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The Hassi Messaoud Oil Field is a hydrocarbon field located in the Ourgula province in northeastern Algeria in the Sahara Desert. The field was discovered in 1965 and was developed by the national oil company Sonatrach. The oil field has proven reserves of 6.4 billion barrels with a daily production of 400000STB/ day. The vast field is a faulted dome structure covering an area of 1600km2. The discovery occurred at a depth of 3337m (10945 ft.). The field yields 43° API oil and has about 500 producing wells. The primary-recovery mechanism at first was gas expansion in undersaturated oil; later, the field gas has been injected in order to maintain the pressure and to improve the final recovery ratio.The field development went from natural depletion to miscible gas injection and water injection in different sectors. Miscible gas injection is the main process throughout the field with 0.55 million m3/day injection rate.

Regional background
The regional geology of Algeria can be divided into 3 different tectonic-stratigraphic domains. The West African Craton which is a Precambrian granitized basement and neoproterozoic mobile belts. The eastern limit of West African Craton borders the Tuareg Shield. The region has been mostly affected by the the Pan-African tectonic-thermal event. In the northern area of Algeria the west African craton croups out in the Reguibat shield. The Reguibat shield is comprised of two parts : the Archaean shield in the west and the Eburnearn shield in the central and eastern part of the country. The Archean shield in the west has an abundance of migmatites and evidence of very high grade metamorphism. In the Reguibat Shield intrusive granites and volcanic formations are dominant. The tectonic style of the Atlas Mountains forms broad synclinal basins and narrow anticlinal pinches. The Bechar-Timimoun Basin, Illzi and Ghadames basins provide most of the hydrocarbons found in this region. These hydrocarbons are due to structural traps, which were formed due to movement in the Caledonian and Hercynian. This produced anticlines and faulted anticlines which formed major hydrocarbon traps.

Geologic setting
The geologic setting is in the central part of the Algerian Sahara. The majority of the producing wells are from Cambrian sandstone reservoirs. The Hassi Messaoud super field cover an are of 1600 km2. The discovery of the field occurred in 1956 on well MD1 which was drilled across the reservoir in a Cambro-Ordovician sandstone at a depth of 3337 m. The Cambrian deposits are represented by sandstones and quartzite. The Trias/Ghadames Province contains the giant Hassi Messaoud oil field, in the Tanezzuft-Oued Mya Total Petroleum System. The province area encompasses approximately 390,000 km2 (square kilometers) and generally coincides with the Triassic Basin. The Oued Mya and Berkine basins contain 85% of oil and 80% of the gas found within this area.

Tectonic history
The Hassi Messaoud oil field is found mainly in the Oued Mya and Berkine basins. The complex structural setting of the basin and basin evolution is due to a heavy tectonic influence during the Cambrian-Ordovician, Carboniferous-Early Permian,Triassic-Jurassic and early Cretaceous. The berkine basin is a intracratonic, circular, extensional basin located in eastern Algeria which extends into Tunisa. During the Cambrian-Ordovician the Saharan platform underwent and intense rifting event. The Berkine basin can be characterized as a sag feature from this event. The plate collision with North America during the Carboniferous-Early permian caused the Hercynian Event in the atlas area which finalized the early Paleozoic extensional phase. Two tectonic events occurred in the Carboniferous. An additional Hercynian event occurred during the early Permian. The compression from this event is the source of the dominant structural relief in the western and northwestern ends of the basin. During this compressional event the Biod-Hassi Messaoud arch were uplifted. During the Permian-Triassic the Paleozoic rocks were eroded to various levels after the Hercynian orogeny. This erosion cut as deep as the Cambrian in some areas. This accounts for the Triassic-Jurassic unconformity. During the early Mesozoic another extensional phase affected the basin. This phase was related to the rifting event that formed Tethys ocean and the opening of the Central Atlantic. The Austrian orogenic phase produced east-west compression which caused high amplitude structures along north-south-trending reverse faults. The creation of the fold belts in northern Algeria are due to the relative movements of Africa and Eurasia during the latest Eocene. The effect of this compressional event caused the inversion of Hercynian normal faults which subsequently produced traps such as Rhourde El Rhouni and El Borma structures.

Major structures
Although the stratigraphy is continuos across north africa, the generation migration and entrapment of each petroleum system is due to the tectonic history of the individual basin. Deformational events in the region, most of them minor, are recorded by unconformities reflecting basin tilting, uplift, and erosion of intracratonic structural axes at various times throughout the Phanerozoic. The main deformational events occurred in the Precambrian to Early Cambrian (Pan African event), Late Silurian to Early Devonian, Late Devonian (Frasnian event), Carboniferous to Permian (Hercynian event), Early Jurassic, Early Cretaceous (Aptian, Austrian event), Late Cretaceous, and Tertiary  (Eocene to Oligocene, Pyrenean event). At reservoir depth the structure is a large flat anticline which is irregularly undulating eroded at the top. It is oriented southwest - northeast. The trap is a combination, not he flank the closure is structural and on the crest it is associated with an unconformity located at the base of the triassic. The structure was formed by a late phase of Hycernian ( late paleozoic) orogeny follow by erosion that cut to the Cambrian on the crest of the field.

Major stratigraphic subdivisions
The Cambrian is divided into four major stratigraphic divisions which are R:3 which consists of 300m of poorly consolidated microconglomertic clay sandstones intercalated with clayish siltstone levels that cannot be exploited due to its poor matrix properties and its deep position below the water table. R:2 is exploitable when in high position it is composed of clayish coarse sandstones with intercalated levels of clay siltstones. The top portion of this reservoir this thickness is 40m has the best matrix properties .Ra is the main reservoir with a thickness that varies from 100m to 130m, it consists of two superimposed units that are the lower Ra and the upper Ra. The Lower Ra is 70-95m thick and consists of 40-60 m of medium to coarse sandstone's with interbedded siltstone levels. The Upper Ra is 40-60m thick it is composed of relatively fine clayish sandstones containing skolithos with many siltstone levels.

Regional stratigraphy
The regional stratigraphy is continuous across North Africa, but petroleum generation, migration, and entrapment within each total petroleum system have been controlled by the tectonic history of individual basins. Deformational events in the region, most of them minor, are recorded by unconformities reflecting basin tilting, uplift, and erosion of intracratonic structural axes at various times throughout the Phanerozoic.The Berkine basin is mainly dominated by a thick section of Paleozoic and Mesozoic-Cenozoic sediments .North of the Hoggar Precambrian shield is comprised of paleozoic sedimentary rocks which crop out in this area. Coarse siliciclastic rocks overlain by sandstones and subordinate shales comprise the Cambrian-Ordovician. The main reservoirs in the Hassi Messaoud are composed of the Cambrian fluvial to estuarine sandstones. Graptolitic shales grading upward to shales and sandstones comprise the Silurian. The upward grading sequence of these sandstones into intercalated shales reflect a marine origin along with a cycle of transgressive-regressive deposition periods. The first hydrocarbon source rock in the basin is a silurian radioactive shale. The next hydrocarbon source rock are the Devonian shales. Over the Bio-Hassi Messaoud axis the Triassic sequence contains oil and gas reservoir rocks. Thick shallow marine sediments consiting of massive anhydrite and salt beds form a hydrocarbon seal. Lagoonal dolomites and shales comprise the thin layer of the Upper Jurassic sequence. Cretaceous sediments are of continental origin, these involve shales and evaporites in the lower parts.

Reservoir: seals and migration
The main reservoir is of Cambrian- Ordovician age. The fluvial to marine quarztitic sandstone and conglomerate is divided into four layers, Ri, Ra, R2 and R3. The total thickness of the reservoir is 270 m or 885ft. The environment at the time of sediment deposition was an alluvial fan. The reservoir porosity ranges from 2 to 12 % with an average of 8% permeability ranges from 0 to 1000 md. The seals on the main reservoir are due to a regional shale and an evaporite trap substantial undiscovered oil. There is a complex migration in the reservoir, the truncation of older rocks by the Hercynian unconformity were key in the migration distribution of source rocks and path ways between source rock and reservoirs. Late mesozoic generation and expulsion of petroleum from source rocks coincided with the development of large structural traps and massive accumulations of oil and gas.

Oil field production and regional politics
. There are many oil companies that do not invest in the giant Hassi Messaoud oil field due to the severe political unrest in the area. The region has been plagued by violence and destruction of oil field pipelines. The civil unrest in the area has prevented the region from developing as fast as it should for a reservoir of that size.