Abstract
The Upper Bahi Formation studied samples are arkosic to subarkosic arenites, deposited under continental and marine conditions. The sandstones are strongly affected by diagenesis. During the eodiagenesis, stage the fluvial sands were subjected to formation of early kaolin, pyrite and heamatite cements, whereas bioturbation, precipitation of calcite and dolomite cements, glauconite and localized pyrite took place in the marine sands. Pressure dissolution (chemical compaction) took place during the mesodiagenesis stage, which supplied silica for the precipitation of first quartz over growths, and thus induced a considerable reduction of primary porosity. Cementation by feldspar overgrowth and carbonates contributed to a considerable reduction of the intergranular porosity. Partial
to complete dissolution, albtization and kaolinization of feldspar grains also occurred and resulted in the enhancement of the
intragranular porosity. Secondary porosity has also been developed by the dissolution of interstitial cements such as carbonates. However, this porosity is partially occluded by clay minerals such as kaolin, illite, and chlorite. These processes were followed by precipitation of quartz overgrowth that engulfed feldspar overgrowths and authigenic clay minerals, as well as local development of non-ferroan dolomite and an hydrite cements. Overburden sediments increased the compaction effects, and the action of formation fluids resulted further alteration and dissolution of the framework grains which enhanced the porosity, but precipitation of late cements had a reversed effect.This study has shown that the diagenetical teration have had a considerable impact on reservoir quality evaluation and on the original composition of the sandstones.