ABSTRACT: Over 200 core samples from 2 wells in the Gialo High of the Sirt Basin, Libya, have been selected for micropaleontological study. A number of silicified marine microfossils have been discovered for the first time in the Nubian Sandstone that have been long considered as of terrestrial origin prior to Neocomian time. Silicified fossils show that in the Gialo area the Nubian Sandstone is moderately to severely cemented by silica were deposited in subtidal to supratidal environments with low amplitudes of sea-level fluctuation. The main body of the Nubian Sandstone is dated as Albian using silicified microfossils and nannofossils, although a few Aptian deposits might exist in the lowermost part of Well I. Four types of silicified microfossils have been recognized. Type I bears calcareous cortex, and is partially replaced by amorphous silica with distinct original features of the microfossils. Type II is characterized by replacement by amorphous silica and by moderately preserved interior and exterior features on smooth surface. Type III is characterized by complete replacement by microcrystalline quartz and by poorly preserved features on surfaces. Type IV is characterized by complete replacement by megacrystalline quartz and by the total loss of interior and exterior features. These silicified microfossils are considered to have formed during different stages of diagenesis. Type III and IV (particularly Type IV) are associated with severe cementation of silica in the coarse sandstones. Well developed primary interstitial conduits between the coarse sand grains may have allowed the movement of large volumes of silica-bearing fluid resulting in more severe precipitation, cementation and recrystallization of silica than in fine grained sandstones. Fossil silicification in the Gialo High appears to be linked to the regional and local tectonic and hydrothermal activities related to frequent phases of faulting and volcanism. Quartz cementation and overgrowths in the Nubian Sandstone during the post-Cretaceous indicated by silicified microfossils have caused great reduction or demise of porosity and permeability, which negatively affect reservoir quality. The discovery of the silicified microfossils in the Gialo opens a new window for biostrati-graphic study, paleoenvironmental reconstruction, prediction of porosity and permeability, and basin modeling for oil and gas exploration success in the Sirt Basin, and possibly the other rifting basins elsewhere in the world.