Deformation in overburden adjacent to salt diapirs: implications for penetration mechanisms of Central Graben North Sea diapirs

Active and passive diapirism control the deformation and geometry of hydrocarbon traps in the overburden, and a more detailed understanding of this process will help reservoir prediction and hydrocarbon recovery. Cores studies of seven Central Graben diapirs indicate Zechstein salt penetrated Late Cretaceous chalk by extreme tectonic thinning with high-angle (> 70 degrees to bedding) normal faulting, tensile fracturing and pressure solution. Attenuation of the chalk significantly weakens the overburden, allowing buoyancy forces to dome up the overburden, Doming created enough topography for downslope sliding of chalk slabs on slip planes parallel to bedding or, in the case of the Kyle diapir, for chaotic debris flows of lithified chalk. Significant extensional bedding-parallel faults and slump folds are developed within Palaeocene shale on the diapiric flanks. Inter-granular slip in unconsolidated elastic material was probably the dominant deformation mechanism. Diapirs have penetrated the Palaeocene elastic sediments by maintaining topographic relief, so that unlithified sediment continually slid off the crest, producing translated intact rafts up to several tens of metres in thickness. (C) 2000 Elsevier Science Ltd. All rights reserved.