Petrography and palynology of quartzites in the Sirte Basin, central Libya

Abstract

Petrographic and palynological analysis and wireline log correlations of the so-called pre-Upper Cretaceous ‘quartzites’ in the central Sirte Basin permits, for the first time, the recognition of the development of six types of quartzites and sandstones of Late Cambrian, Late Devonian, Devonian-Carboniferous and Early Cretaceous age. The Late Cambrian section age, dated by the presence of associated marine acritarchs Vulcanisphaera turbata and Retisphaeridium dichamerum, comprises two shallow marine quartzite units separated by a varicoloured argillaceous feldspathic sandstone unit (Red Beds). The two quartzite units show tidal bedding and mud couplets. They were probably deposited in a subtidal environment. They form fractured reservoirs in the giant Attahaddy Gas Field. The varicoloured sandstone unit (Red Beds) is rich in Skolithos and was probably deposited in an intertidal environment. A Late Devonian (Strunian) shallow marine to continental sandstone unit age dated by the marine acritarch Gorgonisphaeridium solidum and the land-derived spore Verruciretusispora cf. famenensis has been identified in wells V1-59 in the southwest of the study area and EE1-59 in the southeast. A Carboniferous (Visean) sandstone unit age dated by the land-derived spores Aratrisporites saharaensis and Calamospora sp. has been identified in wells C3-6, KKKK1-6, BBBB2-6 and V1-59. The Early Cretaceous continental fractured quartzite unit dated by tricolpate pollen produces oil in the Wadi Field. The study reveals that the Devonian and Carboniferous sandstones, with good primary porosity, previously considered Nubian and Cambro-Ordovician, respectively, form an attractive hydrocarbon-bearing trend in the southern part of the study area.