Middle Miocene palaeoenvironments of the Niger Delta

Abstract

In order to recognize the palaeoenvironments of the deltaic sequence of the Middle Miocene oil-bearing E2.0 Reservoir in the Kolo Creek field, investigations of sediment grain size, textures, structures, trace fossils, mineralogical composition, diagenetic imprints, palynodebris types, palynomorphs and foraminifera have been carried out on materials from two boreholes. Using this multidisciplinary approach the following have been identified in the reservoir: ten lithofacies, two palynofacies associations, a total of 103 palynomorph taxa and a low diversity assemblage of benthic foraminifera. These criteria have provided information on the positions of the deposits relative to the palaeocoastline as well as the physical and ecological conditions within the delta. The overall depositional conditions were found to be broadly similar to those in the present-day Niger delta, although the climatic conditions were somewhat drier in the Middle Miocene, as reflected in the abundance of the grass pollen Graminidites. Seven, or possibly eight, deltaic sub-environments of deposition have been recognized based on an interpretation of gamma ray and resistivity log profiles as well as sedimentological and microfossil data from cores of two boreholes in the field. These depositional settings are the distributary channel-fill, tidal channel-fill (?), lagoon/tidal flat, lagoonal delta, flood-tidal delta in the lower delta plain, the coastal barrier, and the delta fringe and prodelta in the outer delta. The two palynofacies associations A/C and B/D which emerged from average linkage cluster analysis of the palynodebris are environmentally significant.