Petrography, Diagenesis and Reservoir Development of Sandstones in the Asu River Group, Lower Benue Trough, Southeastern Nigeria

Minapuye E. Odigi
niversity of Port Harcout, Port Harcourt, Nigeria

The continental to deltaic Asu River Group in the Afikpo basin, southeastern 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 cements. Three lithostratigraphic intervals can be identified: an arkosic basal conglomeratic unit at the base; a sequence dominated by dark shales and sandstones in the middle; and a section dominated by calcareous arkosics at the top. Overlying the succession is an extensive intrabasinal conglomeratic unit, leached limestone bed and hardground surface of Mid- Cenomanian age.

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 bioiclastic grainstones and packstones respectively deposited in a shallow shelf marine environment.

The sandstones and 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. Decrease in clay content is recorded in the calcareous sandtones. Difference in clay mineralogy can be explained by the difference in permeability and fluid flux, with high permeability occurring in the calcareous intervals with secondary porosity. Porosity is best developed where there is alteration of feldspar, early silicification that has enhanced grain support and which has minimised the effects of late compaction.