Sedimentary facies, paleoenvironments and reservoir potential of the Afikpo Sandstone on Macgregor Hill area in the Afikpo Sub-basin, southeastern Nigeria

Abstract

Sedimentary facies, petrographic and textural analyses of sections of the Afikpo Sandstone on Macgregor Hill area in the Afikpo Sub-basin, southeastern Nigeria, were carried out to evaluate the paleoenvironments and reservoir potential. Result of the sedimentary facies analysis revealed eight lithofacies grouped into estuarine and subtidal sand ridge/foreshore lithofacies associations. The facies include planar cross-stratified sandstone, bioturbated sandstone, trough cross-stratified sandstone, parallel laminated sandstone, bioturbated cross-stratified sandstone, sandy bioturbated heterolith, conglomerate and laminated mudstone. The estuarine lithofacies associations show tripartite subdivision into a mudflat deposits, fluvio-estuarine sandstones in bayhead delta environments and central estuarine dominated by intertidal bayfill and lagoonal muddy deposits. Skolithos–Cruziana ichnogenera of high-energy shallow water were recorded in the bioturbated lithofacies. They showed moderate to high intensity of bioturbations, dominantly and intermittently distributed feeding structures. Petrography of the sandstones shows that samples from different lithofacies differ in compositions and textures. Generally, the sandstone consists of mainly quartz and subordinate weathered feldspar and metamorphic rock fragments as framework elements. The textural characteristics of the Afikpo Sandstone using univariate parameters of mean, standard deviation, skewness and kurtosis and the multivariate discriminate functions suggest the sediments were deposited in high-energy fluvio-marine setting. The sandstones showed good porosity range from 0.485 to 0.499 with an average value of 0.489 and the empirical permeability of 26.03–42.64 md with an average value of 39.86 md, which reveal the sandstones as good reservoirs indicating the Afikpo Sandstone as potential for hydrocarbon reservoir for subsurface analogue.