Abstract

Surface evidence of hydrocarbons has long been known in many of the countries bordering the North Sea. In England and Scotland the shows are mainly in Upper and Lower Carboniferous; follow-up exploration resulted in production of more than 2 × 10⁹ kg (2 million tons) of oil from small oilfields in Millstone Grit and Coal Measures. In Germany the known oil shows in an area of salt tectonics were in Jurassic and Lower Cretaceous and gas was found in the Permian; these led to the establishment of 86 oilfields and subsequently as many gas fields; total oil production now approximates to 10¹¹ kg (100 million tons). In the Netherlands oil has been developed in the east, and the Groningen gas field in the Permian, found in 1959, proved to be one of the world’s largest gas accumulations.

Although the North Sea was earlier recognized as a prospective basin the small scale of fields in the adjoining countries discouraged offshore exploration. Following the Groningen discovery, interest in the North Sea potential was sharply increased, licensing and exploration following formal agreement on national sovereignty of the offshore area (1964).

Interest was initially concentrated on the gas prospects of the southern North Sea and with the aid of greatly improved seismic techniques major discoveries were made, mainly in the basal Permian sands in British waters; subsequently in the Dutch offshore also. When the major southern prospects had been explored activity was transferred to the northern North Sea, with the initial interest directed to the very thick Tertiary basin filling. Early discoveries were made in basal Tertiary chalks and the Paleocene sands of Norwegian waters. The Forties oilfield, the first large discovery in Scottish waters, was in thick Paleocene sands and the Frigg stratigraphic trap gas field on the mid-line was in Eocene sands. Other Tertiary discoveries were small, but investigation of the sub-Tertiary rocks resulted in the discovery of large reserves in the counter-fault traps in the underlying Jurassic, to the east of the Shetlands; it led to the discovery of the Brent and Statfjord reservoirs (Dogger-Lias) off the Scottish and Norwegian coasts. Subsequently, extensive exploration was directed towards the Jurassic lying beneath the unconformable Tertiary and Upper Cretaceous. Thick oil sands occur in the lower, middle and upper Jurassic.