3D simulations of salt tectonics in the Kwanza Basin: Insights from analogue and Discrete-Element numerical modeling

摘要

The Kwanza Basin is located at the passive margin of West Africa in the Central Segment of the South Atlantic, and is an important area for oil and gas exploration. The distribution and deformation characteristics of thick Lower Cretaceous (Aptian) salt deposits in the basin have significant impacts on hydrocarbon accumulation. However, reports on their 3D interpretation, together with an understanding of the genesis and evolution of salt tectonics in this area, are scarce in the literature. In this paper, 3D analogue experiments and 3D Discrete-Element numerical simulations are carried out to analyze the zonal characteristics (extension-translation-contraction), the main control factors, and to develop a kinematic model of salt tectonics in the basin. According to our modeling, we make the following conclusions. (1) The zonal characteristics of salt tectonics formed in the basin reflect the superposition of gravity gliding and gravity spreading. The main control factor of basin-scale salt deformation is basement structural activity, which can be summarized into three points: early post-salt basement extension, initial presence of local variation of base of salt gradient and more recent differential subsidence between landward side and seaward side. (2) Salt deformation is episodic, having experienced one stage of regional deformation (Late Aptian to Albian) and two stages of inherited developmental deformation (Campanian, and Eocene to Miocene), mainly resulting in forward expansion towards the ocean. (3) During the early stage of salt deformation, faults developed in suprasalt layers in the extensional and translational zones of the basin, resulting in local high-velocity areas and a large vertical upward velocity component in the salt layer. Later, due to blocking by the outer high region and increasing overburden, salt in contact with the outer high developed a vertical upward velocity component, gradually forming thrust structures. (4) 3D analysis highlights the pervasive lateral deformation of the salt. The main factors governing it are the distribution of accommodation in the salt basin and the initial morphology of the salt layer. In the early stages, salt layer had a velocity component to the salt-scarce side (from north to south), whereas in the later stages, it developed a velocity component to the wide side of the basin (from south to north), leading to the gradual accumulation of salt in the north of the basin. We have shown that 3D simulations can complete the analysis of salt deformation both parallel and perpendicular to the slope in a passive-margin basin. This may provide guidance for the exploration of oil and gas reservoirs in the context of salt tectonics, especially in asymmetrical basins where seismic imaging is poor and has to be supplemented by strong geological concepts including understanding 3D movement of salt.