Structural settings inferred from gravity, magnetic and seismic data for Durban Basin, east coast offshore of South Africa

Abstract

The South African east coast offshore consists of two sedimentary basins namely, the Durban Basin and the Zululand Basin. The rift Mesozoic Durban Basin covers an area of about 10 000 km² between Port Shepstone Arch and the Zululand Basin. Structurally, it is complex with multiple fault systems and folds. Its development is a result of early extension of the East African plate, break-up of Godwana and initial movements of the AFFZ. The most prominent geological structures in the Durban Basin is the AFFZ, Tugela Cone, Tugela Canyon, Tugela Ridge and Naude Ridge. Only four wells have been drilled on the continental shelf of the Durban Basin i.e. Jc-A1, Jc-B1, Jc-C1 and Jc-D1. The structural setting of the Durban Basin for this study is inferred using gravity, magnetic and seismic data through an integrated approach of analyzing, processing and interpreting available gravity, magnetic and seismic 2D data. To properly map the basin extent, structures, estimate depth and thickness, pick horizons (basement, 1AT1, 17AT1, Oligocene/Miocene and seafloor), the followings were applied to gravity, magnetic and seismic 2D data to improve data interpretation: 1) Filters were applied to gravity and magnetic data i.e. reduced to the pole (RTP), 1st order horizontal derivative, 1st order vertical derivative, and power spectrum for depth estimation; 2) seismic well tie was applied to establish time-depth relationship of seismic data to pick formations and faults using well tops of Jc-C1 well and sonic log; 3) Velocity modeling using seismic stacking velocities to convert time to depth surfaces and estimate sedimentary cover thickness. The Durban Basin extent in the study area was mapped and subdivided into shallow and deep basin, occupying the southwestern, northern part and southeastern part of the study area respectively. The estimated thickness of the sedimentary cover is 2000 m-600m for the thinnest and > 2000 m-6000 m for the thickest part of the study area. The deepest and shallow part of the study area is the central to SW-SE and northern part. Structurally the study area is dissected by normal faults and graben structures with a S-N, W-E, SW-NE and SE-NW directional trends. These structures could be related to pre-rifting, rifting, AFFZ movements, compaction, abnormal gravitational load and contraction processes. The gas chimneys mapped could be evidence of an active petroleum system in the Durban Basin.