overlies Autunian-Stephanian clastic sediments. In eastern Libya and Egypt there are two discrete unconformities, one, Early Permian and the other, Late Carboniferous in age (Grabowski et al., 2002) while further east in Arabia, Westphalian to Artinskian clastic sediments unconformably overlie older Carboniferous and pre-Carboniferous strata. Palaeozoic facies patterns in northern Egypt illustrate a history of progressive uplift that began in the Ordovician and culminated in the Late Carboniferous. It is likely that this uplift, rather than far-field Hercynian compression, is primarily responsible for the development of unconformities and disconformities in ‘Hercynian’ eastern North Africa. The uplift may be associated with the progressive development of long­-lived alkaline magmatic provinces in eastern Egypt and northern Sudan and/or with an early abortive attempt at rifting in the same area. (Grabowski et al., op.cit.).

The reservoir properties of some of the more deeply buried Cambrian and Ordovician sandstones have been locally enhanced by weathering and fracturing during the uplift and erosion of these axes during the Hercynian orogeny.

The present-day maturity levels of the main Palaeozoic hydrocarbon source rocks decrease eastwards across North Africa in parallel with the decrease in the intensity of the Hercynian deformation (Macgregor 1996). Silurian source rocks in Morocco and western Algeria are commonly overmature and typically generated hydrocarbons prior to Hercynian structuration (Macgregor 1995; Makhous et al. 1997), while the same source rocks in the Kufra Basin in southeast Libya are only marginally mature or, possibly, have never entered the oil window. The concentration of large oil and gas fields in Algeria may be partly a function of location with respect to this regional variation in the intensity of Hercynian deformation which resulted in a favourable timing of maturation and trap formation in this area. The Ahnet Basin in central Algeria appears to lie at the western limit of this favourable Hercynian ‘window’. Here, oil generated in pre-Hercynian times was lost because there were few suitable pre-Hercynian traps, but a dry gas charge contemporaneous with early Hercynian inversion, or remigrated pre-Hercynian gas from temporary ‘holding-tanks’, has allowed trapping of large gas reserves in dominantly Hercynian-aged structures. New fission track analyses also indicate that pre-Hercynian hydrocarbon generation and migration could have occurred in eastern Algeria and western Libya between the more deeply buried Ghadames Basin in the north and the Murzuq Basin in the south, prior to uplift of the Qarqaf Arch (Glover, 1999) and between the Murzuq Basin and the Illizi Basin to the west, prior to the uplift of the Tihemboka Arch. Maturity levels in the southern part of the Ghadames Basin and the north-west Murzuq Basin are abnormally high for their present-day depth of burial. The fission track data suggest that the Qargaf Arch may have been uplifted by as much as 2-3km during the Hercynian Orogeny (Glover et al.,1999). Graviational collapse of the Hercynian Orogenic Belt and associated widespread Permo-Carboniferous volcanism in north-west Africa have also affected the Palaeozoic petroleum systems in North Africa. The volcanism may have been associated with a superplume impinging on the base of the lithosphere (Doblas et al., 1998) with the magmatism acting as an 'exhaust valve' releasing the heat accumulated beneath the Pangaean supercontinent by insulation and blanketing processes which triggered large-scale mantle-wide upward convection and general instability of the supercontinent. The resulting transient high heatflow may have played an important role in the local maturation history of the Palaeozoic source rocks in NW Africa.

5. Mesozoic extension (c. 270 Ma to c. 130 Ma)

The tectonic evolution of the Palaeozoic sequences in North Africa during the Mesozoic was dominated by the development of a northward thickening and backstepping wedge of Triassic, Jurassic and Cretaceous sediments associated with the opening of the Central Atlantic and Tethys in the area of the present-day Atlas Mountains. Rifting began in the Late Permian although evidence for sediments of this age is generally limited to the north and west of the region. However, the opening of the Central Atlantic in Triassic-Liassic times (e.g. Oyarzun et al. 1997) and the contemporaneous separation of the Turkish-Apulian terrane from north-east Africa (Figure 39) led to a significant extensional phase across much of North Africa (Gealey 1988) including activity in the Atlas region (Guiraud et al. 1987, Lowell 1995), rifting from Palestine to Cyrenaica (Sestini 1984, Cohen et al. 1990, Taha, 1992; Robertson et al. 1996; Keeley and Massoud 1998) and extension in offshore Libya (Del Ben and Finetti 1991) and in the Oued Mya and Berkine basins in central and eastern Algeria (Figure 40). These rifts were linked to the collapsing Hercynian-Appalachian belt and subsequent oceanic spreading centre in the Central Atlantic to the west and north-west by west north-west - east south-east trending transfer zones that locally offset the graben margin. In Morocco and northern Algeria, the Missour Basin and the High and Middle Atlas developed as rift basins during the Mesozoic, while the north-west – south-east trending Sirt Basin in northern Libya exhibits at least three cycles of Mesozoic extension, one each in the Triassic, the Early Cretaceous and the Late Cretaceous with the main basin-forming phase occurring in the Early Cretaceous. Extension in most of these Mesozoic rift basins began at the end of the Permian, continued through the Triassic and then accelerated during the Jurassic in parallel with the opening of the western Tethys Ocean and the North Atlantic. The break-up of Pangea and opening of the central Atlantic Ocean during the Early Triassic resulted in rifting and volcanic activity throughout the Saharan Platform.

Sinistral strike-slip movements between the Cobequid-Chedabucto-Gibraltar fracture zone in the north and the Bahamas fracture zone in the south during the Late Triassic and Early Jurassic led to the development of north north-east to north-east elongated rift basins on both sides of the Atlantic rifted zone (Manspeizer, 1988), while almost simultaneously, the westward propagation of the Tethys sea floor spreading led to the development of another system of north-east – south-west to east-west trending basins. The Doukkala, Essaouira, Souss, Ifni and Tarfaya-Layoune basins located along the western Atlantic coast of Morocco are considered to belong to the Atlantic rift system, while both the Atlas rift system (Atlas, Inter-Atlas Rif and Pre Rif basins) in Morocco and northern Algeria and the Jabel Akhdar - Western Desert rift system in northern Libya and north-western Egypt probably represent “failed” attempts at oceanic spreading associated with the eventual opening of the Tethys Ocean (Figure 40).

Most of North Africa remained subaerially exposed from Permian to mid Triassic times. Continental red clastic sediments (sandstones, shales, conglomerates) represent the most important lithologies. The Permian of Morocco is largely restricted to a series of intramontane basins located around the margin of the central Moroccan Hercynian massif.

Following Hercynian uplift, the east-west trending Qargaf Arch separating the Ghadames and Murzuq basins in Libya became a barrier to marine transgressions from the north. Consequently, deposition in the Murzuq Basin to the south of the arch was entirely continental in character from Permian to Early Cretaceous times. Thick sequences of Mesozoic sediments were deposited both to the north and the south of the Qargaf Arch in broadly east-west trending depocentres (Klitzsch and Ziegert, 2000) which overprint the earlier “pre-Hercynian” north north-west – south south-east structural trends.

The main facies in the Triassic TAGI (Trias Argilo-Gréseux Inférieur) in the eastern Algerian Berkine (Ghadames equivalent) Basin to the north-west of the Qargaf Arch are fluvial channel sandstones, floodplain silts and palaeosols, crevasse splay deposits, lacustrine sediments and shallow marine transgressive deposits (Figure 41). Fluvial sandstones of the TAGI are the main oil and gas reservoirs in the Algerian Berkine and Oued Mya basins, including the giant gas field of Hassi’R Mel. Similar Triassic sandstones also serve as hydrocarbon reservoir in the Sirt Basin, sourced stratigraphically downwards from Cretaceous source rocks.

Marine Triassic sediments are restricted to the northernmost margin of central and eastern North Africa. During the Late Triassic to Early Jurassic, evaporites were deposited in rift graben of the Atlas Gulf associated with the opening of the Atlantic, and the separation of the Turkish-Apulian terrane from North Africa. Characteristic “salt provinces” are located offshore along the Moroccan Atlantic coast, through northern Algeria/Tunisia and offshore eastern Tunisia/north-west Libya. In most of these areas, halokinesis commenced in the Jurassic (locally in the Early Jurassic) or Cretaceous and was reactivated and enhanced during subsequent Late Cretaceous and Tertiary compressional events.

The Late Triassic/Early Jurassic evaporites and shales in the north-east part of the Algerian Saharan Platform are up to 2 km thick and form a hydrocarbon cap rock for the Triassic reservoirs. In some cases, through the Hercynian unconformity, they also form the top seal for Palaeozoic reservoirs.

The Triassic succession in these basins begins with a basal sequence of non-marine conglomerates, sandstones and sandy mudstones, overlain by gypsiferous mudstones with basalt flows. This sequence forms a thin veneer over the eroded Hercynian basement, frequently infilling topographic lows on the Hercynian unconformity surface and locally absent over the crests of major palaeostructures such as the Hassi Messaoud and Rhourde el Baguel fields in eastern Algeria (Mitra and Leslie, 2003; Figure 41). This suggests that the latter features remained relatively high during the Triassic period (Boudjema, 1987). Younger, 1.0 to 1.5 km thick sequences of reddish brown paralic facies conglomerates, sandstones and overlying mudstones infill east north-east and north-east-trending troughs and basins. The clastic sequences grade laterally into thick salt deposits toward the western and northern continental margins (Van Houten, 1977). The salt is particularly well developed in the Rharb, Saiss, Khemisset and Boufekane basins and in the Essaouira Basin and its continuation offshore. The upper part of the red-bed evaporite sequence is overlain by, or intercalated with, basaltic lava flows up to 500m thick (Figure 41). These have been radiometrically dated at 180-200 Ma, suggesting an early Liassic age (Van Houten, 1977).

With the onset of rifting during the Middle and Late Triassic the former Hercynian foldbelt in Morocco, now extensively peneplained, was affected by sets of north-northeast to south south-west trending normal faults and east-west to north-east – south-west trending strike-slip faults as a consequence of left-lateral displacements along the Gibraltar fracture zone and the South Atlas fault zone (Figure 40). The two sets of faults were reactivated several times during the deposition of the Triassic syn-rift sequences. In north-west Morocco, four large basins, the Doukkala, Abda-Safi, Essaouira and Haha-Souss (western High Atlas) basins, developed at this time. In each basin, listric normal faults, facing either east or west, bound half-graben sub-basins on either side of a central horst (Figure 42). Many of the active faults in the High Atlas region at this time probably reactivated earlier Hercynian structures and the listric normal faults bounding the main Triassic sub-basins probably sole-out downwards in former low angle Hercynian thrust faults. The Atlasic and Inter-Atlasic basins developed along a major east-northeast trending crustal discontinuity that extends along the High Atlas trend from the Moroccan Atlantic coast in the west to Tunisia in the east. This major zone of crustal weakness developed during the Hercynian Orogeny at the boundary between the Hercynian thrust belt in the north and the Anti-Atlas foldbelt and the Saharan Platform to the south. The Hercynian fold belt forming the Anti-Atlas to the south remained as a prominent structural high throughout the Mesozoic and was the provenance of much of the detrital sediment that filled the adjacent rifts.

Further south, in Algeria, the age-equivalent rift-controlled TAGI sandstones and overlying shales and evaporites are important reservoirs and seals, respectively, for hydrocarbons generated from Silurian and Devonian source rocks. The gas of the Meskala field in the Moroccan Essaouira Basin is trapped in a Late Triassic to Early Jurassic horst and was possibly sourced by Palaeozoic source rocks (Broughton and Trepanier 1993). The architecture, thickness and quality of the Triassic Lower and Middle TAGI sandstone reservoirs in fields such as Hassi Berkine South (HBNS) in the Berkine Basin of Algeria are strongly controlled by the accommodation space that developed as the peneplaned Hercynian surface began to subside preferentially over Palaeozoic thicks early in the Triassic (Pink et al., 1999). The reservoir horizons in these lower units typically onlap the topography (palaeo-valleys and fault-controlled lows) that developed on the Hercynian Unconformity surface (Tounsi, 1996; Turner, 2001), but the architecture of the Upper TAGI reservoirs is much more strongly influenced by north-east to south-west trending extensional faults that formed as Tethyian rifting became well established in late Early, Middle and Late Triassic times. One such major Triassic normal fault, for example, defines the western margin of the Rhourde el Baguel field in eastern Algeria (Mitra and Leslie, 2003) and many of the fields along the prolific “Berkine Trend” in the Ghadames/Berkine Basin (Figure 43) exhibit similar contemporaneous fault control. Active faults are also thought to have controlled the location and distribution of Triassic volcanics in the northern part of the Ghadames Basin, on the Hassi Messaoud Ridge and in the adjacent Oued Mya Basin to the west. The provenance area for most of the sands deposited in the northern Ghadames, Berkine and Oued Mya basins at this time was mainly to the south and southwest, most likely the uplifted Hercynian highs of the Amguid Spur and the Hassi Messaoud Ridge, although other Hercynian highs such as the Dahar Dome and the Chanan Uplift in the north may also have provided local sediment input to the Triassic fluvial system (Echikh, 1988). Extensional fault movements continued in many of the intra-plate areas during the deposition of the succeeding Late Triassic-Early Jurassic evaporite succession (including the massive Triassic S3 salts, the interbedded salt and anhydrite of S1 and S2, the Liassic salt, anhydrite and dolomite and the Dogger Lagunaire anhydrites) and in many areas (e.g. Berkine Basin; Rhourde el Baguel) there are significant variations in the thickness of the various evaporite units between upthrown and downthrown fault blocks.

Localised extension associated with the opening of Tethys continued during the Late Jurassic with the development of a series of north-east to east north-east trending rifts in the Western Desert of Egypt, including the Kattaniya Basin (Moustafa, 2004), some rifting in the adjacent Cyrenaica area of north-east Libya (El-Armauti, 2004) and further development of the Atlas rift system.

Another important Mesozoic extensional phase in North Africa occurred during the Early Cretaceous, related to “mantle plume” - driven rifting of the African Plate and to the formation of the Benue Trough and the opening of the South and Equatorial Atlantic Ocean. This resulted in the formation of complex failed rift systems across North and Central Africa (Maurin and Guiraud, 1993; Van Houten 1993) with the development of half-graben in north-west Egypt, in the Sirte Basin in Libya, and in Tunisia and eastern Algeria (Maurin and Guiraud 1993, Guiraud and Bellion 1995, El-Makhrouf 1996). The rifting in central Africa continued until the Central Atlantic entered the “drift” phase. Continued opening of Tethys during the Late Jurassic and Early Cretacoues also resulted in renewed north-south extension in North Cyrenaica with the deposition in Jabel Akhdar of a thick sequence of Lower Cretaceous sediments which thicken northwards offshore above an underlying Jurassic hinge-zone; the simultaneous development of the Sarir-Hamaemat – Abu Garadig graben system in north-east Libya and north-west Egypt between north-east – south-west to north north-east – south south-west trending bounding transfer faults (precursers of the so-called Levant and East Murzuq Shear Zones) and the development of a widespread transtensional regime across much of the remainder of North Africa. The Early Cretaceous extension reactivated pre­-existing basement structures and mylonitic wrench zones throughout North Africa (Maurin and Guiraud 1993) and disrupted the Palaeozoic succession (e.g. Figure 44), creating new hydrocarbon traps and further modifying source rock maturity. In the Sirte Basin, for example, much of the Palaeozoic succession has been eroded on the shoulders of graben formed by Mesozoic rifting but is better preserved along the graben axes (Figure 45). Local preservation of the lower Silurian Tanezzuft Formation is particularly important in this respect, as it may have sourced some of the petroleum occurrences in the Sirte Basin.

6. Mid Cretaceous (‘Austrian’) and Late Cretaceous-Tertiary (“Alpine”) Orogeny (c. 130 Ma to c. 23 Ma)

The ‘Alpine Orogeny’ was a consequence of the collision between the African and European plates. It resulted in the closure of Tethys, uplift of the Atlas Mountains and the development of an overall pulsed compressional regime across North Africa from mid Cretaceous to recent times (Figure 46). Changes in the collisional process, such as subduction of oceanic crust after accretion of a seamount, produced localised stress-neutral or even extensional pulses within the overall compressive regime (Mart 1994, Lüning et al. 1998, Bosworth 1999). The structural boundary between the Atlas Mountains and the Saharan Platform in Morocco, Algeria and Tunisia is defined by the South Atlas Fault. This fault, which extends as a continuous feature from Agadir to Tunis, separates a zone to the north where the Mesozoic and Tertiary cover is shortened and mostly detached from the basement, from a zone to the south where the cover is less deformed and remains attached to the basement. The Atlas fold and thrust belt to the north of the fault is structurally elevated by about 1.5 km above the Saharan Platform to the south of the fault as a result of the “Alpine” deformation (Figure 47).

The ‘Alpine Orogeny’ sensu strictu is restricted to Late Cretaceous and Tertiary orogenic activities, but a Barremian to Aptian compressional event (the “Austrian Orogeny”) which affected parts of North Africa may be considered as a precursor to the ‘Alpine Orogeny’ although it affected a greater area than the true ‘Alpine’ deformation, inverting Early Cretaceous rift systems and reactivating older structures as far south as Central Africa (Maurin and Guiraud 1993). North-south trending Pan-African faults were the most strongly reactivated lineaments during this phase, with the Hassi Touareg Horst, the Tihemboka Arch and the Hassi Messaoud ridge all undergoing significant uplift. Other, more minor, north-south and north-west to south-east trending structures, including those in the Murzuq Basin (Aziz, 1998; Beswetherick et al; 1998), also experienced transpression and/or growth at this time. This activity probably modified pre-existing hydrocarbon traps in Hercynian-aged structures and may have led to local redistribution of hydrocarbons in the Ghadames, Illizi and Murzuq basins (Echikh, 1988). The Aptian compressional phase may have been controlled by large-scale changes in the intracontinental stress field due to the combined effects of the Africa-Europe plate collision and the gradual opening of the Atlantic Ocean. Aptian-age anticlines occur in the Berkine/Ghadames Basin in eastern Algeria (Figure 48) and result from sinistral transpression along the north-south trending Transaharian fracture system (Maurin and Guiraud 1993). A similar mid-Cretaceous compressional event has also been interpreted in the Oued Mya / Mouydir Basin in Algeria (Figure 49) and in the Murzuq Basin in Libya. The broadly west north-west to east south-east directed Barremian to Aptian compression mainly resulted in the reactivation and folding of already uplifted fault blocks, particularly in parts of eastern Algeria (eg. the Rhourde el Baguel fault block) while associated erosion truncated the Barremian, Neocomian and parts of the Upper Jurassic sequences across the uplifted blocks below an intense, but only very locally developed, “Austrian Unconformity”.

The main (“Laramide”) phase of the Alpine compression in North Africa commenced in the Santonian (Figure 50) and culminated in the Early Palaeocene (Danian). Many of the late Triassic-early Jurassic graben were inverted during the final closure of western Tethys. The Atlas Mountains in Morocco (including the Rif) (Brede et al. 1992 Lowell 1995; Beauchamp et al., 1996, 1999; Bernini et al. 1999), western Algeria and Tunisia (El Euchi 1996, Mickus and Jallouli 1999), the ‘Syrian Arc’ Fold Belt in north-east Egypt (Lüning et al., 1998; Abdel Aal et al. (1992), Ayyad and Darwish (1996)) and north-west Arabia (e.g. Cohen et al. 1990, Chaimov et al., 1992), the Cyrenaica Platform in north-east Libya (Röhlich 1980) and parts of the Murzuq Basin in south-west Libya were all affected (Figure 51). A widespread Santonian erosional unconformity marks the end of the main phase of inversion in many areas, noteably in the Western Desert of Egypt (eg. Abu Garadig Basin) and in Cyrenaica. Tethyan terranes collided with Africa in the Early Eocene, marking the start of the inversion of the Atlas Mountains (Figure 52) that culminated in the Late Oligocene to Miocene, but elsewhere in North Africa the effects were comparatively minor. Sinistral Alpine transpressive reactivation of the basement shear zones appears to be responsible for many of the late stage fault-related anticlines that affect Palaeozoic and Mesozoic strata across much of North Africa.

The onset of North Atlantic rifting during the Late Cretaceous led to an abrupt change in the motion of the European Plate which began to move eastwards with respect to Africa. The earlier sinistral transtensional movements between Laurasia and Africa-Iberia were arrested and replaced by a prolonged phase of dextral transpression. Guiraud and Bosworth (1998) attributed Late Cretaceous deformation across North Africa to a rapid Santonian (85-83 Ma) event reflecting a change in the pole of rotation for the opening of the Atlantic. This phase is thought to be responsible for the inversion of the High and Middle Atlas mountains (Guiraud et al. 1987) and the formation of the Syrian Arc fold belt in northern Sinai, Egypt (Guiraud 1998, but see Lüning et al. 1998 where a Coniacian major compressional phase is described). Mild Santonian and strong Paleocene inversion of the north Cyrenaican passive continental margin, resulting in uplift of the Jebel Akhdar anticlinorium (Figure 53), can probably be related to the collision of the Hellenides-Rhodope orogenic wedge with the passive margin of the composite Pelagonian-Apulian-Taurus Platform, which at that time was still separated by an oceanic basin (Ziegler et al. 1998). Today, the Cyrenaican promontory is assumed to be already in the initial stages of collision with the European Plate (Macle et al. 1999).

Most Late Cretaceous movements were accommodated along the north Tethys margin (Savostin et al. 1986), although Guiraud et al. (1987) recognised dextral reactivation of east-north-east-striking faults within the North African Megashear System. Hammouda (1980) inferred dextral movement along east-north-east-striking faults in the Libyan Ghadames Basin and comparable kinematic data have been recorded from parallel fault systems on the Qarqaf Arch in Libya (Glover 1999 & Glover et al. in prep). Compressional deformation inverted earlier-formed north-east-striking Triassic-Liassic graben along the eastern margin of the Amguid-El Biod Arch in the western Berkine/Ghadames Basin. North-south-striking graben along the Amguid El-Biod Arch and north-west-striking faults in the Murzuq Basin of Libya underwent sinistral transpressional reactivation at this time (Glover 1999). The fault bounding the F-field (Elephant structure) in the northern Murzuq Basin has a significant component of Alpine movement, as indicated by folding of the youngest Lower Cretaceous reflectors (Figure 15). North-west to south-east to north-south convergence between the African and European plates slowed considerably at the end of the Cretaceous due to a decrease in spreading rates in the South Atlantic. Collision along the northern margin of Africa caused further dextral strike-slip reactivation of the North African Megashear System from Paleocene to Middle Eocene times. Dextral reactivation of the North African Megashear System promoted sinistral transtensional reactivation of north-west-striking, collision-parallel faults in the Sirte Basin and in the Termit Trough and Tenere Graben portions of the East Niger graben system (Kumati and Anketell 1982; Janssen et al. 1995; Zanguina et al. 1998).

By mid Eocene (Lutetian) times, the Alpine orogeny was well advanced, with shortening and inversion of many of the basins in North Africa. Erosional unconformities at the base of the Eocene (Ypresian) transgressive sequence and between the Ypresian and Lutetian sequences in Cyrenaica (El Hawat, 2004) and similar Eocene unconformities in the Termit Trough (Guiraud et al. 1987) document an important compressional event at this time that is recognisable throughout much of North Africa. The effects of this so-called ”Pyrenean” or “Cyrenaican” phase of activity during Early and Mid Eocene times associated with the progressive closure of western Tethys are recorded in the Berkine Basin of Algeria (Boudjema 1987, Boudjema & Tremolliers 1987), in northern Libya (Kumati 1981), the Atlas basins of Morocco and Algeria and the Benue Trough in West Africa by Guiraud et al. (1987) and Bellion & Guiraud (1988). Transpressional strike-slip movements during Early and Mid-Eocene times generated north-east – south-west trending folds in north-east Libya (El-Arnauti, 2004) while further compression in the Late Eocene led to renewed activity and uplift of the Cyrenaican “Syrian Arcs” and uplift, local subsidence and west-east and north north-west – south-east dextral movements in northern Tunisia associated with the development of a prominent base Oligocene unconformity. Another pulse of compression during the Miocene led to further inversion in north-east Libya and northern Tunisia, with strike slip reactivation of older east south-east – west north-west trend faults, the development of the Eastern Mediterranean Ridge (through aborted ophiolite obduction, time equivalent to the obduction of the Oman Ophiolite) and of an associated “fore-trench bulge” in the area of Jabal Akhtar and the local development of a strong base Pliocene Unconformity (Sengor, 2004). Some older ‘Austrian’ and Hercynian structures in the northern part of the Ghadames Basin were modified in this phase, including the prominent El Borma structure, the crest of which migrated northwards during the Miocene (Ghenima, 1995). Apatite fission track data suggest that large parts of Libya and Algeria were uplifted by 1-2km during the Alpine Orogeny (Glover 1999; Figure 54). This uplift has a significant effect on the maturation history of the Palaeozoic source rocks and associated migration pathways as large areas may have been uplifted out of the hydrocarbon generating window during the Tertiary. These findings have allowed a re­interpretation of the migration history paths in the Murzuq Basin, where long-distance migration models have traditionally been favoured over local sourcing models due to the shallow present-day depth of the Silurian organic-rich shales. The new model may also upgrade the Palaeozoic plays in the Kufra Basin where low thermal maturities of the Palaeozoic source rock candidates has previously been a major concern (see Lüning et al. 1999).

7. Neogene to Recent uplift and volcanic activity (c. 23 Ma to present-day)

The third and final rifting phase which affected the North African area led to the development of the East African Rift, Gulf of Aden and Red Sea Rift system during the Oligo-Miocene (e.g. Rosendahl et al. 1992). As these movements are relatively less important for the Palaeozoic-sourced hydrocarbon plays in North Africa, a detailed description of this tectonic phase is not given here. The extension occurred under north-west to south-east compression, but the collision of the Indian and Eurasian plates caused an abrupt change in the direction of approach of African and Eurasian plates which imposed a westerly-directed push on the northern tip of the Arabian Plate (Savostin et al. 1986). As the southern end of the plate was free from compression, collision promoted the rotation of the Arabian Plate and extension along the Red Sea - Gulf of Aden Rift and associated east north-east – west south-west extension of north-west-trending fault systems across much of eastern North Africa (e.g. Figures 55 and 56).

One important aspect of the Late Tertiary tectonic evolution of North Africa that does have a significant impact on Palaeozoic-sourced hydrocarbon plays is the uplift of the Hoggar Massif. This uplift is estimated to have removed up to 4000-5000m of overburden from the region to the south of the Illizi and Mouydir basins and to the west of the Murzuq Basin, an area now occupied by basement outcrops of the Hoggar Massif. Vitrinite reflectance data show that these areas formed palaeo-depocentres where hydrocarbons may have been generated prior to the Tertiary uplift. The uplift and erosion destroyed these palaeo-kitchens, tilted existing traps in the basins flanking the Hoggar Massif and led to meteoric flushing and the development of a locally steep hydrodynamic gradient across the southern Illizi Basin and southern and western parts of the Murzuq Basin. The southern part of the Illizi Basin, in particular, has been flushed of all hydrocarbons as a result of the uplift through a combination of the high hydrodynamic gradient (resulting from higher rainfall during the Tertiary and early part of the Quaternary, uplifted recharge areas, and the nature of the Palaeozoic reservoir) and the low relief of the structural traps in this area. Further north in the Illizi Basin, northward-tilted oil-water contracts exist in fields such as Tin Fouyé that are located along northward-trending structural noses.

Intense volcanic activity accompanied rifting in central and eastern North Africa from the Late Miocene to the Late Quaternary, and in places this commenced as early as the Late Eocene. Volcanic

features associated with this phase of activity include the plateau basalts in northern Libya, the volcanic field of Jebel Haruj in Central Libya, the Tibesti volcanoes in southeast Libya and northeast Chad, and volcanism in the Hoggar (south Algeria, northeast Mali and northwest Niger). Most workers interpret this continental volcanism as a hot spot product related to a deep-seated mantle plume, but some invoke intraplace stress originating from the Africa-Europe collision leading to melting at the lithosphere/asthenosphere interface by adiabatic pressure release. This volcanic activity, together with other earlier phases that occurred during the Mesozoic, had significant local effects on the thermal maturity of main source rock intervals and hence on the hydrocarbon prospectivity of the Palaeozoic successions across North Africa.

The present-day stress field over much of onshore North Africa is dominated by E-W compression associated with “ridge-push” from the Atlantic and Indian Oceans (the “Central Africa Intraplace Stress Field”). Offshore, the present-day stress field is dominated by N-S compression (the “Mediterranean Convergence Stress Field”) and there is a “transition zone” between these competing two stress fields passing through present-day northern Libya, northern Tunisia and northern Algeria (Bosworth, 2004).

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