A. INTRODUCTION

The natural boundaries of Algeria are the Mediterranean Sea to the north (1200km of shoreline), Morocco to the west, Tunisia and Libya to the east, Mauritania and Western Sahara Tunisia and Libya to the east, Mauritania and Western Sahara.(Fig. 1). The prime meridian (Greenwich) goes through the city of Mostaganem.

From the standpoint of its area (2,381,741 km² ), Algeria is the second largest country in Africa and the Arab world after Sudan.

Distances are great, some 2000 km from the Mediterranean coastline to the Hoggar range and 1800 km from In Amenas in the east to Tindouf in the west.

B. GENERALE MORPHOLOGY

The country is divided from north to south into four zones:

• The Tellian Atlas (or the Tell) is made up of the northern-most steep relief flanked by rich coastal plains such as the Mitidja in the centre, the Chelif to the west and the Seybouse plains in the east;
• The High Plateaus;
• The Saharan Atlas as a succession of NE-SW oriented reliefs spreading from the Moroccan border to Tunisia;
• The Sahara desert south of the Atlas yields most of Algeria's hydrocarbon resources. the desert is composed of large sand dunes (East and West Erg) and gravel plains (regs) with dis-persed oases such as El-Oued, Ghardaia and Djanet . The hypothetical line linking the Eglab range in the west and the Hoggar mountains in the east, marks for all practical pur-poses the southern limit of the Algerian Sahara.

The hypothetical line linking the Eglab range in the west and the Hoggar mountains in the east, marks for all practical pur-poses the southern limit of the Algerian Sahara.

C. STRUCTURAL ASPECTS

Algeria is subdivided into two major structural units, sepa-rated by the South Atlas fault(Fig. 2) :

• Northern Algeria, mainly affected by Alpine tectonics;
• The Saharan Platform, a relatively stable area where tectonic activity is low.

1 - NORTHERN ALGERIA

Northern Algeria is delineated by the following features:

• The Saharan Atlas, a mountain range of Alpine origin to the south;
• Large plains in the centre such as the Oran Meseta to the west and Ain Regada highs to the east;
• The Tellian Atlas, a complex area comprising a succession of nappes set in place during the Lower Miocene to the north. Late Neogene basins such as the Chelif Basin or the Hodna Basin were deposited over these nappe structures.

2 - THE SAHARAN PLATFORM

It is a Precambrian basement unconformably overlain by transgressive phanerozoic deposits. Various tectonic events delineate the boundaries of sedimentary basins having their own more or less complete sedimentary column.

D. THE HYDROCARBON PROVINCES

Four more or less mature hydrocarbon provinces can be dis-tinguished:

• Eastern Sahara, although well known for its oil and gas fields still has potential for major discoveries;
• Central Sahara, until recently considered as being essential-ly gas prone (only gas fields of variable sizes were known) has triggered renewed interest with recent oil strikes;
• Western Sahara is said to be chiefly gas prone but remains practically unexplored;
• Northern Algeria is geologically very complex and its hydro-carbon potential remains only partially known despite small oil and gas strikes.

E. GEOLOGICAL FRAMEWORK

The geological history of the sedimentary basins (Fig. 3) part of the global geodynamic process of plate tectonics which contributed to the division of Algeria into two distinct domains:

• Alpine Algeria in the north;
• The Saharan Platform in the south.

1 - ALPINE ALGERIA

The northern domain is made up of young mountains formed during Tertiary times by the Alpine orogeny. Alpine Algeria consists of a number of structural-sedimentary units, from north to south:

• Off-shore Algeria is a reduced continental shelf bearing Tertiary and Quaternary sediments (1000 to 3500 m, Mio-Pliocene target) overlaying a metamorphic basement;
• The Tellian Atlas is the nappe domain, with mountain basins (i.e. Chelif Basin), and a sedimentary column ranging from Jurassic to Miocene. It has yielded a number of fields of vari-able commercial interest such as Ain Zeft, Tliouanet and Oued Guettirini. The main oil targets are Middle Cretaceous and the allochthonous Miocene and Eocene formations;
• Hodna Basin is a fore-deep basin whose sedimentary infill begins with Eocene and Oligocene continental deposits over-S lain by marine sediments of Miocene age. The main hydrocar-bon target is the Eocene;
• The High Plateaus are the fore-land of the Alpine range bear-ing a thin sedimentary cover (Liassic target). Local distension mechanisms allowed the formation of intra-mountain basins such as those of Telagh and Tiaret;
• The Saharan Atlas was formed from an elongated trough pinched between the High Plateaus and the Saharan Platform. During the Mesozoic times, the trough was filled-in by thick (7000 to 9000 m) sedimentary deposits. Later, the Tertiary compressive tectonic stresses modified the former extension trough into a number of reverse structures which led to the creation of the mountain range. The main target of this area is the Jurassic.
• The Chott Melrhir basins in the SE Constantinois, where the structures developed during Tertiary times. Its 5000 m of Cretaceous sedimentary deposits have yielded hydrocarbons in the Cretaceous (Djebel Onk, Rass Toumb, Guerguet El Kihal North).

2- THE SAHARAN PLATFORM

It is located to the south of the alpine domain and is part of the North African Craton. It comprises a Precambrian base-ment unconformably overlain by a thick sediments, struc-tured during the Paleozoic into a number of basins separated by high zones. These basins, from west to east are:

• Tindouf and Reggane Basins located on the northern and north-eastern fringe of the Reguibat shield. The sedimentary column reaches over 8000 m in Tindouf Basin and 6500 m in Reggane. This area is little explored, but its Paleozoic rocks, which could be gas and oil prone, constitute a good prospec-tive target;
• The Bechar Basin bound to the north by the High Atlas and to the south and west by the Ougarta range. Its sedimentary infill reaches 8000 m. The reservoirs are located within Lower Paleozoic clastics and Carboniferous reefs;
• The Ahnet-Timimoun Basin bordered to the north by Oued Namous shoal, to the west by the Ougarta range, to the south by the Touareg shield and to the east by the Idjerane-Mzab ridges. Sediment accumulations reach some 4000 m. In the south the Ordovician and Devonian reservoirs are gas prone. In the north, oil has been discovered through all the Paleozoic formations in the Sbaa trough;
• The Mouydir and Aguemour-Oued Mya basins are bordered to the west by the Idjerane-Mzab ridge and to the east by the Amguid-El Biod dorsal. In the Mouydir area to the south, the Paleozoic sedimentary sequences outcrop. The Aguemour-Oued Mya Basin to the north is infilled by thick Paleozoic and Meso-Cenozoic sediments, approximately 5000 m at Oued Mya. Major fields are located within the Cambrian in Hassi Messaoud and the Triassic formations of Hassi Rmel;
• Illizi-Ghadames syncline is bound to the west by Amguid-El Biod dorsal and to the east by the Tihemboka uplift and the Algeria-Tunisia-Libya border lands. In the Ghadames Basin the sedimentary cover, exceeding 6000 m, yields hydro-car-bons both in Paleozoic and Triassic sediments.

F. GEODYNAMIC EVOLUTION

The development of the Mediterranean Alpine Ranges and of their Algerian section is the result of the rotation of the lAfrican continent with respect to the Eurasian continental plate. This rotation is a slow drifting of the continents towards one another . The continental convergence which is believed to have started in Lower Jurassic times, is only clearly recognizable during the Upper Jurassic (150 MY).

The tectonic environment is that of the collision between Africa and Europe. It produced the Algerian Alpine ranges which still lay on the northern fringe of the African plate.

Even though the first signs of drifting of Africa and Europe only became clear during the upper Jurassic, the phenome-non was initiated during early Liassic times (180 MY), and is associated with the opening of the North Atlantic.

G. STRATIGRAPHY

It is subdivided into the same two previously defined domains. The Saharan Platform is a large and stable area with a sedimentary history which dates from the early Paleozoic. It can be divided into three hydrocarbon provinces. Northern Algeria, formed during the Tertiary by the Alpine phases com-prises a number of units whose stratigraphic characteristics differ from one another.

1- THE SAHARAN PLATFORM

A number of factors, formation thickness (1000 to 8000 m), lithologies, tectonic deformations and subsidence have led to the creation of three distinct sedimentary basins distributed on the Saharan Platform in the Western Province, the Eastern Province and the Triassic Province.

a. THE WESTERN PROVINCE

This includes the Bechar, Tindouf, Reggane, Ahnet, Mouydir, Timimoun and Sbaa basins. These depressions contain Paleozoic deposits ranging from Cambrian to Namurian. Mesozoic and Cenozoic sediments are scarce (Fig. 3a). Sediment thickness varies from 3500 m to 8000 m. Tindouf and Reggane basins are asymmetrical troughs located on the N and NE of the Eglab shield. The Paleozoic sediments reach-es thicknesses of over 8000m in the Tindouf Basin and 6000 m in Reggane. The Bechar Basin is bound to the north by the High Atlas, to the south and west by the Ougarta range and to the east by the Meharez shoal. Sediment thicknesses can reach 8000 m.

The Ahnet-Timimoun Basin is bordered to the north and to the west by Oued Namous shoal and the Ougarta range while to the south and east the limits are the Touareg shield and the Foum-Belrem and Mzab dorsals. The Mouydir Basin is bor-dered to the west by the Foum Belrem dorsal and to the east by Amguid-El Biod dorsal. Sediments reach over 3000 m.

Figure 3a	Figure 3b	Figure 3c

b. THE TRIASSIC PROVINCE

This province, in fact a large basin located on the Northern part of the Saharan Platform, is an E-W oriented anticlinorium which can be broken down into the following features:

• The Tilrhemt dome and the Talemzane shoal;
• The Djemaa-Touggourt structural system;
• The El Agreb-Messaoud dislocation system;
• The Dahar uplift.

These features are separated by depressions such as the Oued Mya where typical Triassic province formation are encoun-tered (Fig. 3b) and where the Paleozoic deposits are often eroded down to the Ordovician or Cambrian levels.

The Mesozoic formations, unconformably overlying the Paleozoic, are present from Triassic to Upper Cretaceous. Cenozoic sediments are represented by Mio-Pliocene clastics.

c. THE EASTERN PROVINCE

It is also known as the East Algerian Syncline and consists of the Illizi and Ghadames basins separated by the Ahara ridge.The typical section (Fig. 3c) shows sediments lying unconfor-mably on Precambrian basement and displays most stratigra-phic units from Cambrian to present day. In some areas the late Mesozoic is overlain by Mio-Pliocene formations.The thin and discontinuous Quaternary deposits, constitute the final stage of sedimentation.

d. ILLIZI BASIN

Paleozoic sediments (some 3000 m) outcrop on the Southern fringes of the basin where they constitute the Tassilis. The Mesozoic formations outcrop to the centre of the basin while Tertiary deposits are essentially located on the north-west flank.

e. BERKINE BASIN (Ghadames Basin)
This depression was filled by a sedimentary column which attains a thickness of 6000 m. Its Mesozoic formations are characterised by a thick succession of salts and anhydrites deposited in the north and northeastern parts of the basin.

2. NORTHERN ALGERIA

Northern Algeria involved the geological evolution of the Mesogean basin. The foreland is subdivided into four paleo-geographical and structural units :

• The Oran Meseta;
• The Saharan Atlas taken as a whole;
• The south eastern Constantine area;
• Tthe Tellian domain.

The stratigraphy of Northern Algeria has been established in each of those basins from field or drilling data.