The Influence of Edge Fresh Water on the Giant Zelten Oil Filed, Central Sirt Basin, Libya

Abstract

The Zelten oil field in the Sirt Basin, central Libya, primarily consists of Upper Landenian age fossiliferous limestone, introduced by Barr and Weegar in 1972. It occasionally contains anhydrite dolomite and shale. It's bounded by the Khalifa or Gir Formation below and the thinner Harash Formation above. Zelten Formation's depositional cycle is similar to Harash Formation but relatively thinner in thickness. The Zelten oil field, part of the Zelten platform, is a giant field in Concession 6, about 180 km south of the Mediterranean coast. It comprises three main oil pools, discovered in 1959 by Esso oil company. The north pool's discovery well, C1-6, reached peak production of 1/2 Million BOPD. Cumulative production by June 1997 includes 2.254 BBls oil, 2.945 BBls water, and 1.497 TCF of gas, with a recovery rate of 47.1% of the original oil in-place. Zelten Formation's porosity is influenced by sea level fluctuations and carbonate build-up facies, reaching 40% around the crest area of the three structures. Hydrocarbons are sourced from Cretaceous shales in the Hagfa Troughs. The Zelten Formation reservoir age is Paleocene limestone, with average net reservoir thickness of 325ft, 22% porosity, and permeability up to 1500 md. Reservoir pressure remained stable at around 2,250 psi for over 60 years, thanks to the bottom-edge fresh water from the Domran Formation, faulted down adjacent to the Zelten Formation reservoir, providing a strong edge water drive. This drive maintains pressure and can be used for enhanced oil recovery. Over 13 wells in the Zelten field, including horizontal wells, have contributed to hydrocarbon production.