ENVIRONMENTAL SETTING AND DIAGENESIS OF LOWER PERMIAN PALAEOAPLYSIND BUILD‐UPS AND ASSOCIATED SEDIMENTS FROM BJØRNØYA: IMPLICTIONS FOR THE EXPLORATION OF THE BARENTS SEA.

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Abstract
Lower Permian palaeoaplysinid build‐ups, inter‐bedded with inter‐build‐up logoonal sediments occur both on Bjornoya † and in the subsurface of the Barents Sea. They formed on a low‐energy, broad and shallow shelf, and show a cyclic development which was probably related to minor eustic sea‐level changes. During periodic lowering of sea level, build‐ups were subaerially exposed, meteoric diagenesis with Karstification took place within the build‐ups, and hypersaline conditions prevailed in the lagoons.Most diagenetic changes took place early and include fresh‐water dissolution, dolomitization, evaporite precipitation, and pseudomorphic replacement of evaporites. Fracturing and associated hydrothermal calcite, barite and galena cementation occured during the Early Tertiary opening of the Norwegain‐Greenland Sea, and affected the entire western part of the Barents Shelf.In the Barents Sea, the palaeoplysinid build‐ups have composite thickness on the order of several hundred metres and are restricted to positive structural elements. They are regarded as potential reservior rocks in this area, partly because of the high porosity of similar build‐ups in Central Spitsbergen. The reservoir quality of the build‐ups on Bjornoya and from the single cored build‐up complex in the Barents Sea is, however, relatively poor. Early Tertiary fracturing increased the permeability of the bulk rock, but also caused leakage of accumulated hydrocarbons.