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Regionally, well-defined belts of lowest Eocene (Ypresian) Metlaoui carbonates trend northwest-southeast. On the northeast is an open-marine, basinal facies of planktonic foraminiferal micrite and marl. Thick bars of shallow marine nummulitic wackestone, packstone, and grainstone trend northeastward at an angle to the paleoshelf. Lagoonal or supratidal carbonates are widespread between the shelf deposits and thick evaporites that crop out in intermontane basins.
The reservoir is confined largely to nummulitic packstone, and visible effective porosity is best developed between forams in zones filled with sand-size debris where secondary solution-enlargement has occurred. Porosity within nummulite chambers, while abundant, is ineffective, although a few open fractures were observed in cores. This lithology tested oil in 2 recent wildcats and is a commercial reservoir at Sidi El Itayem and Ashtart fields.
Distribution of Zebbag carbonates of Late Cretaceous (Turonian) age is more complex. A northwest-southeast-trending platform is bounded on 3 sides by basinal shale and micrite with planktonic forams which grade into a transitional facies of micrite and wackestone that shows some evidence of shallow-water deposition, such as dolomitization, bioclasts, rare ooliths, etc. Predominately back-reef and lagoonal bioclastic wackestones and packstones occur in narrow belts, apparently controlled at least locally by block faulting. The rest of the platform lithology comprises mostly dolomite and dolomitic limestone.
The most significant porosity is interparticle (generally solution-enlarged) in foram packstones, but intraparticle porosity in forams and rudists commonly enhances the reservoir. Intercrystalline porosity in dolomitized zones is common, and fenestral porosity occurs in a few places. All are modified by nonfabric-selective channel and vuggy porosity and in some instances by fractures.
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