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Palaeozoic Hydrocarbon Systems
of North Africa
CRAIG, J.1; RIZZI, C.1; SAID, F.2, THUSU, B.3; LUNING, S.4; ASBALI, A. I.5, KEELEY, M.L.3; BELL, J. F.6, DURHAM, M.J.6, EALES, M.H.6, BESWETHERICK, S.6 AND HAMBLETT, C.6
The Late Precambrian to Phanerozoic structural development of North Africa is characterised by at least seven major tectonic phases: the Pan-African Orogeny, Infracambrian extension, Cambrian to Carboniferous alternating extension and compression, mainly Late Carboniferous ‘Hercynian’ intraplate uplift, Late Triassic-Early Jurassic and Early Cretaceous rifting, Late Cretaceous - Tertiary ‘Alpine’ compression and Oligo-Miocene rifting. Parts of north-west North Africa were strongly affected by plate collisions during Hercynian and Alpine times, but much of the region was subject to only subtle intraplate processes involving repeated transpressional and transtensional reactivation of major fault systems that often follow Proterozoic and older zones of weakness.
The continental crust of North Africa initially developed through oblique collision between the West African Craton, the East African Craton and numerous island arcs during the Pan-African orogeny (700-600 Ma). This was followed by major extensional movements in “Infracambrian” times (Late Neoproterozoic - Early Cambrian, 640-520 Ma) with structures interpreted as forming through shearing along the Trans-African lineament, as pull-apart basins along the eastward continuation of the Arabian Najd fault system and/or as half-graben associated with the extensional collapse of the Pan-African orogen.
The post-Infracambrian, pre-Hercynian structural evolution was complex and heterogeneous. Local transpressional and transtensional reactivation processes dominated, resulting from the complex interaction of intra-plate stress fields and the geometry of pre-existing fault systems.
The Late Carboniferous continental collision between Gondwana and Laurasia led to uplift and thrusting in north-west North Africa and folding and inversion in neighbouring intra-plate areas. The intensity of deformation decreased eastwards, with Hercynian folding and erosion in the north-west replaced by subtle, low-angle unconformities and disconformities further south and east.
The opening of the Central Atlantic in Triassic to Liassic times and contemporaneous separation of the Turkish-Apulian terrane from north-east Africa led to significant extension and associated volcanism in North Africa. A second important Mesozoic extensional phase occurred during the Early Cretaceous, related to the opening of the South and Equatorial Atlantic Ocean. This resulted in the development of a complex suite of failed rift systems across North and Central Africa.
The onset of rifting in the northern North Atlantic during the Late Cretaceous led to an abrupt change in the motion of the European Plate. The earlier sinistral transtensional movements were replaced by a prolonged phase of dextral transpression, resulting in the collision between the African and European plates and an overall compressional regime in North Africa from mid Cretaceous times. This “Alpine compression” caused further folding and thrusting in north-west North Africa and intra-plate inversion and uplift of Late Triassic-Early Jurassic graben elsewhere.
The final major rifting phase to affect North Africa occurred during Oligo-Miocene times and was associated with the development of the Red Sea, Gulf of Suez and Gulf of Aqaba rift systems.
Overall, this polyphase evolution produced a complex system of interconnected and superimposed Precambrian and Palaeozoic basins separated by long-lived, stable palaeohighs. The petroleum potential of these basins varies as a function of their differing tectonic, sedimentary and thermal evolutions and the complexity of structural styles. These factors combine to provide particularly favourable conditions for hydrocarbon generation and preservation in the basins of eastern Algeria and western Libya.
North Africa lies at the northern margin of the African Plate and, for the purposes of this discussion, comprises the countries of Morocco, Algeria, Tunisia, Libya and Egypt and the northern portions of Mauritania, Mali, Niger and Chad (Figure 1). The Precambrian and Palaeozoic geology of this vast region is remarkably consistent, reflecting its common geological history during most of the Phanerozoic period. Geological analysis in the region benefits greatly from excellent large-scale desert outcrops and a voluminous subsurface database derived from a long history of hydrocarbon exploration. Today the region contains some 4% of the world’s remaining oil and gas reserves (mainly in Algeria, Libya and Egypt) together with substantial amounts of fossil groundwater, phosphate and other mineral resources.
Palaeozoic hydrocarbon systems provide effective plays in most of onshore North Africa, although these are less significant in Egypt where Mesozoic and Tertiary systems dominate. The basis for the successful Palaeozoic plays are rich petroleum source rocks, of which the Silurian (Rhuddanian) and Late Devonian (Frasnian) organic-rich shales are considered to be the most important (Boote et al. 1998, Lüning et al. 2000, Lüning et al. 2003). Hydrocarbons generated from these source rocks migrated into various reservoir horizons, including Cambrian (e.g. super giant Hassi Messaoud oil field), Ordovician (e.g. Murzuq Basin), Silurian (e.g. Libyan Ghadames Basin), Devonian (e.g. Berkine Basin) and Triassic (e.g. Berkine Basin) sandstones, sealed by overlying shales and, in some cases, by (Triassic) evaporites.
The timing of the hydrocarbon trap development across North Africa is complex. Multiple re-activation of structures is common, often following Pan-African basement zones of weakness, and many faults were subjected to alternating compression/transpression and extension/transtension during the Phanerozoic. Some fault systems are thought to be part of transcontinental shear zones which may have been active since Precambrian times (Arthaud and Matte, 1977; Neev 1977, Neev et al. 1985). Due to this complexity, the detailed structural history of many composite present-day structures is still unclear. An improved understanding of the structural development is needed to provide better control on the relative timing of hydrocarbon generation and trap development. The petroleum potential of the Palaeozoic basins varies across North Africa, largely due to differences in structural styles and basin histories.
In this review we subdivide the Late Precambrian - Phanerozoic structural history into seven major tectonic phases, describe the associated structural styles and continental-scale trends, and discuss the origin of the stresses involved in a plate tectonic framework. All Phanerozoic structural developments which may have affected the Palaeozoic hydrocarbon systems of North Africa are included in this review, but the tectonic development of purely non-Palaeozoic hydrocarbon systems, such as the Gulf of Suez, is not examined in detail. This paper is based on published literature and unpublished but publicly available material together with proprietary interpretations undertaken by Eni, its subsidiaries and its partners during some 50 years of exploration and development in North Africa. Previous overviews of the structural history of North Africa have been given by Macgregor (1996), Selley (1997) and Coward et al. (2003).
North Africa has formed part of a single lithospheric plate throughout the Phanerozoic and, hence its structural development has been controlled to a large degree by intraplate processes (e.g. Ziegler et al. 1998). An exception to this is the extreme north-western part of the region (Moroccan Meseta, Atlas Mountains) which was involved in two phases of plate collision between Gondwana/Africa and Laurasia/Europe during ‘Hercynian’ and ‘Alpine’ times. In addition, a graben system occurs along the Mediterranean margin, which is related to the separation of various terranes from North Africa during the Triassic-Jurassic.
The timing of tectonic events affecting basins in North Africa corresponds closely with the major events in the fragmentation of Gondwanaland and Pangaea and the relative movements of the African, Laurentian and Eurasian plates. The Mesozoic-Cenozoic structural history of North Africa can be related to three successive rifting events that correspond with the development of the Equatorial, South and North Atlantic and the associated opening and subsequent closure of the Tethyan Basin. This polyphase tectonic evolution resulted in the development of the present-day system of partly interconnected Palaeozoic sedimentary basins, separated by intervening uplifts (Klitzsch, 1971; Schandelmeier, 1988). The nature and ages of the main tectonic structures and regimes in North Africa are illustrated in Figure 2, overlain on the Hercynian subcrop map on which the Palaeozoic basin outlines can be clearly seen, and in a series of regional geological cross-sections through the North African continental margin in Figure 3. The major oil and gas fields within the Palaeozoic succession in North Africa occupy complex structural, stratigraphic or combination traps as a direct result of the polyphase tectonic history of the region. The complex pattern of oil, condensate and dry gas accumulations at different levels in different reservoir horizons reflects variations in source rock maturity, maturation history, migration, reservoir architecture, faulting, cross-fault juxtaposition, fault seal, folding and truncation produced by the superimposed tectonic events (Figures 4 and 5). The subtlety and complexity of many of these events is only now being unravelled with the aid of high quality 3D seismic data, but much still remains unresolved. The late Precambrian to Phanerozoic structural development of North Africa can be divided into seven major tectonic phases:
(1) The Pan-African orogeny
(2) Infracambrian extension
(3) Cambrian to Carboniferous alternating extension and compression
(4) Mainly late Carboniferous ‘Hercynian’ intraplate uplift
(5) Late Triassic-early Jurassic and early Cretaceous rifting
(6) Mid Cretaceous ‘Austrian’ and Late Cretaceous - Tertiary ‘Alpine’ compression
(7) Neogene to Recent uplift and volcanic activity.
1. Pan-African Orogeny (c. 900-600 Ma)
The North African continental crust developed during oblique continent to continent collision between the West African Craton, the East Saharan Craton and numerous island arcs during the Pan-African orogeny (Caby et al. 1981; Vail 1991; Boullier 1991; Black et al. 1994; Greiling et al., 1994; Jacobs and Thomas, 2004). The sustained arc accretion and continent-continent collision during the Pan-African Orogeny eventually resulted in the “fusing-together” of various cratonic nuclei and the development of two major orogenic belts, the East African Orogen and the Chaine Pan-Africaine (or Tran-Saharan Megabelt) that together form the basement to the Palaeozoic successions in North Africa and Arabia (Figure 6). The oldest areas are represented by the West African, Nile and East Saharan cratons that have been stable since the Middle Proterozoic (2100-1800 Ma; Schandelmeier et al. 1987; El Makhrouf 1988). Pan-African overprinting (700-600 Ma) is mainly restricted to the oceanic volcano-sedimentary assemblages with locally preserved ophiolites and subduction related batholithic intrusions that surround the stable cratonic blocks (Vail 1991; Figure 7). The oceanic assemblage that encircled North Africa is interpreted as a single heterogeneous structural feature which links the African-Arabian Shield of Egypt and Saudi Arabia with the Pharusian Belt of Algeria (Vail 1991).
East African Orogen
The northern part of the East African (-Antarctic) Orogen is represented today by the Arabian-Nubian Shield in the western part of the Arabian Peninsula and in eastern Egypt and Sudan. The Shield formed around 640-510 Ma as a result of the gradual closure from 900 Ma of the Mozambique Ocean that separated East and West Gondwana. The Mozambique Ocean was characterised by oceanic arcs and back-arc basins. These collided to form superterranes and subsequently amalgamated to form the Arabian-Nubian Shield (Condie, 2003), which consequently consists mainly of accreted island arcs and a few microcontinents, but lacks major continental components (Boger and Miller, 2004; Genna et al., 2002; Jacobs and Thomas, 2002; Johnson and Woldehaimanot, 2003; Miller et al, 2003). This contrasts with the southern part of the East African-Antarctic Orogen which originated through continent to continent collision (Jacobs and Thomas, 2002). During the Pan-African tectonism in the East African Orogen, sediment eroded from the newly formed orogenic belt was deposited in associated foreland and intracratonic molasse basins (Genna et al., 2002).
The second Pan-African orogenic system in North Africa is represented by the Chaine Pan-Africaine (or Trans-Saharan Megabelt) in parts of Algeria, Mali and Niger (Figure 8). This formed between 750 and 520 Ma through the collision of more than twenty terranes between the West African and East Saharan cratons (Azzouni-Sekkal et al., 2003; Bournas et al., 2003; Caby, 2003). The Chaine Pan-Africaine is exposed in the Anti Atlas, Ougarta, Pharusian-Tuareg (Hoggar Massif), Gourma and Dahomeyan belts (Condie, 2003). The Touareg Shield consists of “Pharusian” microplates from the so-called Pharusian Ocean, which lay between the West Africa Craton and Eastern Sahara Craton (Hallet, 2002). Pan-African orogenic belts appear to have completely surrounded the West African Craton during the Neoproterozoic. Outcrops of Pan-African rocks are known from the western side of West African Craton in the Mauretania, Bassaride and Rokelide belts (Condie, 2003).
The boundary between the Chaine Pan-Africaine and the West African Craton can also be identified in the Moroccan Anti-Atlas, which has been divided into three structural domains (Fekkak et al., 2004): the southern Anti-Atlas which represents the northern border of the West African Craton; the central Anti-Atlas which represents the suture zone with an accretionary complex; and the eastern Anti-Atlas where only Neoproterozoic rocks occur. In contrast, Ennih and Liégeois (2001) contend that the whole Anti-Atlas belongs to the West African Craton. According to these authors, the Pan-African units in the Anti-Atlas represent ocean slices that were thrust onto the Craton about 685 Ma ago as a result of Pan-African accretion tectonics.
Intramontane molasse basins are also developed in the Chaine Pan-Africaine. Large amounts of Pan-African molasse accumulated in the north-west Hoggar where it infills residual basins and graben with up to 6000m of red and green clastic sediments together with limestones/dolomites (Caby and Monié, 2003). On the nearby stable West African Craton, Infracambrian deposition was also influenced by the tectonic activity in the Chaine Pan-Africaine. For example, there are considerable variations in the thickness of Neoproterozoic sediments within the Taoudenni Basin, ranging from 1000m in Adrar to 100m in Algeria (Moussine-Pouchkine and Bertrand-Sarfati, 1997).
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