Petrology and Diagenetical Processes in the Hawaz Formation, a Middle Ordovician Clastic Reservoir of the Murzuq Basin, Libya

Dolors Barsó¹, David Gómez-Gras², Mariano Marzo³, Emilio Ramos¹, Khairi Tawenki⁴, and Nestor Bolatti⁵. (1) Stratigraphy, Paleontology and Marine Geosciences Dep, University of Barcelona, Martí Franqués, s/n, Barcelona, 08028, Spain, phone: (34) 93 402 15 96, fax: (34) 93 402 13 40, dolo88es@yahoo.es, (2) Departament de Geologia, Universitat Autònoma de Barcelona, Facultat de Ciències, Bellaterra, 08193, Spain, (3) Stratigraphy, Paleontology and Marine Geosciences Dep, University of Barcelona, c/ Marti i Franques s/n, E-08028 Barcelona, Spain, (4) REMSA, Trípoli, Libya, (5) Libya Finding Team, REPSOLYPF, Spain

The Hawaz Formation is a Llanvirnian-Llandeilian aged oil bearing detritic unit which constitutes a part of the Paleozoic sedimentary infill in the Murzuq and Kufrah basins, in Central and South Libya. It crops out in the basin margins as tectonic uplifts. This work is based on outcrops of the Gargaf high, in the northerly boundary of the Murzuq Basin, where the Hawaz Formation is mainly composed of sandstones deposited in shallow marine, shoreface and estuarine environments and include a number of thin beds of volcanic ash.

The Hawaz Formation sandstones are fine to medium grained, well sorted and subrounded with low matrix contents. Compositionally, these sandstones are mainly quartzarenites made up of quartz grains and minor amounts of K-feldspar, plagioclase, mica and metamorphic rock fragments. Primary porosity is mostly intergranular and ranges from 0 to 22.5 % having the highest values towards the lower part of the Formation. Secondary porosity is uncommon. Diagenetic processes promote the reduction of initial intergranular porosity via precipitation of authigenic minerals and chemical and mechanical compaction.

The diagenetic sequence starts with the precipitation of early quartz and minor K-feldspar overgrowths prior to the development of mechanical and chemical compaction. Ferruginous coatings precipitated subsequently to this compaction, followed by the neoformation of clay minerals (illite-smectite and kaolinite). Finally, dolomite cement filled the residual porosity. However, the main reduction of the primary porosity was produced by chemical and mechanical compaction. The highest compaction values have been measured in the middle and upper parts of the Formation.

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