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The structural history of North Africa is characterised by at least seven major tectonic phases, each of which played a critical role in determining the varying potential of the Precambrian and Palaeozoic hydrocarbon systems across North Africa. Structural styles and the intensity and timing of deformation vary spatially across the region. The main structural traps comprise a variety of four-way dip and fault-and-dip closures associated with compressional anticlines, roll-over anticlines and tilted fault-blocks that developed or were modified during the Silurian-Devonian, Late Carboniferous-Early Permian (“Hercynian”) and Mid Cretaceous and Late Cretaceous-Tertiary (“Alpine”) compressional and Mesozoic extensional phases (Figure 57). These structures contain hydrocarbons trapped in widely distributed reservoir horizons of predominantly Cambrian, Ordovician, Silurian-Devonian and Triassic age and more locally distributed reservoir horizons of Carboniferous age (Figure 57). Stratigraphic and combined structural-stratigraphic traps are almost certainly widely developed in all these reservoir levels, but to date have rarely been the target of specific and concerted exploration compaigns. As such, they undoubted comprise a major prize for future exploration in the region.
The polyphase evolution of North Africa resulted in the development of 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 evolution and the complexity of their structuration. One of the key controls on hydrocarbon prospectivity appears to be the distribution, maturity and maturation history of the main Palaeozoic source rocks – the Ordovician, basal Silurian and the Devonian (Frasnian) organic-rich shales. In general, the present-day maturity of these Palaeozoic source rocks appears to be closely controlled by the thickness of the overlying Carboniferous and/or Mesozoic successions, but for the key basal Silurian source at least, there is a clear eastward “younging” in the onset of hydrocarbon generation, and a similar eastward decrease in the level of present-day maturity, that appears to reflect a corresponding eastward decrease in the intensity of both the “Hercynian” and the Alpine deformation across the region (Figure 58). Hence, at a regional scale the basal Silurian “hot-shales” (where present), appear to have been largely already either in the “gas window” or overmature by the early to mid Mesozoic in the northwest (e.g. Essaouira and Tindous basins), while in the southeast (e.g. Murzuq and Kufra basins) they are currently either within the oil generation window or possibly, in the case of the Kufra Basin, have only relatively recently entered the “oil window” (Figure 58). At a more local scale, the situation is, of course, somewhat more variable. In Morocco, for example, the Lower Palaeozoic source rocks are generally presently in the “oil window” in the Doukkla Basin, where the Permo-Triassic, Jurassic and Cretaceous sections are thinner that in the adjacent Essaouira Basin where the Silurian and Ordovician source rocks are presently in the “wet gas window”. Even at a basin scale in this area, the maturity of the Palaoezoic source rocks in the Doukkla Basin and the maturity of the gas in the Essaouira Basin tend to increase with increasing thickness of the Permo-Triassic sections. These relationships suggest that there is a strong element of post-Hercynian maturation in these north-western Moroccan Basins.
Elsewhere, the interaction between trap styles, timing and intensity of deformation, maturation history of the source rocks and the hydrocarbon potential of the Precambrian and Palaeozoic systems is more complex. For the primary basal Silurian/Late Devonian sourced Palaeozoic plays, at least, these factors appear to combine to provide particularly favourable conditions for hydrocarbon generation and preservation in a core area centred on the basins of eastern Algeria (northern Oued Mya, Illizi, Berkine) and western Libya (Ghadames, northern Murzuq). Here, overprinting of extensional and compressional tectonic episodes has led to a high trap density with multiple trap styles and commonly two, and locally three or more, significant pulses of hydrocarbon expulsion, namely before or during the “Hercynian” (Late Carboniferous-Early Permian), the “Austrian” (mid-Cretaceous) and the Alpine (mid to late Tertiary) phases of uplift and exhumation (Figure 60) and/or, locally, associated with late Triassic/early Jurassic and Tertiary volcanic activity. Most pre-Hercynian palaeo-accumulations would probably have been dispersed during later periods of uplift and erosion, but it is possible that some pre-‘Hercynian’ charged accumulations might have survived in the most robust traps in these ares. There appears to be little doubt, however, that the majority of the traps in this core area were finally charged with hydrocarbons during the Late Cretaceous and Early Tertiary, prior to the main phases of the Alpine Orogeny. Mid and Late Tertiary uplift and unroofing of the southern flanks of this core area (e.g. southern Illizi, southern Ghadames and Western Murzuq basins) caused both local and regional structural tilting, late faulting and the basin-margin exposure and related recharge and flushing of Mesozoic and Palaeozoic aquifers (e.g. Pallas, 1980; Hammuda, 1980). In these marginal areas, the combined effect of tilting, late faulting and pressure changes associated with aquifer recharge would have encouraged the spillage, breaching and dispersal of any hydrocarbon in “pre-Alpine” accumulations (Dardour et al., 2004).
The core area of the established basal Silurian and Late Devonian sourced play fairway is flanked by areas to the west where ‘Hercynian’ tectonic events appear to have a dominant control on hydrocarbon trap styles (Ahnet and Reggane basins) and to the east by an area where trap styles in the Palaeozoic (and, potentially, Precambrian) systems are dominated by Early Cretaceous extensional events (Sirte Basin). These areas are flanked, in turn, by “frontier areas” (Figure 60), where the traditional Palaeozoic hydrocarbon plays are currently unproven and there are regional play risks associated with the timing of trap generation versus the timing and phase of hydrocarbon generation and expulsion (e.g. Tindouf Basin), or of source presence (southern Murzuq Basin), or of both source presence and, possibly, source maturity (Kufra Basin). As the traditional structural traps in the core area of the Palaeozoic play fairway become increasingly well explored, it is inevitable that stratigraphic traps in general and the under-explored “frontier areas” (and their counterparts further south in Mauritania, Mali, Niger and Chad), will become the focus of increased and renewed exploration activity, in particular, for the Palaeozoic plays and the largely untested potential of the Precambrian plays in North Africa region.
Over the last 20 years, a combination of factors have contributed to an increased pace of exploration and exploitation of the Palaeozoic hydrocarbon systems of North Africa. These include:
This acceleration in activity shows every sign of continuing, particularly now that the Libyan Government has launched a series of exploration rounds following the lifting of US sanctions. Field rehabilitation and enhanced recovery activity is also likely to pick up significantly.
With increased exploration, appraisal and development across the region, the focus of explorers is turning increasingly to deeper and more remote plays and basins; focussing more and more on the Palaeozoic and possibly the Precambrian potential.
Understanding the factors which control the emplacement history and distribution of hydrocarbons in Precambrian and Palaeozoic systems is clearly of fundamental importance. This review has highlighted, in particular, the significance of trends in the intensity of Hercynian deformation across North Africa.
Hercynian compression was dominant in trap formation in west and central Algeria, whereas further eastwards, other factors such as pre-Hercynian transpression and transtension, Mesozoic extension or Alpine inversion became more important.
This trend is also reflected in the parallel decrease in the maturity of the prolific Silurian and Late Devonian source rocks.
The juxtaposition of source, carrier and reservoir beds, the distribution of seals, the relationship between hydrocarbon expulsion, trap formation and preservation and the diagenetic history of the reservoir horizons will only be unravelled by detailed data acquisition and interpretation – but the prize arising from this understanding remains enormous.
Funding for the study was provided by Eni. We thank the following national oil companies and oil companies for permission to use data in this contribution: NOC, Sonatrach, Eni, AGOCO, BHP-Billiton and Repsol. We would like to acknowledge valuable contributions by the late Professor Mike Coward (Ries-Coward Associates), Andy Fisher (ConocoPhillips), Bashir Hussin (AGOCO), Edwige Zanella (EnCana), Lindsay Davidson (Eni), Tim Glover (British Gas), Khalid Echikh (independent consultant), H. Seddiq (NOC), A. Khoja (NOC) and M. Shelmani (ALEPCO). Drafting was provided by Wave Graphics. The authors particularly wish to thank Khalid Echikh for his thorough and insightful review of the final manuscript.
AUTHORS PRESENT ADDRESSES
J.F. Bell Amerada Hess Ltd., 33, Grovenor Place, London, SQ1X 7HY
M.J. Durham Northern Petroleum plc., No. 1 Cornhill, London, EC3V 3ND
M.H. Eales Agip-KPO, KPDL Karachaganak London Office, 4 Millbank, Westminster, London SW1P 3JA
C. Hamblett Eni Exploration and Production Division - Via Emilia, 1, 20097 San Donato Milanese, Italy
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