Federal University of Lafia, Nasarawa State, Nigeria
Abstract
The Eocene Nanka Formation at Ogbunike type localtiy was studied to ascertain the lithostratigraphy, petrophysical properties and depositional environment. Petrophysical properties of reservoir rocks are influenced by current velocity, degree of current variance and grain size parameters. These properties are partly controlled by facies characteristics which in turn are related to depositional environment. Field mapping utilizing bearing and pacing, and spot sampling based on lithologic variation was adopted along six lithogic profiles. Measurement of azimuths and dips of cross beds were taken for paleocurrent analysis. A total of 18 representative samples were collected for granulometric, thin section and heavy mineral petrographic analyses. Detailed sedimentology field study and lithologic description revealed the section to be 14.78m thick (vertical thickness) with a lateral extent of about 82m consisting of ironstone/sandstone, pyritic grayish shales, interstratified claystone/mudstone and heterolite. Azimuths of NE-SW and NW-SE directions described the paleocurrents of the formation with dominant polar-unimodal orientation, high vector strength (mean of 14.18o) and low variance ranging from 248.5 to 368.9. The sediments are bimodal to polymodal suggesting different parent sources, predominantly positively skewed, leptokurtic and dominantly moderately sorted suggesting fair porosity, fair permeability, heterogeneous reservoir unit, moderate energy of transportation medium and fluvial environment. Heavy minerals found indicated mixed provenance and mineralogical maturity. The study revealed that the sediments are ferrugineous, texturally sub-mature to mature, mineralogically super mature,fine to medium grained, moderately sorted and deposited in a near-shore fluvial environment under a moderate energy condition. The sediments are characterized by fair permeability and effective porosity, heterogeneous toslightly homogeneous reservoir unit
