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Exploration trends in marginal basins from Skagerrak to Stord

1995; Elsevier BV; Linguagem: Inglês

10.1016/s0928-8937(06)80039-1

2212-1390

Steinar Sørensen, O.H. Tangen,

Geological and Geophysical Studies

This paper is a compilation of several internal Elf Petroleum Norge studies during the past years. The study area involves several structural provinces along a belt rimming the Norwegian mainland. Of these marginal basins hydrocarbons have been found in the Egersund Basin and sta Graben. Of the virgin exploration areas, the Skagerrak Graben, where sediment thicknesses vary dramatically and in places reach thicknesses in excess of 12 km, might be interesting for exploration. The sediments range in age from Cambrian to Recent, with a variety of sedimentary deposits preserved. The main concern for the area is the level of maturation of the potential source rocks. However, observations from nearby areas indicate that the level of maturation might be significantly reduced compared to the Oslo Graben. Exploration has shown that a thick sedimentary package has been affected by tectonic movements giving room for several play concepts. Another virgin exploration area is the Farsund Basin, which achieved its present shape during Jurassic times. The area was affected by severe fault movements and halokinetic movement during several phases, the latter probably starting in Late Triassic times. Several speculative source rock intervals have been interpreted, with the most favourable ones being potential Upper Carboniferous black shales and Lower Jurassic marine shales. Reservoir rocks are expected within the Palaeozoic and Mesozoic intervals. Several structures have been identified, particularly along the flanks of the basin, in the form of classical tilted fault blocks and stratigraphic traps. The Egersund Basin and the Asta Graben are structurally separated by an east-to-west ridge at approximately 58°N. In these two areas four hydrocarbon discoveries have been made. Upper Jurassic (Tau Formation) source rocks are proven. The best reservoir rocks found are the Middle Jurassic sandstones (Sandnes and Bryne Formations). The least risky prospect types are classical rotated fault blocks and roll-overs at Jurassic level, adjacent to mature kitchen areas. For the Palaeozoic plays the risk is increased due to lack of data; however, structures at Palaeozoic levels are bigger than those defined at the Mesozoic level. In the Stord Basin, which is structurally linked to the Asta Graben, only one exploration well has been drilled (dry), but Jurassic rocks were found. The problem is the level of maturation, due to limited burial. The presence of Palaeozoic source rocks is speculative, but from onshore areas to the northeast, Devonian sediments are well known. At Mesozoic levels several reservoir rock intervals can be expected, mainly within the Jurassic sequence. The least risky plays are rotated fault blocks at Jurassic level which have experienced short distance migration from the potential locally mature kitchen areas. More risky are the prospects mapped at Palaeozoic levels, but their size might justify drilling to evaluate their potential.