Abstract Hassi Messaoud field, in the eastern part of the Algerian Sahara Desert and 350 mi (560 km) from the Mediterranean, produces 375,000 bbl/day, or about 40 percent of Algeria's production. The field is operated by SN REPAL and CFP(A). Oil is produced from a Cambrian sandstone reservoir in a large dome. Producing depth is about 11,000 ft (3,300 m), the oil column is about 900 ft (270 m), and the productive area is 500 mi 2 (1,300 km 2 ). A total of 178 wells had been drilled to the end of 1967, 39 of which were dry or produced water, and 30 of which were producing at a rate greater than 6,000 bbl/day. In-place reserves throughout the Hassi Messaoud field are approximately 25 billion bbl; cumulative production to the end of 1967 reached 720 million bbl. Reservoir porosity ranges from 2 to 12 percent, with an average of 8 percent; permeability ranges from 0 to 1,000 md. Accumulation is in a combination trap. On the flank, closure is structural; on the crest, it is associated with an unconformity at the base of the Triassic. The structure was formed by a late phase of Hercynian (late Paleozoic) orogeny followed by erosion that cut to the Cambrian on the crest of the field.