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

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et al., 1994). Post-Pan-African deposition in the Anti-Atlas region of Morocco was also associated with extension and half-graben development (Figure 14), with volcanic activity between 580-560 Ma accompanied by north-west to south-east directed extension and sinistral oblique slip on faults oriented 100°-120°, parallel to the earlier Pan-African suture (Algouti et al., 2000; Azizi Samir et al., 1990; Soulaimani et al., 2003; El Archi et al., 2004).

The Pan-African orogeny was followed by a period of continental-scale uplift and erosion throughout Northern Gondwana. This resulted in the development of a vast peneplain, cross-cutting the exhumed Precambrian basement (Avigad et al., 2003), and extending from Morocco in the west to Oman in the east. The final phases of the Pan-African orogeny imposed a strong north-west - south-east structural grain across much of North Africa and this influenced deposition of the succeeding Cambro-Ordovician sequences that accumulated on the peneplained “basement” surface during the next phase of the tectonic evolution (eg. Hallett, 2002).

3. Cambrian to Carboniferous alternating extension and compression

The structural evolution of the Palaeozoic basins of North Africa between the Infracambrian extensional phase and the late Carboniferous 'Hercynian Orogeny' is complex. The region lay on a wide 'ramp-like' northward-facing passive margin of Gondwana throughout the Palaeozoic. Deposition was mainly controlled by glacio-eustatic sea level variations and, during the Cambrian to mid–Silurian at least, major structures exerted relatively little control on the distribution of sedimentary facies. There were no major plate collisions or separations at this time and local transpressional and transtensional reactivation processes dominated the region as a result of the interaction of intraplate stress fields with pre-existing fault systems of varying orientation and geometry. Periodic minor growth on the major pre-existing structural elements in the region caused subtle variations in stratigraphy and eventually sub-divided the margin into a series of subtle sags and intervening uplifts. Inter- and intra- basin highs exerted a greater control on deposition from the mid-Silurian onwards and the Tihemboka, Qargaf and Dahar uplifts, in particular, had a significant influnce on the facies distribution of the Late Silurian Akakus and Devonian sandstone sequences. In the Murzuq Basin of south-west Libya, several Palaeozoic successions exhibit abrupt thickening of between 100 to 300m across steep faults, indicating Cambro-Ordovician to Carboniferous extensional/transtensional and compressional/transpressional events. These tectonic processes played an important role in the formation of hydrocarbon traps in the Murzuq Basin, including the growth of the structure containing the giant ‘Elephant’ oil field (Davidson et al., 2000).

During most of the Early Palaeozoic, the Saharan Palaeozoic basins were part of a large, inter-connected North African shelf system that was in a sagging stage after the Infracambrian extensional phase. Some relief was possibly already associated with increased sagging above former rifts, leading to thickness changes, e.g. thinning of Cambro-Ordovician strata towards the margins of the south-east Libyan Kufra Basin. The Saharan basins differentiated more distinctly from the Late Silurian/Early Devonian onwards when the intervening ridges were uplifted, but the differentiation became less pronounced again during the Late Devonian and Carboniferous (Klitzsch, 1970: Klitzsch and Ziegert, 2000). Throughout the Palaeozoic, the Saharan basins filled with thicker sequences of generally finer grained clastic sediment while the intervening ridges accumulated thinner, slightly coarser-grained sequences with numerous local unconformities.

3.1. Cambro-Ordovician extension in central North Africa (c. 525 to 444 Ma)

The Cambro-Ordovician succession in North Africa is mostly represented by continental and shallow marine siliciclastics, dominantly sandstones with a few siltstone and shale intervals. Deposition occured on the wide shelf in a generally low accommodation setting. The main sediment source was the large Gondwana hinterland to the south with prevailing south-to-north and south-east to north-west directed palaeocurrents (Figure 15). The main reservoir horizons of the giant Hassi Messaoud oil field and the nearby Rhourde el Baguel oil and gas field in eastern Algeria are located in Upper Cambrian to Arenig quartzitic sandstones, including the Lower Ordovician Hamra Quartzite (Mitra and Leslie, 2003).

The Cambrian sandstones were deposited in a variety of braided stream, deltaic and shallow marine environments over eroded basement and, in general, become finer-grained, better sorted and more mature higher in the sequence. They are overlain by repeated transgressive and regressive sequences of sandstone and shale deposited during the Ordovician, including the so-called, “Zone des Alternances”, “Argile d’El Gassi”, “Larroque” and “Quartzite du Hamra” formations and their equivalents.

A major, shortlived (500,000 to 1 million year) glaciation occurred in Western Gondwana during the latest Ordovician with the centre of the ice sheet located in central Africa (Figure 16). Features characteristic of glacigenic deposition include glacial striations, glacial tectonics, diamictites, microconglomeratic shales and systems of km-scale channels. Similar co-eval glacial deposits are also known from the Arabian Peninsula. The uppermost Ordovician glacigenic sediments are an important hydrocarbon reservoir horizon in Algeria (Unit IV) and Libya (Memouniyat Fm.).

The major NNW-SSE trending structures which dominate the early Palaeozoic evolution of eastern Algeria and western Libya (including the South Haruj, the Tripoli-Tibesti, Tihemboka and Amguid El Biod arches) seem to have been initiated as horsts in the late Cambrian (Klitzsch and Ziegert, 2000) and subsequently formed the cores of later (“Caledonian”) uplifts. These uplifts separated similarly trending troughs or graben (including the Dor el Gussa-Uri and Murzuq-Djado troughs) along which there were repeated marine transgressions during the early Palaeozoic.

The Cambro-Ordovician extension in central North Africa was associated with the break-up of the pre-Palaeozoic Rodinia supercontinent (Rogers et al. 1995). Extensional disintegration of the North African margin of Gondwana at this time created the continental terranes of Avalonia, Armorica and Baltica (Figure 15). Estimates of the timing of separation of these terranes range from late Cambrian to late Ordovician (Caradocian) depending on the differing weight given to the faunal, palaeomagnetic and facies evidence (Cocks & Fortey 1988; McKerrow & Scotese 1990; Torsvik et al. 1996; Tait et al. 1997; Prigmore et al. 1997). Rifting of Avalonia from Gondwana occurred over the present-day West African coast creating the Rheic Ocean, whilst Armorica was situated along the area of the present-day North African coast (McKerrow & Scotese 1990; Tait et al. 1997). Four distinct late Cambrian to late Ordovician subsidence events have been identified in subsidence analyses of the Avalonian continent (Prigmore et al. 1997). These authors proposed that Avalonia separated from Gondwana during Arenig to Llanvirn times, which correlates well with palaeomagnetic estimates of separation and with the occurrence of endemic faunas (Tait et al. 1997). They also documented Tremadocian subsidence, possibly as a transtensional precursor to arc initiation, and Caradocian transtension and/or extension of Avalonia long after the two continents were separated. These Caradocian transtensional events correlate with the age of deformation in the Ahnet Basin in Algeria (Beuf et al. 1971) and in the Illizi-Ghadames and Murzuq basins in east Algeria and west Libya where movement on the major structures, including the Gargaf (Qarqaf) arch, the Tihemboka Arch, the Amguid Spur and the Ahara High is marked by onlap onto and the development of local unconformities on, these north-south, north-west to south-east and east-west trending early structural highs. The onset of southerly directed subduction, volcanism and transtension (Prigmore et al. 1997) along the northern margin of Avalonia during the Tremadoc marks the change from a passive to active plate margin (Tait et al. 1997).

Volcanic layers in the Brides area on the western flank of the Ghadames Basin and in the Illizi Basin (Mereksene, Stah and Dimeta Fields) reflect this phase of activity (Echikh, 1998). Cambro-Ordovician extension also produced rifting of the Armorican terrane assemblage (comprising Iberia, Armorica and Bohemia) from the North African margin of Gondwana sometime between the Llanvirn and the Caradocian, creating the proto-Tethyan Ocean (Tait et al. 1997) (Figure 15). With the separation of Avalonia and Armorica from Gondwana during the Ordovician, the North African margin of Gondwana evolved into a back-arc setting. Extension of the northern margin of Gondwana was accompanied by basaltic volcanism and a northward tilting of the Saharan Platform (Beuf et al. 1971). The tilt is reflected both in palaeocurrent directions and in facies changes which indicate an increasing marine influence to the north. Primary depositional trends in the Lower Palaeozoic successions are dominantly towards the north and north-north-east and in the southern part of the region were strongly controlled by the basement fabric and the periodic uplift/inversion of Pan-African structures.

The general stability of the shelf across North Africa throughout this period resulted in the deposition of broadly similar Cambrian and Ordovician successions across the entire region. The Cambrian succession consists of cyclic sequences of thick, regionally extensive transgressive, fluvial to estuarine sandstones. These pass upwards into a lower and middle Ordovician succession of interbedded shallow marine sandstones and shales that can be broadly divided into four distinct correlatable sequences that reflect second order eustatic cycles superimposed on a gradual and progressive rise in global sea level throughout the Ordovician (Figure 17). This rather simple picture is complicated by the effects of instability at various times and in various places during the Ordovician. Instability during the early Ordovician is indicated by the absence of Cambrian strata over several of the main Palaeozoic uplifts in central North Africa including the Ahara Arch and the Tihemboka Arch (Attar, 1987) and by the local development of an angular unconformity between the Cambrian and Ordovician successions in the outcrops which define the eastern margin of the Murzuq Basin. Tectonic activity reached a peak in the late Llanvirn (formerly Llandeilo) particularly in the Illizi Basin, around the southern flanks of the Ghadames Basin and close to the Gargaf Arch, where active faulting and localised erosion led to the development of a series of overlapping deep erosional troughs which then filled with late Ordovician (Hirnantian) glacigenic deposits (Echikh, 1998). Active faulting controlled the thickness and facies of the syn- and post-tectonic successions deposited during the early and mid-Ordovician and a widespread 'Taconic' polygenetic unconformity surface developed in the late Ordovician as a result of the combined effects of the glacially induced sea level fall and of subglacial erosion (Craig et al., in review). The Taconic unconformity locally cuts down through older Ordovician strata to at least the level of the El Gassi Shales on the Dahar High along the northern edge of the Ghadames Basin (Echikh. 1998; Figure 18) and locally through the Lower Ordovician Haouaz Formation to the underlying Cambrian Hasawnah Formation in the Jebel Eghei region on the western margin of the Kufra Basin further south-east. The unconformity is overlain by an extremely variable succession of periglacial and glacial sediments (Bir Tlacsin, Melez Chograne and Memouniat formations and their equivalents) that were deposited during the Hirnantian glaciation at the close of the Ordovician Period (Sutcliffe et al., 2000; Craig et al., in review; Figure 19). These include fluvio-glacial sandstones that fill deeply incised palaeovalleys in the southern Illizi Basin, in the Mouydir Basin and in the Murzuk and Kufra basins (Le Heron et al., 2004) and pass laterally into glacio­marine 'micro-conglomeratic' shales (poorly-sorted sandy clays) that infill palaeotopography on the Taconic Unconformity surface further north in the northern part of the Murzuq Basin, in the Illizi Basin, in Cyrenaica, in Ghadames Basin, across the Hassi Messaoud Ridge and into the adjacent Oued Mya Basin (K. Echikh, pers. comm.). Seismic and well data indicate that the deposition and present-day distribution of the primary Early Silurian source and late Ordovician reservoir intervals in the Murzuq Basin of south-west Libya are strongly influenced by Cambro-Ordovician tectonism. Locally, Late Ordovician glacigenic reservoir sandstones of the Memouniat Formation were eroded from the crests of early-formed fault blocks prior to the main post-glacial Silurian marine transgression.

The Upper Ordovician reservoirs in North Africa contain at least 5 billion barrels of oil equivalent in more than 50 fields scattered across a broad area from the Murzuq Basin in south-west Libya (Figure 20) to the Ahnet Basin in central Algeria. Most of these reservoirs were deposited in glacially-influenced, generally shallow marine settings, on the continental shelf, beyond or at the margins of the continental ice sheet.

The deposits of the late Ordovician glaciation are extensive and occur at outcrop and in the sub-surface in North and South Africa, the Arabian peninsula, South America, and parts of southwestern Europe. The ice sheet was centred over Central Africa and expanded outward onto the surrounding continental shelves. At its maximum extent, it was of comparable size to the present day Antarctic Ice Sheet and may have extended over 65º of palaeolatitude, reaching as far north as 30ºS.

Growth of the late Ordovician ice sheet paradoxically occurred during a period of elevated CO2 levels that lasted throughout most of the Early Palaeozoic. The glaciation caused a eustatic fall in sea level of 50-100 m, produced ventilation of the world’s oceans and triggered the second largest mass extinction in Phanerozoic history, during which 85% of all extant species were eliminated.

The initial stage of ice-sheet growth was terrestrial and commenced at the start of the extraordinarius Zone of the Early Hirnantian (latest Ashgill). This initial stage was coeval with the ‘first strike’ of the Late Ordovician mass extinction that resulted in the evolution of the Hirnantia fauna (Figure 21). This distinctive fauna is preserved immediately beneath and within Upper Ordovician glacigenic rocks of North Africa, indicating that the glaciation here is almost entirely Hirnantian in age. In the later stages of the extraordinarius Zone, the ice sheet advanced onto the continental shelf and deposited ‘glacially-influenced’ sediments across most of North Africa.

Unequivocal evidence of the presence of ice on the continental shelf of North Africa is sparse, but includes isolated occurrences of outsized, exotic, faceted and striated (ice-rafted?) clasts in shales and the presence of locally extensive soft-sediment striated ‘ice-pavements’. The Upper Ordovician glacigenic rocks characteristically exhibit very rapid lateral and vertical changes in facies. These make it notoriously difficult to establish a sound sequence stratigraphic framework for the deposits and to correlate them from one area to another, even within a single field. However, new work has enabled the Upper Ordovician glacigenic rocks of North Africa to be subdivided allostratigraphically into ice-contact, glacimarine shelf, and rebound units, based on an analysis of the facies and facies associations preserved within the Upper Second Bani Formation in Morocco, the Hassi el Hadjar Formation in Algeria, and the Melez Chograne and Memouniat Formations in Libya. Fourteen facies and six facies associations characterise these rocks. Together they indicate ice-contact to distal glacimarine shelf settings on a high-latitude shelf influenced by an extensive grounded ice sheet. The fining-upward transition from ice contact to glacimarine shelf architectural elements characterises a glacial-retreat succession.

Two glacial-retreat successions are preserved in the Upper Ordovician rocks of North Africa and both are underlain by sub-glacially deformed, coarsening upward, glacimarine shelf allostratigraphic units deposited during glacial advance. Related cycles of eustatic sea-level fall and rise are recorded in age-equivalent sequences in Canada, the Welsh Basin, the Prague Basin and Portugal suggesting that the two cycles of advance and retreat of the ice sheet produced global effects. The upper glacial retreat succession in North Africa is overlain by a rebound allostratigraphic unit that represents the collapse of the ice sheet from the shelf and the associated isostatic rebound. Full-glacial conditions ended abruptly in North Africa near to the base of the Late Hirnantian
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