Minapuye I. Odigi and G. C. Soronnadi
The continental to deltaic Asu River Group in the Afikpo basin, south-eastern Nigeria, is mainly composed of arkosic sandstones with minor proportion of volcanic rock fragments and calcareous subarkosic sandstones. The arkosic sandstones are cemented mainly by calcite with minor quartz cement; while the calcareous sandstones have calcite as major cements. Petrographically, the sandstones and calcareous sandstones can be divided into four facies: the basal conglomerate to coarse grained sandstones, medium-fine grained sandstones deposited in a fluvial non-marine environment; calcareous siltstones and calcareous subarkosic sandstones. The calcareous sandstones are bioclastic grainstones and packstones respectively deposited in a estuarine to shallow shelf marine environment. Petrographic investigations indicate that diagenetic processes which have modified the Awi and Awe Formations respectively include micritization, cementation, dissolution, neomorphism and compaction. The calcareous sandstones have undergone diagenetic alteration under low temperatures and pressures. Alteration started with pore-space reduction by compaction and was followed by pore-filling cement. Dissolution at the surface, however, has caused secondary porosity. The sandstones have a lower porosity due to a higher degree of cementation. The higher porosity in the calcareous sandstones is due to dissolution of feldspars; and are better sorted and more loosely packed. The diagenetic history can be divided into three stages: marine, near surface, and burial. Each of these is characterized by differing degrees of porosity formation and cementation. Porosity in the Asu River Group is general either primary (intergranular and intragranular) or secondary, enhance by dissolution and fracturing (during tectonic movement) of the calcareous sandstones. It varies from zero to 6.5% (based on visual estimation) and includes both fabric – selective and non fabric-selective types. Fabric-selective porosity includes mouldic, intergranular and intragranular types. Difference in porosity within the facies is attributed to variations in the nature of the diagenetic fluids with depth. Dissolution in the Asu River Group probably reflects circulation of aggressive fluids which were undersaturated with respect to calcite (presumably meteoric waters). An influx of meteoric waters can be linked to exposure of the Asu River Group Cenomanian uplift. The outcrop reservoir quality studies can be used as a model in the predicting of the extent of diagenesis and reservoir quality in the subsurface Niger Delta.
