Sedimentology and Ichnology of Late Oligocene Delta Front Reservoir Sandstone Deposit, Greater Ughelli Depobelt, Niger Delta

Abstract

Sedimentological and ichnological study of cored reservoir sands correlated with wireline logs between two wells, 0.8 km distance apart along dip direction, enabled vertical and cross sectional facies variability assessment aimed at determining intra sand-body continuity in longitudinal or down-dip direction; and sub-environments of deposition and factors that controlled depositional processes. Ten lithofacies described 62 meters cores of the reservoir sands and seals. Sub-environments of deposition identified with lithofacies associations include proximal delta front-mouth bar, distal delta-front, prodelta-offshore, transgressive marine sandstone and tidal flat. High mica content, poor sorting, very coarse quartz grains, high angle bedding contact, micro-slump folds and absent to sparse bioturbation at the base of an upward-coarsening sequence indicated mouth bar deposition and direct link to a distributary channel. The study of vertical and lateral intra-reservoir depositional trends indicated that sediment structural and textural (grain sizes and biogenic features) heterogeneities in the deltaic deposit were controlled by variations in physical energy and mixed interactions of seal level changes, tide, wave, fluvial influx, storm, food supply and oxygen levels. Consequently, there is down-dip lithofacies heterogeneity, pinch out of lithofacies or gradation from coarse grains to finer grains and better sorting. Though ichnodiversity is fairly uniform between the two wells, ichno-abundance and burrow sizes decrease down-dip especially at the proximal delta front-mouth bar deposit. The results of this study improve our knowledge of the characteristics of a mouth bar deposit in a mixed-processes deltaic environment and it can be applied in the characterization of delta front deposit elsewhere with similar depositional processes and tectonic setting.