Depositional Environment and Diagenesis of the Eocene Jdeir Formation, Gabes‐tripoli Basin, Western Offshore, Libya

Abstract

The late Ypresian (early Eocene) Jdeir Formation was deposited in the Mesozoic‐Cenozoic Gabes‐Tripoli Basin, offshore Libya. The basin developed on the northern passive margin of the African Plate and was relatively unstable being affected by syn‐sedimentary tectonic movements. Deposition was coeval with a relative rise of sea‐level and the subsequent highstand. A lower, thinly‐developed nummulitic bank facies with restricted distribution records the transgressive event and is succeeded by more micritic sediments that record the time of maximum flooding. The succeeding sea‐level highstand is represented by a thick, and widely developed, progradational‐aggradational nummulitic sequence that displays lateral changes across WE‐ESE trending facies belts. Three major lithofacies are recognized in the Jdeir Formation: Nummulites packstone‐grainstone, Alveolina‐Orbitoliteswackestone‐packtone, andFragmental‐Discocyclina‐Assilina wackestone‐packstone, depositedin 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, a major 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. 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.

Hydrodynamic behaviour of Nummulites: implications for depositional models

“…• The Nummulites accumulation results from a high prolific biocoenose and is not affected by bottom currents as suggested by Nemkov (1962), Arni (1965), Kulka (1985) and Anketell and Mriheel (2000) • The Nummulites accumulations are controlled by a reworking processes, but high-energy primary structures are destroyed by bioturbation, which is often observed (Moody and Grant 1989;Loucks et al 1998;Moody et al 2001;Racey 2001;Racey et al 2001) • The Nummulites deposits are formed in low-energy environments, i.e., either below the storm wave base or in a protected area that could have been created by the presence of a physical barrier or by the presence of dense seagrass meadows (Blondeau 1972).…”

“…These banks, which are characterised by a mono-specific association of Nummulites, separate a restricted area (back-bank environment) from an open marine zone (fore-bank settings). This model has been applied to the Tatra Eocene of Poland (Kulka 1985), to the Middle Eocene build-ups in Egypt (Aigner 1983), and to the Jdeir Formation in offshore Libya (Anketell and Mriheel 2000) • Low-relief banks or sheets which are developed along a broad, gently dipping homoclinal ramp (Comte and Lehman 1974;Loucks et al 1998;Moody et al 2001;Hasler 2004). The petrographic composition of the sedimentary body is controlled by physical processes such as winnowing of both the matrix and smaller A-forms (Aigner 1982(Aigner , 1985Racey 2001).…”

“…The reservoir qualities are mostly induced by the preservation of intraskeletal porosity of Nummulites tests. Numerous sedimentological studies have been made on these deposits in order to better understand the geometry of subsurface reservoirs (Arni 1965;Comte and Lehman 1974;Fournié 1975;Bishop 1985;Moody 1987;Bailey et al 1989;Moody and Grant 1989;Bernasconi et al 1991;Loucks et al 1998;Anketell and Mriheel 2000;Racey et al 2001;Jorry et al 2003a,b;Vennin et al 2003;Hasler 2004;Jorry 2004).…”

Evolution of shallow benthic communities during the Late Paleocene–earliest Eocene transition in the Northern Tethys (SW Slovenia)

“…Today, most of the species belonging to this genus colonize the deep environments, down to the lower limit of the photic zone (Langer and Hottinger 2000). Fossil orthophragminids have been described from a diverse array of environments within the photic zone including shallow back-and fore-reef/shoal environments (e.g., Anketell and Mriheel 2000;Ghose 1977) to deeper, outer ramp environments (e.g. Gilham and Bristow 1998).…”

Geochemistry of carbonate cements in Cretaceous sandstones, southeast Benue Trough, Nigeria: Implications for geochemical evolution of formation waters

“…10), reflecting diminishing early cement values and increasing proportions of later cement. Progressive decrease in oxygen-isotope composition primarily reflects increasing temperature or fractionation of the precipitating waters (Land et al, 1975;Anderson and Arthur, 1983;Ankertell and Mriheel, 2000).…”