Provenance and Paleoenvironments reconstruction of Sandstones from Onshore Dahomey Basin, Southwestern, Nigeria: Implications for Reservoir Potential

Abstract

The aquiferous sandstone units in Daniel-1, Olambe and Yakoyo boreholes located in the onshore part of the eastern Dahomey Basin, Southwestern Nigeria, were investigated through grain size, petrography and geochemical studies to evaluate their provenance, paleoenvironments and the preliminary aquifer or reservoir quality of the sandstones. Average mean, standard deviation (sorting), skewness and kurtosis across the three boreholes ranges from 0.37 mm – 1.37 mm, 0.54 mm – 0.82 mm, 0.18 mm – 0.84 mm and 0.89 mm – 2.07 mm respectively suggesting dominance of medium to coarse grain, and moderate to moderately well sorted sandstones. The mineral grain sizes, sorting and sub-angular to sub-rounded shapes indicate a sub-mature texture. Mineralogy reveals average 78 % quartz, 9 % feldspar and 13 % rock fragments typical of sub-arkose to quartz arenites sandstones. Major oxides of SiO₂ ranges from 90.70 – 99.62 %, Al₂O₃ 0.49 – 3.34 %, Fe₂O₃ 0.07 – 1.65 %, while other oxides are less than 1.0 % further suggesting dominance of quartz mineral grains. Bivariate plots of Th, La, Sc, Co trace elements further revealed dominance of quartz minerals sourced from felsic granitic igneous rock of a passive margin continental basement. The sub-mature to mature textures and mineralogy coupled with their shallow depths of the sandstones at 130 ft – 160 ft are suitable factors that enhances good porosity and permeability in an aquifer or reservoir rock. These are probably the positive attributes favouring the storage and efficient water production of the aquiferous sandstones. They have similar textural and mineralogy features with the Turonian – Coniacian Afowo sandstones reservoir producing oil and gas in the deep subsurface offshore area of the basin and the onshore Tar sand in Nigeria and part of Benin Republic, West Africa