Integrated Sedimentological Characteristics and Hydrocarbon Prospectivity of the Ogwashi–Asaba Formation, Southeastern Nigeria

Abstract

The Ogwashi–Asaba Formation in southeastern Nigeria comprises interbedded sandstone, shale, and lignite units that record variations in depositional processes and provide a basis for preliminary assessment of hydrocarbon prospectivity within the Anambra Basin. This study evaluated the sedimentological characteristics, petrographic features, organic matter content, and depositional significance of selected outcrop successions. Field observations were integrated with grain-size analysis, petrographic examination, and total organic carbon determination using the Walkley–Black wet oxidation method. The sandstone units are predominantly fine- to medium-grained and variably sorted, indicating fluctuations in depositional energy. Petrographic examination shows quartz-rich sandstones with minor feldspar, rock fragments, and accessory minerals, consistent with compositional maturity, sediment recycling, and prolonged transport prior to deposition. Shale and lignite intervals contain appreciable organic matter and represent comparatively low-energy depositional conditions favourable for organic matter preservation. Integrated sedimentological and geochemical evidence supports deposition within a dynamic transitional system influenced by fluvial, deltaic, swamp, lagoonal, and shallow marine processes. Organic-rich shale and lignite intervals indicate preliminary source-rock potential, whereas compositionally mature sandstone units may have reservoir relevance. However, the available evidence is insufficient for definitive petroleum-system evaluation, and further analyses, including kerogen typing, Rock-Eval pyrolysis, vitrinite reflectance, and biomarker studies, are required to constrain hydrocarbon generation potential, thermal maturity, and petroleum-system significance.