The Hassi Messaoud oil field was discovered in 1956 by S.N. REPAL on the Saharan permits of the CFP(A) and S.N. REPAL Association, following a seismic-refraction study, which was the only method able to give, at that time, valuable data beneath the thick Mesozoic cover.

Regional and local geological studies showed that this high zone of the Saharan platform has remained relatively high during geologic time. The field structure covers an area of about 600 sq mi of this high platform zone.

At reservoir depth, the structure is a large, flat anticline, irregularly undulating eroded at the top; and its general orientation is south-southwest--north-northeast.

The stratigraphic succession may be described as follows: (1) On the granitic basement, the Paleozoic is represented by the Cambrian "Hassi Messaoud sandstones," about 1,700 ft thick; it is divided into three units: R3, R2, and Ra. Ordovician sediments are present around the structure; the most complete Ordovician deposits are on the flanks. (2) The Mesozoic formations cover the field to the ground surface. About 10,000 ft thick, they are unconformable on the Paleozoic. (3) The Tertiary is represented, away from the structure, by Eocene and Mio-Pliocene deposits.

The main Cambrian reservoir, Ra, is formed by sandstone-quartzite, the thickness of which ranges from 0 to 480 ft, according to the degree of pre-Triassic erosion. The average thickness is 200-330 ft.

Oil was trapped in the Cambrian reservoir mainly during the Mesozoic Period; had entrapment occurred earlier, hydrocarbons would have escaped during the pre-Triassic erosion.

The Hassi Messaoud oil field is operated jointly by S.N. REPAL and CFP(A); it has produced, since 1958, more than 600 million bbl of oil.

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. Compression plants have been operating since January 1964. Tests of alternate injection of gas and water have been made.

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