--> Abstract: Structural Styles and Prospectivity in the Palaeozoic Hydrocarbon Systems of North Africa, by J. Craig, B. Thusu, S. Luning, E. Zanella, L. M. Davidson, F. Said, A. I. Asbali, T. Glover, K. Echrickh, H. Seddiq, W. Zwawi, and M. Shelmani; #90923 (1999)

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CRAIG, J., Lasmo; B.THUSU, Agoco & Univ. College London; S. LUNING (Royal Holloway Univ. London), E. ZANELLA, L. M. DAVIDSON (both Lasmo), F. SAID, A. I. ASBALI (both Agoco),T. GLOVER (Univ. Aberystwyth), K. ECHICKH (Saga), H. SEDDIQ (Agip Oil),W. ZWAWI (NOC), M. SHELMANI (Alepco)

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



The late Precambrian to Phanerozoic structural development of North Africa is characterised by at least six major tectonic phases, namely the Pan-African Orogeny, infracambrian extension, (local) Cambrian to Carboniferous alternating extension and compresssion, mainly late Carboniferous 'Hercynian' intraplate uplift, late Triassic-early Jurassic and early Cretaceous rifting, and Late Cretaceous - Tertiary 'Alpine' compression. Large parts of Morocco were affected by plate collisions between Gondwana and Laurasia during 'Hercynian' and Africa and Europe during 'Alpine' times, but much of the rest of North Africa was affected by more subtle intraplate processes involving repeated transpressional and transtensional reactivation of major shear systems, that often follow Proterozoic Pan-African and older zones of weakness. This polyphase tectonic evolution resulted in the development of a system of partly interconnected Palaeozoic sedimentary basins with intervening uplifts. The petroleum potential of these Palaeozoic basins varies, depending partly on differences in structural styles. It reaches an optimum in the basins of eastern Algeria and western Libya.

Pan-African Orogeny and Infracambrian extension

The Proterozoic basement in NE Africa developed through the progressive cratonisation and accretion of numerous intraoceanic island arcs and Andean type magmatic arcs during the interval 900-600Ma. Major extensional movements took place across North Africa and Arabia in infracambrian times (Late Neoproterozoic-Early Cambrian) and structures are interpreted to have formed either as a result of shearing along the Transafrican lineament, as pull-apart basins related to the eastward continuation of the Arabian Najid fault system (Husseini and Husseini 1990), and/or as half-graben associated with the extensional collapse of the PanAfrican orogen. Infracambrian black shales are known from outcrops in southwestern Algeria and are interpreted to have been deposited in infracambrian graben (Moussine-Pouchkine & Bertrand Sarfati 1997). Large scale-infracambrian graben have been interpreted on seismic lines from the Kufra Basin in SE Libya.

Post-infracambrian - pre-Hercynian

The structural evolution of the Palaeozoic basins of North Africa between the infracambrian extensional phase and the late Carboniferous 'Hercynian Orogeny' is more complex and heterogeneous. Local transpressional and transtensional reactivation processes seem to have dominated, resulting from the complex interaction of intraplate stress fields and the orientation and geometry of pre-existing fault systems. In the Murzuq Basin in SW Libya, several horizons exhibit abrupt thickening across steep faults, indicating Cambro-Ordovician and Carboniferous extensional/transtensional (Carboniferous: 'Hercynian' transpressional?) events (Fig. 1).The Cambro-Ordovician event may be associated with the supposed separation of the 'Armorica' microplate from central north Gondwana (Tait et al. 1997). Other evidence from seismic, well and field data in the Murzuq Basin and in the Illizi/Ghadames basins suggests that there was a.compressional event in Siluro-Devonian times.The post- Infracambrian to pre-Hercynian tectonic processes played an important role in the formation of hydrocarbon traps in the Murzuq Basin.

Hercynian Orogeny

The late Carboniferous continental collision between Africa and Laurasia led to uplifting and thrusting in NW Africa (Morocco, western Algeria) and also folded and inverted the Palaeozoic strata in neighbouring intraplate areas.The intensity of deformation decreases eastwards away from the collisional zone (Fig. 2) so that significant Hercynian folding and erosion of anticlinal crests in the Algerian Sbaa and Ahnet basins is replaced by subtle, low-angle unconformities to disconformities in the Murzuq Basin in SW Libya.The present-day maturity levels of the main Palaeozoic hydrocarbon source rocks also decreases eastwards in parallel. Silurian source rocks in Morocco and western Algeria are often very mature and typically generated hydrocarbons prior to Hercynian structuration, whereas it is unclear whether the same source rocks in the Kufra Basin in SE Libya ever entered the oil window. The occurrences of large oil and gas fields in Algeria may be partly related to their favourable location with respect to the Hercynian deformational intensity which resulted in a favourable timing of maturation and trap formation. 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 through the absence of suitable pre-Hercynian traps, but a dry gas charge contemporaneous with early Hercynian inversion, or remigrated pre-Hercynian gas from a temporary 'holding-tank' has produced rich gas reserves.

Structural Styles North Africa

Mesozoic extension

The opening of the Central Atlantic in Triassic-Liassic times and contemporaneous separation of the TurkishApulian terrane from NE Africa led to a significant extensional phase in North Africa which included activity in the Atlas Rift, rifting from Palestine to Cyrenaica and extension in the Ghadames Basin of Algeria.Another important Mesozoic extensional phase occurred during the Early Cretaceous with subsidence of half-grabens in NW-Egypt, the Sirte Basin (Libya),Tunisia and eastern Algeria. In the Aptian, the Ghadames Basin experienced sinistral transpression along the N-S trending Transaharian fracture system. A mid-Cretaceous compressional event has also been interpreted in the Murzuq Basin.This Mesozoic extension and compression again reactivated basement structures and disrupted the Palaeozoic succession, creating new hydrocarbon traps and further modifying source rock maturity.

Alpine Orogeny

Many of the Mesozoic grabens were inverted between Late Cretaceous and Miocene times as a result of the 'Alpine' plate collision between Africa and Europe. The Atlas Mountains in Morocco and western Algeria, the Palmyride Fold Belt in NE Africa and NW Arabia and the Cyrenaica Platform (NE Libya) were all affected, as were parts of the Murzuq Basin in SW Libya. Sinistral Alpine transpressive reactivation of the basement shear zones may be responsible for many of the late stage fault-related anticlines affecting Palaeozoic and Mesozoic strata across much of North Africa.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England