Zelten field is located in the center of the Sirte basin, Libya. Hydrocarbon production at depths of 5,000 to 5,300 ft (1,500 to 1,600 m) below sea level is from the Zelten Member of the Ruaga Limestone of Paleocene and early Eocene age. Fifteen carbonate facies are easily recognized from this producing unit, but most of the production is from two highly porous facies: (1) Discocyclina foraminiferal grainstone and packstone and (2) coralgal wackestone. Core porosity ranges as high as 40%.

Primary interparticle and intraparticle and secondary leached matrix and moldic porosity occur in the Zelten reservoir. High primary porosity was preserved in the grainstones by slight amounts of early cement which prevented compaction. In the packstones and wackestones porosity was preserved by the presence of porous carbonate mud which inhibited cementation. This primary porosity was enhanced by extensive secondary porosity formed by subsequent freshwater leaching during a period when most of the carbonate bank was subaerially exposed.

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