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The AAPG/Datapages Combined Publications Database
Journal of Petroleum Geology
Abstract
Journal of Petroleum Geology, vol.
Three major lithofacies are recognized in the Jdeir Formation: Nummulites packstone-grainstone, Alveolina-Orbitolites wackestone-packstone, and Fragmental-Discocyclina-Assilina wackestone-packstone, deposited in bank, back-bank, and fore-bank environments, respectively. The formation passes to the NNE into the pelagic lithofacies of the Hallab Formation; landward, to the south, it passes into shoreline evaporitic facies of the Taljah Formation. The lithofacies were structurally controlled by contemporaneous and/or syndepositional tectonic movements, with nummulitic facies tending to develop on uplifted areas.
Petrographic and petrophysical studies indicate that porosity in the Jdeir Formation is controlled by depositional environment, tectonic setting and diagenesis. The combined effects of salt tectonics, amajor unconformity at the top of the formation and meteoric diagenesis have produced excellent-quality reservoir facies at the Bouri oilfield and in other areas.
Porosity is highest in the nummulitic bank facies and lowest in the Alveolina-Orbitolites micrite facies. Good to excellent reservoir quality occurs in the upper part of the nummulitic packstone-grainstone facies, especially where these sediments overlie structurally high areas. High rates of dissolution found at the crests of domes and anticlines suggest that early diagenetic processes and features are, in part, structurally controlled. Future exploration success will depend on investigation of similar structures within the Gabes-Tripoli Basin.
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* Department of Earth Sciences, University of
Manchester Manchester M13 9PL. [email protected]
** Petroleum Research Centre, Tripoli, Libya.
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Both porosity initiation and preservation are related to early depositional and diagenetic processes. The wide time-gap between hydrocarbon generation and reservoir formation points to the role of the seal in porosity preservation and rules out the assumption that early emplacement of oil had preserved the porosity.
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