Genesis, Migration and Entrapment of Jurassic Oil in the North Sea

Abstract

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Abstract Many major North Sea oil fields occur in Jurassic sandstone reservoirs. Trapping mechanisms are diverse, but the most common types are tilted horsts and block-faulted anticlines. These have undergone complex tectonic evolutions, which in different structures, predate, postdate and were synchronous with the deposition of the reservoir formations. Thus depositional thinning and truncation are commonly present over the crests of these features, but at different stratigraphic intervals in different fields. Several marine and nonmarine shales with source potential occur interbedded with the reservoir sands. It is unlikely, however, that oil generation occurred before the Palaeocene. Some structures were certainly not sealed until well into the Cretaceous and a considerable thickness of Cretaceous cover must have been deposited to enable the geothermal gradient to build up sufficiently for the source rocks to generate their hydrocarbons. Introduction Within the last 5 years several major oil fields have been found in Jurassic sandstone reservoirs of the northern North Sea. The object of this paper is to review the habitat of the Jurassic oil; to discuss its genesis, migration and entrapment, and the nature of the reservoirs. The paper commences with a brief account of the geography and stratigraphy of the Jurassic rocks; this is followed by accounts of the source rocks and reservoir rocks. The penultimate section describes the structural penultimate section describes the structural style of the traps and the paper concludes with an analysis of the time and mechanism of oil generation, migration and entrapment. Jurassic Geography and Stratigraphy Prior to the Cretaceous period, North America, Greenland and Western Europe formed a single area of continental crust. Tensional forces commenced in the Triassic. These caused downwarping of sedimentary basins and fault-bounded rift valleys. Ultimately, the North American, Greenland and European plates were torn apart as new oceanic crust formed in what are now the ocean basins of the Atlantic. It was against this background of tensional tectonic forces that the Jurassic rocks of the North Sea were deposited. Subsidence began in the North Sea basin in the Permian. Thick continental red sandstones and shales were deposited throughout the Permian and Triassic with local intermittent phases of evaporite formation. A fault-bounded axial rift valley developed down the North Sea basin, close to the modern median line.