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Introduction

DUNCAN S. MACGREGOR

BP Exploration Operating Co. Ltd, Kuningan Plaza, S. Tower, P.O. Box 2749, Jakarta 12940, Indonesia

This Special Publication of the Geological 'booked' in Algeria in 1994 than in any other Society documents a series of papers collected country in the world. on the petroleum geology of , Algeria, The importance of the region to the petroleum Tunisia, Libya and the western part of Egypt. industry is, of course, not reflected in the volume This region contains some 4% of the world's of geological literature available on the area, par- remaining oil and gas reserve (Table 1), placing ticularly that in English. This volume represents the area above more traditional producing the first such attempt at a compilation of the pet- regions, including Europe, the USA and SE roleum geology of this region. The main objec- Asia, in its importance as a future petroleum pro- tives of this book are thus to increase the level ducer. In addition to these considerable proven of documentation towards that appropriate for reserves, North Africa is now one of the world's such major petroleum provinces and to facilitate most active exploration areas. This was reflected, the application of analogues between North Afri- for instance, in more new oil reserves being can countries and beyond.

Table 1. North African petroleum reserves and their global significance

Rank Country Reserves (109 BBO)

Oil reserves (remaining) 1 Saudi Arabia 261.2 2 Iraq 100.0 3 United Arab Emirates 98.1 4 Kuwait 96.5 5 Iran 88.2 6 Venezuela 64.5 7 Mexico 49.8 8 Russia* 49.0 9 USA* 29.6 10 Libya 29.5 (2.9% of world total) 11 China 24.0 12 Nigeria 20.8 13 Algeria* 9.2 (0.9% of world total) - Egypt (inc. Gulf of Suez) 3.9 (0.4% of world total) - Tunisia 0.4 ( <0.1% of world total)

Rank Country Reserves (1012 SCF)

Gas reserves (remaining) 1 Russia* 1700 2 Iran 742 3 Oman 250 4 United Arab Emirates 205 5 USA* 164 6 Venezuela 140 7 Algeria* 128 (2.6% of world total) 8 Nigeria 110 9 Iran 110 10 Indonesia 69 19 Libya 46 (0.9% of worm total) - Egypt (inc. Gulf of Suez) 22 (0.4% of world total)

*Areas with significant Palaeozoic- reservoired component to reserves. Source: BP Statistical Review of Worm Energy, 1996, data as at end 1995. BBO, Billion barrels of oil; SCF, stan- dard cubic feet.

MACGREGOR,D. S. 1998. Introduction. In: MACGREGOR,D. S., MOODY,R. T. J. & CLARK-LOWES,D. D. (eds) 1998. Petroleum Geology of North Africa. Geological Society, London, Special Publication No. 132, 1-6. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

2 D.S. MACGREGOR

It has been our mission, in seeking papers for category. Overview papers are presented for this volume, to try to obtain a full and represen- each of the two stratigraphic sections thus tative set of papers covering different regions and defined and are followed in each case by regional themes. Thus we sought both full coverage of the papers, covering topics such as petroleum sys- stratigraphic column (Fig. 1) and as wide as pos- tems, stratigraphy and regional structure, and sible geographical coverage (Fig. 2). Particular those relating to individual reservoirs, fields encouragement was given to authors from and play fairways. Because of its unique struc- North Africa to submit previously unpublished tural history and play types, the Atlas region is material on the oilfields and gas fields of the assigned a separate sub-section within the Meso- region. In this regard, we particularly acknowl- zoic- category. The Gulf of Suez and edge the contribution from Sonatrach, which, Nile Delta petroleum provinces are not strictly with six papers, is overwhelmingly the predomi- included in the region covered by this book, nant contributor to the volume. but are touched on in some papers, particularly As the aims of this book are to encourage the that by Keeley & Massoud. use of analogues over national boundaries, we have avoided grouping the papers geographi- cally. Instead, we have first split the papers stra- Palaeozoic and sub-salt tigraphically into 'Palaeozoic and sub-salt' and '-Cenozoic' sections (Fig. 2). This divi- The North Africa region is perhaps best known sion is justified both by the structural history of in international geological circles for its Palaeo- the area, which would suggest a partition at the zoic reservoirs and source rocks (Figs 1 and 3). Hercynian unconformity (Fig. 3), with a modifi- The Palaeozoic and sub-salt contribute nearly cation to this boundary suggested by reviewing half the oil (43%) and the vast majority (84%) source-reservoir associations, such that discus- of the gas reserves of the region (Figs 4 and 5), sions on the sub-salt Triassic reservoir of Algeria with most of this petroleum originating from are most rationally included in the 'Palaeozoic' and source beds (Fig. 3).

Fig. 1. Chronostratigraphy of the Phanerozoic of the main North African petroleum provinces, as compiled from various papers in this volume. The most significant oilfields, reservoirs and source rocks are labelled. Papers in this volume are split into those pertinent to the petroleum systems below the salt in the west of the region and those on the Mesozoic systems in the east of the region. (For line of section, see Fig. 2.) Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

INTRODUCTION 3

Fig. 2. Geographical coverage of the papers in this volume. The main petroleum basins are stippled and numbered as follows: 1, Ahnet; 2, Oued Mya; 3, Ghadames; 4, Illizi; 5, Gabes-Pelagian; 6, Sirt; 7, Abu Gharadiq; 8, Murzuq.

Fig. 3. Composite petroleum system diagram for the North African provinces. The boxes are sized according to the relative importance of the various reservoirs, source rocks and events. It is possible to differentiate two main plate tectonic cycles, to and Triassic to Recent, which are also reflected in cycles of petroleum generation, entrapment and destruction. *Existing hydrocarbon-bearing traps only. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

4 D. S. MACGREGOR

giant fields that may still lie undiscovered in this region. Fekirine & Abdallah concentrate on the stratigraphic development and sequence stra- tigraphy of the Algerian Palaeozoic, work which emphasizes the unusually strong role that eustasy plays in controlling facies development in this cratonic area. A complementary paper by Echikh attempts a similar understanding of the Ghadames Basin; this study extends into Libya and notes a number of key differences with the parts of Algeria reviewed by Fekerine & Abdal- lab, particularly the more significant role of the Fig. 4. Pie chart illustrating distribution of oil reserves Caledonian event in the Ghadames area. The amongst the main reservoir fairways of North Africa. The predominance of Mesozoic-Cenozoic reservoirs extension of the Palaeozoic petroleum systems should be noted. productive in Ghadames and Illizi into the fron- tier basins to the west is the subject of a paper by Logan & Duddy, who emphasize the significance in these areas of timing of generation relative to trap formation. This paper emphasizes the con- tribution that modern techniques, in this case apatite fission track analysis, can make to fron- tier exploration in North Africa. The interpreta- tion by these authors of a late-stage heating event of large regional extent is one which has major significance for our view of the prospectivity of western Algeria. Cambro- clastic reservoirs contri- bute 26% of the oil and 13% of the gas in North Africa (Figs 4 and 5), with the region having higher reserves in rocks of this age than Fig. 5. Pie chart illustrating distribution of gas reserves any other region of the world. Crossley & amongst the main reservoir fairways of North Africa. McDougall open the review of Palaeozoic reser- The predominance of Palaeozoic and Triassic voirs by summarizing some of the key points con- reservoirs should be noted. The more gas-prone nature trolling reservoir quality and distribution in the of these reservoirs is related to the state of maturity of Lower Palaeozoic, based on literature and their Palaeozoic source and rocks, and timing relationships own outcrop studies in Morocco. These authors between that maturity and the formation of traps. also emphasize the potential for subtle trap dis- coveries in the future, on grounds independent of those used by Macgregor. Most of the Cambro-Ordovician reserves lie in a single field, Hassi Messaoud. The controls on With the exception of the Amal and a few asso- the producibility of these reservoirs in the area ciated fields in the Sirt Basin (Gras & Thnsu), of that field are reviewed by Djarnia & Fekirine. Palaeozoic production comes almost exclusively This work illustrates the importance of clay from the western part of the region, particularly mineralogy as a control on permeability, and Algeria (Figs 1 and 2). Boote et al. open this sec- identifies relationships to clay type and content tion with a comprehensive overview of Palaeo- similar to those described in the Rotliegendes zoic petroleum systems, based on a series of of the Southern North Sea. detailed play maps delineating the main produ- The clastic section ranging from latest Silurian cing plays in the Cambro-Ordovician, Early to Early Carboniferous delivers 11% of North Devonian and Triassic, sourced mainly from African oil and 13% of gas (Figs 4 and 5), with the Early Silurian and Late Devonian. Traut et most of this contribution coming from the al. then detail the long exploration history of Early Devonian of the Illizi area (Fig. 2). These these plays. In another overview paper, devoted reservoirs are addressed by three papers. In mainly to the largest petroleum fields within each case, reservoir quality is shown to be spora- these Palaeozoic petroleum systems, Macgregor dic and generally facies controlled, although in examines regularities in the main producing his overview of these reservoirs, Echikh also fields, and considers the possible nature of the emphasizes the importance of diagenetic pro- Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

INTRODUCTION 5 cesses, for instance the favourable influence of al., respectively, document the established petro- grain coating chlorite. The F6 reservoir of the leum systems of these two relatively underex- Tin Fouy&Tabankort field, which straddles the plored parts of North Africa and highlight Silurian-Devonian boundary, is the subject of a many new potential plays. paper by Alem et al. In addition to sedimentolo- The Mesozoic reservoirs and fields section gical controls, the key feature of this hydro- describes many of the features, problems and carbon occurrence is the hydrodynamic opportunities specific to the two main fairways influence on oil entrapment. A further subtle within the Mesozoic cycle, namely the Early Cre- trap analogue in the Middle Devonian of the taceous-Cenomanian clastic deposits, produc- Alrar stratigraphic trap is presented by Chaouchi tive in Libya and Egypt, and the Late et al.: here facies variations control not only the -Paleogene carbonates, productive distribution of reservoir quality but also entrap- in the Tethyan margin basins (Fig. 2) extending ment itself. from Egypt to Tunisia. These reservoirs form There is no specific paper in this volume deal- part of a large-scale transgressive cycle overlying ing with the sub-salt Triassic fluvial reservoir the rifts described by Guiraud (Figs 2 and 3). The system, which delivers 11% of the oil and a stag- clastic rocks, which seem to be sourced primarily gering 59% of the gas of the region. The giant from underlying syn- or post-rift sources, contri- Hassi R'Mel field is here the main contributor. bute 20% of North African oil and 6% of gas, Most of the key controls on this fairway are, whereas the carbonates contribute 36% of however, covered in the overview and regional North African oil and 9% of gas (Figs 4 and papers, with Boote et ai. summarizing the petro- 5). These reservoir papers are ordered from east leum system involved, Macgregor the features of to west (Fig. 2), following a general trend of pro- the main fields and Echikh the regional sedimen- ductivity in progressively younger portions of the tary model for these reservoirs and the sourcing stratigraphic column (Fig. 1) and from clastic model for the recent finds in the central Gha- deposits to carbonates. dames Basin. This model of long-distance migra- Richardson et al. focus on the key control of tion from Silurian and Devonian subcrops to the fault seal in the clastic fairways of the Egyptian Hercynian unconformity forms an important Western Desert. Other important factors in this analogue for other areas which might not area are presented in the regional overview by appear prospective at first glance. Keeley & Massoud. By far the predominant area of Mesozoic clastic production, however, is the eastern Sift Basin, which is reviewed by Gras & Thusu. This paper compiles many of the Mesozoic-Cenozoic section new concepts proposed for this region in recent years, including the probability of contributing The Mesozoic section agains opens with an over- lacustrine source rocks in the Triassic and the view paper (Macgregor & Moody), aimed at Early Cretaceous, the recent assignment of introducing the reader to the Mesozoic-Ceno- much of the productive reservoir section in the zoic tectonic cycle and the basics of its petroleum giant Amal field to the Triassic (rather than geology. Mesozoic prospectivity extends over all Cambro-Ordovician) and the possibilities for coastal and offshore areas of North Africa and new structural plays in features such as relay over the Atlas belt, which is dealt with in a sepa- ramps. rate sub-section. The structural history of Meso- Late Cretaceous-Paleogene carbonates are the zoic basins is better controlled than for the predominant producers in a belt extending from Palaeozoic and is presented in two complemen- the northern Sirt Basin through offshore Tunisia tary papers by Guiraud and Wilson & Guiraud. into the Atlas. Two papers deal with the complex These papers provide invaluable compilations reservoir models for these carbonates, emphasiz- of our understanding of the rifting patterns ing the wide variety of environments under which which led to the Mesozoic basin and the asso- these carbonates are deposited, the complexities ciated volcanism, and will be invaluable refer- in reservoir distribution and the contribution ences for these involved in predicting the deep that sequence stratigraphy and sedimentological stratigraphy of many basins and the history of studies can make to identifying areas of reservoir heat flow development within these. These two development. Spring & Hansen examine the mul- papers are followed by two structural and petro- titude of environments within a Paleocene carbo- leum system overviews, covering northeast nate sequence in northern Libya and outline Africa (Egypt, Eastern Libya) and northwest reservoir distribution models pertinent to the Africa (Morocco) respectively. These two two types of margin observed in the region, papers, by Keeley & Massoud and Morabet et ramps and rimmed carbonate margins. Loucks Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

6 D.S. MACGREGOR et al. describe and interpret the depositional that it is only with improved seismic definition, environment of and reservoir development with together with the application of modern explora- the Eocene nummulitic of offshore tion techniques and concepts, that the barriers to Tunisia, these being the main producing reser- success in the Atlas will be broken. voir in that country and in its largest field, Ash- We hope this volume, in addition to increasing tart. The paper by Zaier et al. is also concerned interest in the region, will help provoke inter- with this region, concentrating on the structural national debate on many ill-understood issues controls on sedimentary patterns that influence affecting the petroleum geology of the region. both reservoir and source rock development at Examples of such issues highlighted in this Paleogene level. volume include the potential for subtle traps Despite a not inconsiderable exploration effort throughout the region and stratigraphic over the years, large discoveries have not materi- column, the structural origin of many of the alized in the Atlas, and this region does not even Palaeozoic basins, the possibility of deeper feature in the statistics presented in Figs 4 and 5. plays associated with precursor rifts to Sirt and As pointed out by Macgregor & Moody, a revita- other 'Cretaceous rifts', the relationship between lization of interest may occur in this region fol- timing and trap formation in many regions, the lowing recent successes in directly analogous debate between thin- and thick-skinned tectonics regions of the Alpine belt in Europe, particularly in the Atlas, and the complex depositional and in the Apennines. It is therefore now critical to diagenetic controls on many of the key reser- understand why the wells drilled in this region voirs. We hope this volume will be the first of to date have failed. The three papers presented many attempts to discuss these and other issues on the region contribute to this understanding, openly and to facilitate debate between workers as does the previously mentioned overview of from North Africa itself and those working the Morocco by Morabet et al. The papers by region from elsewhere. Morgan et al. and Brae~ne et al. deal with the We close by thanking all the authors for their alternative tectonic models of thin- and thick- efforts, particularly those from North Africa skinned tectonics applicable to the surface anti- itself, together with the many reviewers of the clines that have been the focus for much of the papers. Because of language problems and lack drilling to date. This distinction is fundamental of drafting support, many of the papers were not only to whether wells penetrated valid clo- rewritten and/or redrafted by the editors, and sures at depth but also to the deep stratigraphy, D.M. acknowledges the support of the BP Indo- including reservoirs and source rocks, that can nesia and Algeria assets in this regard, particu- be expected below such potential traps. Both larly the efforts of Misran and Krisnandi in the groups of authors conclude by identifying differ- BP Indonesia drafting office. Malika Hamraoui ent mechanisms for their individual study areas of BP's Algiers office is thanked for her hard but acknowledging that different models apply work in co-ordinating the submission of papers in different parts of the Atlas. In other words, from Algeria. In addition to financial contribu- there is no commonly applicable model or easy tions from many authors themselves, the follow- solution to the issue. In the final paper, Mekir- ing are thanked for financial contributions eehe et al. focus on many of the problems con- towards drafting for authors who required assis- cerned with reservoir development and tance in this regard: BP (Algeria asset), the Petro- petroleum systems which must be addressed leum Group, Fina, ARCO and Leeds University. while exploring this region. The overall message Amina Mabrouk is thanked for her help with the of these papers for the petroleum industry is French proof-reading.