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

摘要

> Sedimentological and ichnological study of cored reservoirsands correlated with wireline logs between two wells, 0.8 km distance apartalong dip direction, enabled vertical and cross sectional facies variabilityassessment aimed at determining intra sand-body continuity in longitudinal ordown-dip direction; and sub-environments of deposition and factors thatcontrolled depositional processes. Ten lithofacies described 62 meters cores ofthe reservoir sands and seals. Sub-environments of deposition identified withlithofacies associations include proximal delta front-mouth bar, distaldelta-front, prodelta-offshore, transgressive marine sandstone and tidal flat.High mica content, poor sorting, very coarse quartz grains, high angle beddingcontact, micro-slump folds and absent to sparse bioturbation at the base of anupward-coarsening sequence indicated mouth bar deposition and direct link to adistributary channel. The study of vertical and lateral intra-reservoirdepositional trends indicated that sediment structural and textural (grainsizes and biogenic features) heterogeneities in the deltaic deposit werecontrolled by variations in physical energy and mixed interactions of seallevel changes, tide, wave, fluvial influx, storm, food supply and oxygenlevels. Consequently, there is down-dip lithofacies heterogeneity, pinch out oflithofacies or gradation from coarse grains to finer grains and better sorting.Though ichnodiversity is fairly uniform between the two wells, ichno-abundanceand burrow sizes decrease down-dip especially at the proximal delta front-mouthbar deposit. The results of this study improve our knowledge of thecharacteristics of a mouth bar deposit in a mixed-processes deltaic environmentand it can be applied in the characterization of delta front deposit elsewherewith similar depositional processes and tectonic setting.