Turbidite facies identification and analysis using geophysical methods in the salt diapir domain of Lower Congo basin

Abstract

This study applied geophysical methods to investigate the turbidite system developing over salt domes in the Lower Congo basin. As a rift basin, the Lower Congo basin experienced synrift, transition and post-rift tectonic movement. A series of evaporite deposits consisting largely of Aptian salts overlay the synrift half-grabens followed by thick post-rift sediment. The westward tilting triggered massive loading of Congo fan delta and initiated salt movement. In the study area, the major reservoirs are Oligocene and Miocene turbidite sands. The tectono–sedimentary interactions between Oligocene-Miocene deep-water depositional systems and salt-related structural topography can be observed through the salt structure and the deformation of turbidites. Later salt movement created a complex structure and either deformed the original setting of turbidite deposition or provided accommodation of the turbidite flow system. However, the complexity of salt structures and turbidite systems complicates the prediction of reservoir configuration and sedimentary facies in the study area. With high-resolution 3D seismic, we are able to study the sedimentary facies by employing geophysical methods that include seismic amplitude analysis, seismic waveform classification and spectral decomposition techniques. Our interpretations suggest that in the study area, the width of turbidite system ranges from 200 m to 4 km and the total thickness is up to 200 m. The turbidite channels trended from the northeast to the southwest during the Oligocene and turned westwards approximately 24° in the Miocene. Three typical sediment architectures are observed including distributary channel mouth lobe, confined turbidite complex with cut and fill channels and isolated individual channels, which would be the useful models for the reservoir evaluation over the active salt domes.