Petrography and Diagenesis of the Acacus Formation, Ghadamis Basin, Libya

By

Abdussalam M. Sghair¹

(1) Petroleum Research Centre, Tripoli, 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. Sand-shale ratios for the lower member show that it becomes more mud-rich from south to north and a similar relationship is apparent for the upper member although here there is some evidence that additional sources from the east and possibly from the west began to play a part in introducing sand to the basin.

The texture and composition of the sands and muds of the Acacus Formation in the Ghadamis Basin show that they are commonly very iron-rich. In general the thicker sandstones, with and without mud clasts tend to be the most iron-rich although the are interbedded with some relatively clean thin beds of quartz sandstones. These may represent high energy winnowed deposits where all mud has been removed.

The framework silicate grains are dominantly quartz varieties, both mono-and polycrystalline. Feldspar is rare and usually partially dissolved -it is, however, dominantly 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.

The iron content is comprised largely of pore-lining and pore-filling chamosite and pore filling siderite which comprise the main cements. Iron oxides are very important in the red beds which occur at the top of some of the shoaling-up successions. Quartz overgrowth and rare feldspar overgrowth are also present and quartz overgrowth is particularly important in some of the clean sandstones. Apatite and pyrite are important cement, although it is present in small quantity only.