Zelten Oil Field - Sirte-Zelten Petroleum System

Sirte-Zelten Petroleum System

There is one dominant petroleum system in the Sirte Basin sourced by the Cretaceous Sirte Shale. In the Central Sirte Basin, carbonates of Upper Cretaceous, Paleocene and Eocene age produce in 150 fields on significant horst blocks or platforms (e.g., the Beda and Zelten platforms). Reservoirs are largely carbonate build-ups and reefs with some offshore production in the Gulf of Sidra. Carbonate reservoirs are largely related to two Upper Cretaceous and two Paleocene cycles of sedimentation in a syn-rift fill sequence; however, there is minor Mid-Cretaceous calcarenite production. Giant oil fields in this unit include Intistar, Beda, Defa, Waha, Haram, Zelten (Nasser), Hofra and Nafoora.

Upper Cretaceous Sirte Shale of the Rakb Group is the dominant source rock. Geochemical data of 81 oils demonstrate that the Sirte Shale generates a low sulfur, high gravity oil with low gas oil ratios. Generation began in Eocene era and continues to present. Onshore the petroleum is dominantly oil; however, offshore increased thermal gradients and deeper burial suggest higher gas oil ratios and natural gas potential. The Upper Cretaceous carbonate reservoirs are commonly dolomites whereas the lower Paleocene cycles include calcilutites, calcarenites, oolites and skeletal debris. The petroleum system is named after the Zelten Formation, which was deposited in this latter Paleocene transgression.