Seismic Stratigraphy and Development of Deep Fluvial Channel Incisionsin the Middle Miocene Deposits, Ewan and Oloye Fields, NorthwesternNiger Delta, Nigeria

Abstract

Seismic interpretation of depth converted three-dimensional seismic survey from Ewan and Oloye fields; onshore northwestern Niger Delta has helped in the identification of incised valleys (up to 350 m) deep. This study evaluated their morphologies, evolution and the local controls that influence their development. The seismic reflections of the incised valleys are characterized by low-moderate amplitude, variable internal reflections, aggradational, chaotic and progradational-sigmoid reflection patterns overlying by parallel to sub-parallel reflections configuration. The seismic reflection characteristics are probably due to variable sedimentation processes within the valleys which were affected by mass wasting. Asymmetry morphology of the valleys suggests fluvial origin while low sinuosity of the channel may indicate high gradient and high discharge. The occurrence of incised valleys landward of the shelf edge suggests and partially reflects underlying structural control. This might be caused by uplift due to shale diapirism (shelf instability). The magnitude of incision is difficult to explain in terms of sea-level fall alone because ecstaticallydriven sea level variations during the Miocene are generally reported to be less than 100 m. This suggests that the character of fluvial incisions development and depositional facies preserved within the study area is locally controlled by growth faults, rapid relative sea level changes, basin physiography (shelf edge), shelf instability, variation in sediment input, slope collapse (mass flow depositional processes and downward cutting by downslope sediment flow). The incised valleys within the study area probably serve as conduits for sediment transport to the deep water. Seismic delineation of the distribution and morphology of these incised valleys may also provide critical input for reservoir modeling and volumetric analysis.