A Re-evaluation of the Depositional Environments and Sedimentary Facies of Middle Miocene Paralic Deposits (Agbada Formation), Ewan and Oloye Fields, Northwestern Niger Delta

摘要

This study analyses depositional environments and facies of the middle Miocene (Agbada Formation), northwestern Niger delta, based on cores and well log data, and incorporating ichonological data, that has led to a revision and re-evaluation of the facies within the study area. Log motif analysis, calibrated by lithology descriptions based on core photos, was used to define log facies associations and to identify stacking patterns and key surfaces. Six main wireline log facies associations have been recognized using gamma ray log motifs calibrated with core. Within the cored interval two lithofacies associations have also been identified, based on composition, colour, sedimentary structures, and ichnofabric. They are: slope channel sand and shoreface sequences. The interpreted depositional facies model suggests an extensive development of marine dominated slope channel fill and shoreface deposits in the northwestern part of the delta. This suggests that in the studied area, the coastal barrier complex contains a large amount of sand that was originally deposited in river mouth bars, but was laterally removed by longshore current and re-deposited on the shoreline margin and basin ward through developed of canyons on the slope which serves as feeders and conducts for slope and basin fans. The descriptive framework for the ichnofabrics analysis is based on bioturbation index (degree of bioturbation), burrow size, burrow frequency and ichonological diversity. Six ichnofabrics are recognized within the core namely: Planolites-Teichnichnus, Planolites-Phoebichnus, Planolites, Planolites-Thalassinoides, Teichichnus-PhoebichnusPlanolites and Palaeophycus-Planolites ichnofabrics. Observed tiering patterns are both simple and complex, which may reflect gradual aggradation or degradation of the substrate leading to overprinting as a stable ichnofauna structure develops. This type of tiering arrangement indicates a stable environment. The ichnofabrics are named after the dominant trace observed. Sedimentological analyses indicated that the depositional environments range from inner shelf to offshore.