Salt flows downslope, irrespective of overburden. In salt basins on passive margins, the salt will tilt and flow towards the ocean immediately after continental rifting has ended due to thermal subsidence. Using real examples, as well as physical and numerical models, tilting is shown to be relatively rapid, enhanced by isostatic rebound updip and loading downdip where salt pools and inflates behind an outer high. In the Santos, Campos and Kwanza basins, this outer high is represented by an embryonic mid-Atlantic ridge, amplified in height by the differential weight of the inflating salt. Widespread extension and translation of overburden, utilizing both seaward- and landward-dipping normal faults, characterizes the early evolution of the inboard region. Inflation and contraction occur outboard, the effects of which tend to expand in a landward direction over time. Rapid accumulation of salt implies wholesale dewatering of pre-salt sediments, the water possibly permeating the salt once it has reached a burial depth of c. 3 km. The process of thermal subsidence, salt drainage and isostatic amplification is an efficient mechanism for moving sediment on passive margins tens of kilometres seaward during a relatively short period and helps explain why great thicknesses of salt can accumulate there in the first place.

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The passive margin of the South Atlantic shows typical features of a rifted volcanic continental margin, encompassing seaward dipping reflectors, continental flood basalts and high-velocity/density lower crust at the continentocean transition, probably emplaced during initial seafloor spreading in the Early Cretaceous.The Springbok profile offshore western South Africa is a combined transect of reflection and refraction seismic data. This paper addresses the analysis of the seismic velocity structure in combination with gravity modelling and isostatic modelling to unravel the crustal structure of the passive continental margin from different perspectives.The velocity modelling revealed a segmentation of the margin into three distinct parts of continental, transitional and oceanic crust. As observed at many volcanic margins, the lower crust is characterised by a zone of high velocities with up to 7.4 km/s. The conjunction with gravity modelling affirms the existence of this body and at the same time substantiated its high densities, found to be 3100 kg/m³. Both approaches identified the body to have a thickness of about 10 km. Yet, the gravity modelling predicted the transition between the highdensity body towards less dense material farther west than initially anticipated from velocity modelling and confirmed this density gradient to be a prerequisite to reproduce the observed gravity signal. 2Finally, isostatic modelling was applied to predict average crustal densities if the margin was isostatically balanced. The results imply isostatic equilibrium over large parts of the profile, smaller deviations are supposed to be compensated regionally. The calculated load distribution along the profile implies that all pressures are hydrostatic beneath a depth of 45 km.3

The Orange Basin offshore southwest Africa appears to represent a classical example of continental rifting and break up associated with large-scale, transient volcanism. The presence of lower crustal bodies of high seismic velocities indicates that large volumes of igneous crust formed as a consequence of lithospheric extension.We present results of a combined approach using subsidence analysis and basin history inversion models. Our results show that a classical uniform stretching model does not account for the observed tectonic subsidence. Moreover, we find that the thermal and subsidence implications of underplating need to be considered. Another departure from the uniform stretching model is renewed sub-crustal thinning and linked to that uplift in the Cenozoic that is necessary to reproduce the observed phases of erosion and the presentday depth of the basin. The dimension of these events has been examined and quantified in terms of tectonic uplift and sub-crustal thinning. Based on these forward models we predict the heat flow evolution not only for the available real wells but also for virtual wells over the entire study area. Finally, the hydrocarbon potential and the temperature evolution is presented and shown in combination with inferred maturation of the sediments for depth intervals which comprise potential source rocks.

Although the development of passive margins has been extensively studied over a number of decades, significant questions remain on how mantle and crustal dynamics interact to generate the observed margin geometries. Here, we investigate the Orange Basin, located on the south-west African continental margin. The basin fill is considered to comprise a classic rift-drift passive margin sequence recording the break-up of Gondwana and subsequent opening of the South Atlantic Ocean. Based on interpreted seismic reflection data, a 3D geological model was first constructed. Subsequently, an isostatic calculation (Airy´s model) using a homogeneous middle and lower crust was applied to this geological model to determine the position of the Moho for an isostatically balanced system. Isostatic sensitivity tests were applied to the model, and their gravity response were validated against different crustal structures for the basin.The best-fit model requires dense, presumably mafic material, in the middle and lower crust beneath the basin and an abrupt change to less dense material near the coast to reproduce the observed gravity field.

In 2012, a repeat time-lapse seismic survey was acquired over the Halfdan oil field located in the Danish North Sea. The main oil reservoir is the Cretaceous chalk Tor Formation, and the field is developed with a line-drive water flood. The reservoir saturation changes due to water injection and gas breakout lead to large changes in acoustic impedance, which can be imaged with 4D seismic. The 4D acoustic-impedance changes, in conjunction with production and geologic data, were analyzed on a well-by-well basis and resulted in several interesting correlations between the geology and the 4D response. The 4D was used to update the seismic interpretation of some important layers within the static and dynamic model. In some areas, the impact of local porosity variations on the lateral sweep could be seen. The 4D also provided evidence that some faults serve as potential pathways for formation-water entry into the oil-producing layers. In addition, numerous well-intervention opportunities were identified through integrating the 4D with the production, geologic, and dynamic data. Examples from the two-vintage and three-vintage 4D acoustic-impedance data illustrating these key 3D and 4D learnings and a successful well intervention are shown.

Rifts and rifted passive margins are often associated with thick evaporite layers which challenge seismic reflection imaging in the sub-salt domain. This makes understanding the basin evolution and crustal architecture difficult. An integrative, multidisciplinary workflow has been developed using the exploration well, gravity and magnetics data together with seismic reflection and refraction datasets in order to build a comprehensive 3D subsurface model of the Egyptian Red Sea.Using a 2D iterative workflow first, we constructed cross-sections using the available well penetrations and seismic refraction data as preliminary constraints. The 2D forward model uses regional gravity and magnetic data to investigate the regional crustal structure. The final models are refined using enhanced gravity and magnetic data and geological interpretations. This process reduces uncertainties in basement interpretation and magmatic body identification. Euler depth estimates are used to point out the edges of high susceptibility bodies. We achieved further refinement by initiating a 3D gravity inversion. The resultant 3D gravity model increases precision in crustal geometries and lateral density variations within the crust and the pre-salt sediments. Along the Egyptian margin, where data inputs are more robust, basement lows are observed and interpreted as basins. Basement lows correspond with thin crust (lt;12 km), indicating that the evolution of these basins is closely related to the thinning or necking process. In fact, the Egyptian Northern Red Sea is typified by dramatic crustal thinning or necking that is occurring over very short distances ~30 km, and very proximal to the present day coastline. The integrated 2D and 3D modelling reveals the presence of high density, magnetic bodies which are located along the margin. The location of the present day Zabargad Transform fault zone (ZTFZ) is very well delineated in the computed crustal thickness maps, suggesting it is associated with thin crust, and shallow mantle.

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