Structural evolution of fault systems associated with salt diapirs

Background/Rationale Salt-cored diapirs are widely developed in over 200 sedimentary basins and are important structures for exploitation of hydrocarbon, mineral and groundwater resources, and increasingly are being targeted for high grade waste disposal. Many of these diapirs are associated with complex fault systems whose evolution is in some way linked to that of the diapir. These faults have been classified into various families based on geometry, and include radial, concentric and differential compaction faults (extensional), and concentric thrust faults. Beyond this simple classification, however, these fault systems remain poorly understood. In particular, we do not know whether specific fault types form during periods of active or passive diapirism. This important question impacts the wider debate over the origin of salt diapirs, and how their evolution correlates with regional tectonics. Passive diapirism or downbuilding is intrinsically less likely to require straining of the host sedimentary succession than active diapirism because of the much greater shape changes in the diapir that occur in the active mode, and this opens the possibility of using the distribution and timing of fault activity to constrain the mode of salt diapirism. This in turn would be invaluable for resource prediction since it would allow an independent evaluation of diapir type to be made, and this is well known to be critical in prediction of diapir geometry in areas of poor imaging of the margins of the salt body. Aims of the PhD Project: (1) to analyse the structural evolution of the various fault families that are associated with salt diapirs and to place this evolution in the context of the diapir evolutionary mode (active vs passive), and (2) to use this structural evolution to test the hypothesis that specific fault types and the strain fields they represent correlate positively with periods of active or passive diapirism. Database and Methodology The primary method to be used will be 3D seismic interpretation of the fault systems associated with salt diapirs, using workstation interpretation and visualisation facilities in Cardiff and in Shell's Aberdeen office. 3D fault analysis techniques will be used to classify fault types and to reconstruct their growth history. High resolution biostratigraphy from closely spaced development wells will be used to constrain the timing of fault growth and the timing of diapir growth. Structural restoration software will be used to reconstruct the 3d evolution of salt bodies relative to the fault systems to evaluate precisely which fault sets develop with which specific changes in salt geometry. Diapir evolution will be linked into regional tectonic models using basin-scale 3D seismic surveys and fully tested proprietary basin models. Finally, geomechanical evaluation of the fault sets will be conducted using numerical modelling. The project will exploit Shell's unique seismic and well database in the Central North Sea salt diapir province, representing one of the highest quality subsurface databases in any salt basin. The student will interpret regional megamerge surveys covering >10,000km2 and constrain the interpretation with ties to >200 wells located adjacent to some 30 salt diapirs or salt ridges. 3D surveys over Shell assets such as Pierce, Fram, Gannet C, Merganser have been additionally processed using the latest pre-stack depth migration procedures and are available for this study. These provide the best possible imaging of both the fault systems and the complex geometry of the steep-sided salt bodies. Finally, comparisons will be made with case studies in Angola, Brazil and the Gulf of Mexico to test the results and to ensure that they have wide applicability beyond the North Sea Basin.