Overburden deformation patterns and mechanisms of salt diapir penetration in the Central Graben, North Sea

Active and passive diapirism control the deformation and geometry of hydroc arbon traps in the overburden, and a more detailed understanding of this pr ocess will help reservoir prediction and hydrocarbon recovery. Cores studie s of seven Central Graben diapirs indicate Zechstein salt penetrated Late C retaceous chalk by extreme tectonic thinning with high-angle (> 70 degrees to bedding) normal faulting, tensile fracturing and pressure solution. Atte nuation of the chalk significantly weakens the overburden, allowing buoyanc y forces to dome up the overburden, Doming created enough topography for do wnslope sliding of chalk slabs on slip planes parallel to bedding or, in th e case of the Kyle diapir, for chaotic debris flows of lithified chalk. Sig nificant extensional bedding-parallel faults and slump folds are developed within Palaeocene shale on the diapiric flanks. Inter-granular slip in unco nsolidated elastic material was probably the dominant deformation mechanism . Diapirs have penetrated the Palaeocene elastic sediments by maintaining t opographic relief, so that unlithified sediment continually slid off the cr est, producing translated intact rafts up to several tens of metres in thic kness. (C) 2000 Elsevier Science Ltd. All rights reserved.