Abstract
The Lower Cretaceous Nubian Formation forms the basal part of the sedimentary sequence in Abu Attifel oil field. It is unconformably overlain by Upper Cretaceous clastic rocks (Member VIII of Rakb Group), and the lower boundary is not reached. The Nubian Formation represents fluvial deposits of braided and meandering river systems.
On the basis of detailed petrographic analyses several controls on porosity and permeability are designated to explain the present characteristics of the Nubian reservoir in Abu Attifel oil field. Primary controls, represented by the variation in the primary sedimentary texture of the facies, show a clear regular relationship to porosity and permeability values distribution. The highest porosity and permeability are present in the braided river and point bar deposits and, in contrast, the overbank deposits have lower porosity and permeability.
Secondary controls are represented by the product of the diagenetic processes particularly cements (silica, dolomite and clays) and dissolution of unstable constituents (feldspar and matrix). The porosity and permeability in the upper part of the reservoir (braided river deposits) are mostly reduced by precipitation of silica, ferroan dolomite, and formation of kaolinite, whereas in the lower part of the reservoir (meandering river deposits) they are mostly reduced by quartz overgrowth, and formation of authigenic chlorite which can have a remarkably important effect in reducing the permeability. With further burial significant dissolution of feldspar, some authigenic cements (kaolinite), and matrix generate secondary porosity and enhance the permeability.