Sedimentologic study of the Nubian Formation (? Late Jurassic-Early Cretaceous) of the Sirte basin, Libya.

摘要

Five lithofacies were observed in the Nubian Formation of the study area, and were alphabetically designated A through E. Lithofacies A, B, and C were interpreted as fluvial deposits, in which rivers were flowing to the north and northwest, and in which vertical and lateral lithofacies changes resulted from channel migration in response to changes in slope through time. Lithofacies D and E were interpreted as swamp and lake deposits.; The rocks of Lithofacies C are characterized by the best porosities up to 27% and permeabilities up to several thousand md and good vertical continuity of sandstone bodies. Lithofacies B also has good reservoir quality, but the effective porosity and permeability are lower, because of associated shales. Lithofacies A has relatively low porosities and permeabilities. Lithofacies D and E have negligible reservoir potential. The most important diagenetic features responsible for a reduction in reservoir quality are those resulting from compaction, cementation and the genesis of clay minerals.; The upper parts of Members 1 and 3 are dominated to a large extent by the porous and permeable rocks of Lithofacies B and C, especially in the upland areas near the eastern margin of the Sirte basin region and associated with elevated basement rocks south of the Faregh district. Truncation and wedge-out of these lithofacies against paleotopographic high areas and directly below the Upper Cretaceous are thought to have formed stratigraphic and structural-stratigraphic traps for hydrocarbons which merit consideration as prime exploration targets. (Abstract shortened by UMI.)