Lithofacies control on depositional environments in shallow offshore Niger Delta: implication on reservoir quality

Abstract

The KEN field lies within latitude 4⁰52ꞌ44ꞌꞌ N to 4⁰53'04'' N and 6⁰22'50'' E to 6⁰22'26'' E. The geological and geophysical data sets were used to describe reservoir depositional facies and their environment of deposition. A detailed and accurate environment of sediment delineation is a solid basis to enhance characterization and providing measures for improving hydrocarbon reservoirs. This study aims to presents an effective method for accurately defining depositional environment with different data sets. The data used comprises of biostratigraphy, well logs from three wells and 3D full angle stack seismic data. The biostratigraphic data help to ascertain the age of the formation delineated to be middle Miocene to late Miocene based on the marker shale. It also helps in picking the stratigraphic surfaces. Three depositional sequences were delineated and dated with maximum flooding surfaces of 15.0, 12.6 and 11.5 Ma, respectively. Log sequence analysis reveals the internal geometry and stacking pattern of the mapped sequences. The gamma ray signature varies from serrated cylindrical, funnel to the bell-shaped log motif. The seismic stratigraphy involved facies analysis and reflection termination patterns, which aided the mapping of depositional sequence. The internal geometry is composed of Highstand, transgressive, and lowstand systems tracts. The stacking patterns vary from progradational, aggradational and retrogradational. Based on the seismic facies analysis, the integrated results show that the field of study is of pelagic and debris flow origin deposited in shallow marine settings, which also conform with the other data sets used for this study. The depositional environment of the three delineated reservoirs (Reservoir A, B and C) vary from deltaic upper to lower shoreface channels sand. Reservoir C, which is laterally continuous across the three studied wells shows that the connectivity of reservoir C is loosely amalgamated. The integrated data used for this study indicate that the environment of deposition varies from inner neritic to outer neritic environment. The results of this research are essential for reservoir quality, exploration, appraisal and development phases.