SEDIMENTOLOGY OF EOCENE SANDSTONES AT OGBUNIKE AREA, SOUTHERN NIGERIA: INSIGHTS FROM PETROGRAPHIC AND GRANULOMETRIC ANALYSES

Abstract

The sedimentary facies exposed at Ogbunike Area belong to the Eocene Formation that made up the Anambra Basin. This research estimates the lithology, thickness, sedimentary succession, paleo-environment, reservoir quality and source area tectonics using geological mapping, petrography, grain size distribution and statistical analysis. Geological mapping shows that the section is approximately 27m thick and consists of sandstone, shale, siltstone and ironstone lithofacies. Granulometric analysis performed on selected samples shows that the sandstones are fine to medium grained (1.15 to 2.17 ϕ), moderately sorted (average 0.96 ϕ), fine skewed (average 0.16 ϕ) and leptokurtic (average 1.17 ϕ). Bivariate plot of skeweness against sorting and mean against sorting pinpoint a fluvial origin for the sandstones. The results from rose plot point to a bimodal-bipolar paleocurrent pattern while the bi-directional paleo-flow is in the northeastern-southwestern directions, which is an indication of tidal influence suggesting shore environment of deposition. Average permeability values derived from an empirical formula indicate good potential for reservoir rock. Deductions from field observations and petrographic analysis suggests that the sandstones are submature, with angular to sub-rounded quartz. The high percentage of goethite (average 30.4%) in the ferrugenized sandstones samples suggests an environment of low oxidation. Provenance and tectonic assessment reveal metamorphic source, humid climate, high relief, recycled orogeny, and derived primarily from Cameroon Basement Complex rocks.