Origin of anhydrite rocks in the Tromsø Basin, and the evolution of the Tromsø Basin in the light of salt tectonics
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The Southwestern Barents Sea Shelf is estimated to contain 260 300 billion m3 of gas reserves, of which a significant amount have been proven to be associated with salt structures in the Nordkapp Basin. Salt tectonics and their structures therefore have great implication on the petroleum system of Evaporite basins along the Barents Sea shelf. However detailed investigation into salt structures of the Tromsø Basin has received little attention in recent times. This work therefore aims to understand the origin of anhydrite rocks in close relation with salt structures and the overall evolution of the Tromsø Basin in the light of salt tectonics, both of which have great implication on hydrocarbon exploration in the Tromsø Basin. The zero-phased, normal polarity 2D data in combination with detailed interpretation of wireline logs and seisimic attributes (amplitudes, chaos, variance, local structural dip, gray-level co-occurrence matrix) provide excellent investigation into the salt structures of the Tromsø basin. Detailed interpretations have proven salt diapirs as well as anhydrite layers overlying salt diapirs of the Tromsø basin. The thick halite dominated salt diapirs in the basin, almost reaching the seabed, are concluded to have formed by evaporation of sea water from a partially isolated tectonic graben/basin (bared basin model) in an arid and warm climatic conditions during the Late Carboneferous to Early Permian, at which time the Barents shelf was situated in a tropical to subtropical region. Anhydrite layers predominantly occupy crestal and flanking positions of the diapirs and are interpreted to have formed from one or a modification of any combinations of the following; (1.) rotated stratigraphic layering; (2.) late stage diagenetic alteration of halite: (3.) rotated crestal caprock. Anhydrites rocks entrained in diapirs (stringers) are interpeted to originate from complex multi-dimensional salt flow resulting in their subsequent folding and boudinage. Salt diapirism in the basin was initiated by overburden extension related to Early Triassic rifting, initiating reactive rise of salt which later grew actively in the Jurassic. Thick sequences of Cretaceous sediments triggered passive diapir rise and by Early Tertiary, salt flow had ceased and diapirs were buried under thick sediments. Mid-Tertiary compression reactivated most diapirs defining their present tear drop geometry even after Neogen erosional piercement. Flank trap, fold (anticlinal and drag fold) traps, fault traps and upturned pinchouts (wedge-out) are prolific hydrocarbon trapping styles associated with salt diapirs of the Tromsø Basin.