Volume 7 Article informations Pore-lining and grain-coating chlorites in siliciclastic reservoir sandstones (Acacus formation, Ghadamis basin, Libya), nature and implications for their origin Eman AB Moh Taktek , Narjes M Omar Shebli Abstract The Acacus formation is found in western Libya in the Murzuq and Ghadamis basins. It outcrops in the southwest on the margins of the Murzuq Basin and continues northward in sub crop into the Ghadamis basin. The formation thins towards, and is then absent over, the Qargaf Arch which separates the two basins. These relationships reflect the tectonic history of the region and the development of the major Paleozoic and Mesozoic sedimentary basins which dominate the structural and stratigraphical framework of Libya. Evaluation of core samples and composite well logs shows that the Acacus Formation in the Ghadamis Basin can be subdivided into three members. The lower and upper members are sand dominated whereas the middle member is mud dominated. Textural and compositional analysis of the sands and muds of the studied formation demonstrate that they are commonly very iron-rich. The framework silicate grains are dominantly with quart, feldspar including k-feldspar. Muscovite is an important component in some sandstone levels forming thin mica-rich laminae. Glauconite and detrital chamosite occur as replacement of mica and faecal pellets. Chamosite is also present as the cortex of ooids which have been transported into the sands from an adjacent ooid "factory'. These have been classed as inherited grains. Detrital siderite is also present. Phosphate fragments of teeth, bone and scale tend to occur at the top of the iron-rich layers although apatite is also present as rare grain-coating. Chlorite has been found in the formation with the four lithofacies. They fall into three categories: uncoated grains; coated grains or ooids and mica replacement grains. Features of these grains, suggests that the final event in their history was as detrital particles although the genesis of the chlorite component was authigenic. Key words: Mineralogy, Sandstone, Diagenetic features, Chronological relationships, Ghadamis, Libya