The habitat of North Sea oil

Abstract

Beneath the waters of the North Sea lies a large volume of porous petroliferous sediments. In the southern North Sea a thin veneer of Tertiary, Cretaceous and Jurassic rocks overlies a basin of Permo/Triassic sediments. This basin contains a number of gas-fields. Beneath the northern North Sea is a thick basin infilled by Mesozoic and Tertiary sediments. The structure of this basin is dominated by a north-south rift whose major bounding faults lie on either side of the median line separating British and Norwegian waters. The oil-fields which have so far been discovered are concentrated along this rift. This general distribution is due to the existence of a sedimentary depocentre combined with syndepositional structural movement and a high geothermal gradient. Nevertheless, the habitat of oil varies within the northern basin. There are three main types of trap. In the south, oil occurs in Cretaceous Chalk reservoirs on salt-domes. Ekofisk and associated fields in the southern part of offshore Norway are of this type. Farther north, many reservoirs occur in Jurassic sandstones on the crests of faulted horsts and tilted blocks. The Brent and Piper fields are of this type. Thirdly, oil occurs in basal Tertiary sands draped over deep-seated horsts. The Forties, Frigg and Montrose fields are of this type.