The Nordkapp Basin, Norway: Development Of Salt And Sediment Interplays For Hydrocarbon Exploration

Abstract

Abstract Investigation of a particular salt diapir in the Nordkapp Basin, Barents Sea has revealed the following sequence of events:salt started to rise when approximately 1.5 k 0.3 km ofsedimentary cover was present (Carboniferous/Permian time);salt reached the sediment surface when about 3.5 + 0.7 km of sediment had been deposited (Triassic time);the mushroom cap on the salt stock top developed over a period of about 75-100 Ma (i.e, during the time when about another km of sediment had been deposited) (Triassic through Base Cretaceous time);the mushroom cap started to dip down significantly (- I km) into the sediments around Cretaceous to Tertiary erosion time;oil generation started in the deep sediments of the Carboniferous around the time that salt reached the surface (Triassic time) and continues to the present day at sedimentary depths between about 4 to 7 km (currently Triassic and deeper sediments);gas generation started around mushroom cap development time and continues to the present day at sedimentary depths greater than about 6-7 km. Introduction The estimation of hydrocarbon reserves and their likely locations in the Nordkapp Basin poses special problems because of the presence of many salt diapirs, which have tops close to the seafloor and also widespread mushroom salt caps, making seismic identification of underlying sedimentary beds difficult. Apart from a few, generally shallow, wells there is little direct down hole information in the Nordkapp Basin on sub-surface conditions, or petro physical properties. Information available is generally restricted to:regional seismic reflection profiles;gravity surveys;regional geologic background information (e.g. Dalland et al., 1988; Gabrielsen et al., 1990; Johansen et al., 1992). Physical models (see e.g. Koyi, 1991) suggest the likely development of the basin, while numerical models provide measures of sediment fill, compaction, fluid-flow, and salt evolution in relation to basinal tectonic evolution. Background Geological Information Development of the Nordkapp Basin in relation to the largerBarents Sea picture has been addressed (e.g. Rannevik et al., 1982; Faleide et al., 1984; Jensen and Sorensen, 1988, 1992; Gabrielsen et al., 1990; Johansen et al., 1992; P. van Veen et al., 1992; Talbot et al., 1992; Richardson et al., 1992). The basin formed due to crustal extension during the Late Devonian (?) to Early Carboniferous and contains numerous diapirs of Carboniferous salt. Evaporite deposition in the Early Permian has also been suggested by Faleide et al. (1984), After formation, the basin and surrounding areas became tectonically stable and a carbonate platform developed regionally in the Late Carboniferous - Early Permian. From the Late Permian until the end of the Triassic large amounts of dominating tine-grained clastic sediments were deposited in the Nordkapp Basin.