Post-Salt Structural Domains in Lower Congo Basin, Offhsore West Africa

Abstract

The objective of this study is to understand the configuration of different deformation styles in post-salt sedimentary succession of Lower Congo Basin. Emphasis is placed on structural characteristics with respect to the thickness of salt and the geometry of the base of salt detachment. Owing to its weak visco-plastic properties, salt is very effective at decoupling deformation in pre- and post-salt sequences. Gravity and density driven deformation in the post-salt sediments is predominately controlled by salt thickness and changes in dip at the base of salt. Variations in these elements change across the basin which control the nature and timing of trap development. Using regional 2D and 3D seismic depth cubes, interpretation focused on the Aptian Salt and post-salt Cretaceous-Tertiary sedimentary succession. Here we map and identify various deformation styles in salt tectonics across the basin within which we observe consistent trap geometries which are containing hydrocarbons and resulting one of the prolific basins in the world for oil production. The post-salt sediments, present a classic example of gravity driven deformation associated with salt tectonics at a passive margin. Gravity driven structuration of (i) Extension, (ii) Translation and (iii) Compression can be observed along regional dip-sections. Through detailed mapping, various sub-domains provide an insight into the regional structural trends and tectonic evolution in the post-salt succession. Overall, ten structural domains have been identified from Shelf to distal basin along the regional dip section. Well defined structural domains can play a major role in classifying the trapping styles for hydrocarbon accumulations in post-salt successions. Identification of these domains provides a framework to de-risk different trap styles or highlights those traps which carry a higher trap risk. Timing of trap formation due to salt movement also plays a major role to further de-risk these traps. The main source rock for Post-salt section is located in Senonian interval, the charge modelling suggests the peak expulsion of hydrocarbon in Early Miocene time, therefore those traps formed in Oligocene to Early Miocene intervals have higher chance of trapping hydrocarbons, however traps formed from Mid – Miocene to younger levels have higher risk of trapping smaller or no hydrocarbons due to lack of charge availability.