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Integrated Facies-Based Stratigraphic Architecture, Chemostratigraphy and Diagenesis of the Middle-Late (Ladinian – Carnian) Triassic Al Aziziyah Formation, Jifarah Basin, NW Libya


The Jifarah Basin in northwestern Libya is bounded by the Nafasah uplift to the south and the offshore Sabrath Basin to the north. The Middle-Late Triassic (Ladinian–Carnian) Al Aziziyah Formation is comprised of massive grey limestone, dolomite and dolomitic limestone interbedded with shale. In the north of the Jifarah Basin, Al Aziziyah is a potential oil reservoir interbedded with good source rocks. Eight measured sections (Ghryan Dome, Ras Mazal East, Ras Mazal West, Kurrush Dome, Kaf Bates, Al Aziziyah Town, Ras Lafal and Bu Arghop) record a depositional dip cross section of the Al Aziziyah Formation within the Jifarah Basin. The Al Aziziyah is a 2nd-order (10-20 m.y. duration) subtidal carbonate ramp with tidal flats restricted to the southernmost sections. Inner ramp tidal flats were arid with mudcracks, evaporite nodules and stromatolites. Ramp crest shoals were predominantly pellet packstone-grainstone, whereas subtidal open marine facies are thin-medium beds of skeletal wackestone-packstone with hummocky cross-stratification, mechanical lamination and low to high density bioturbation. Stable isotope (C, O) whole rock chemostratigraphy of the Al Aziziyah Formation at the Ghryan Dome and Kaf Bates sections indicate systematic enrichment and depletion in carbon and oxygen isotopes that can be used to correlate between the sections. The d13C values range from −7.8 to +3.9‰ and the d18O values range from −7.4 to +3.4‰. The diagenetic events of the Al Aziziyah Formation were determined by thin section petrography, cathodoluminescence (CL), stable isotope, and trace element analyses. Four types of dolomite and multiple early and late cement types occur within the Al Azizyah Formation. Dolomite 1 is very fine- to fine-crystalline (crystals > 50 μm) and preserves depositional fabric well. Dolomite type 2 is coarse crystalline dolomite with 100 to 300 μm crystal that destroyed the depositional fabric. Dolomite 3 occurs as a clear outer rim to precursor cloudy coarse dolomite crystals. Dolomite 4 is a late stage coarse to very coarse crystalline dolomite that formed in a deeper, high temperature burial environment. The calcite cement types include early formed rim, whereas late cements calcite (blocky cement) and silica. The d13C values for dolomite and multiple early and late cement types range from −0.7 to +1.8‰ and the d18O values range from −7.6 to +4.1‰ recording different cement generations.