Depositional Environments and Reservoir Quality Potentials of Campanian Sediments at Macgregor, Afikpo Basin, Nigeria

Abstract

A road-cut section and a quarry face of outcropped Campanian sediments of Afikpo basin were studied to determine the environments of deposition and the reservoir quality potentials of facies. Lithofacies identified include Shale, interlaminated silt and clay, wavy-laminated sandstone, burrowed cross laminated sandstone, cross bedded sandstone and thick/very thick bedded sandstone. To determine intra-facies grain size trend, samples were collected at the base and at the top of facies. Nine samples were analysed for grain size/granulometric properties distribution. Granulometric parameters/properties were determined using Gradistat software. Grain size distribution curves of all sandstone facies showed that particle grain sizes ranged from fine to pebbles and bimodal which implied the combination of fluvial and marine sourced sediment typical of an estuary. The bivariate plots of intra-facies granulometric parameters however showed the domination of fluvial supply/processes. Facies characteristics and associations indicate that the Campanian sediments were deposited in estuary sub-environments which include Bayhead delta, Tidal inlet fill and Offshore/offshore transition zone. Bayhead delta sandstone is lower coarse- to very coarse- grained, poorly sorted, near symmetrical to positive skewed, varied kurtosis and has an internal depositional surface lined with impermeable mud. Therefore, it has potential for low quality reservoir.  But the shoreface sandstone is generally moderately sorted, very fine- to coarse-grained positive skewed, platykurtic to mesokurtic and therefore has potential for good quality reservoir. While tidal inlet fill sandstone is homogenous, mean upper medium grained, moderately sorted, positive to strongly positive skewed and mesokurtic; and therefore, has potential for the highest quality reservoir.