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Gravity Driven Collapse Systems on Passive Margins: A Field Study in Seismic


Gravity driven collapse systems occur on most of the world's passive continental margins and are becoming an increasingly attractive target for hydrocarbon exploration. Existing models that assume a balanced up-dip extension coupled to a down-dip toe thrust system have recently been called into question because of a growing consensus that there is commonly a mis-balance with an excess of extensional strain. Previous studies have focussed on seismic or experimental data partially due to the lack of onshore field examples on which sub-seismic features can be observed; this has left a gap in our understanding of their temporal and spatial evolution. In this study we consider field examples of a shale detachment gravity driven systems in Northern Spain, in which we observed multiple smaller scale compressive features. These structures are contained within the extensional portion of the collapse system imply considerable internal compaction prior to the formation of a down dip compressional domain. We use these field observations to re-interpret seismic examples of multiple shale driven collapses in the Orange Basin, offshore Namibia and South Africa. These seismic examples show considerable variations in style and complexity including; multiple detachment horizons lines containing only extensional features and numerous reactivations. We undertake multiple restorations of both field and seismic examples to estimate the effect of smaller scale deformation on measurements of extension versus compression. Our results suggest that this sub-seismic scale strain can account for up to 10% of previously unrecognized compression. By applying our observations from the field to seismic examples we are able to interpret structures close to seismic resolution and make predictions about the likely effects of sub-seismic features on reservoir prospectivity, hazard prediction, permeability and porosity