The Intisar "D" reef oil field was discovered by Occidental in October 1967; the discovery well tested 75,000 b/d. The prospect was based on reflection seismic data, which indicated the presence of an isolated reef. Three such prospects were drilled previously with varying degrees of success.

The Paleocene of the Sirte basin is characterized by carbonate rocks and shales deposited in an epeiric sea. The Intisar reefs grew in a late Paleocene embayment bounded on three sides by carbonate banks. Three distinct stages of organic development are recognized.

The Intisar "D" reef is roughly circular in plan and approximately 5 km in diameter. Its maximum thickness is 1,262 ft (385 m). The reef is coral and algal with grain-and mud-supported biomicrites. Porosity averages 22% and is mostly solution and intergranular. Measured permeability is as high as 500 md and averages 87 md. The main reservoir is remarkably homogeneous without noticeable layering typical of other reefs in the area.

The reef was full to spill point with a maximum oil column of 995 ft (291 m). The 40° API gravity oil has a paraffinic base and is low in sulfur. The original gas/oil ratio (GOR) was 509 cu ft/bbl. Original stock tank oil in place is estimated at 1.8 billion bbl. The field currently produces 200,000 b/d oil from 13 wells; 11 water-injection and 7 gas-injection wells are used. Cumulative oil production as of September 30, 1978, totaled 777 million bbl. Ultimate recovery efficiency is expected to approach 75%.

No pressure support was expected. Supplemental recovery operations began early and include pressure maintenance by both water and gas injection. The reservoir pressure is now maintained at the 4,000-psi level, high enough for miscible gas displacement.