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Statoil, Forushagen (UND-GE), Postboks 300, 4001 Stavanger, Norway
The hydrocarbon finds of the Norwegian and British sectors of the North Sea, north of 56°, can be grouped into six discrete plays. The key to all the plays is the presence of organic-rich Upper Jurassic shales (source rocks) and a rift system of the same age. The rifting provided the structures (the traps) and post-rift cooling caused the subsidence necessary for hydrocarbon generation. The pre-Jurassic Play is of least importance. The Lower-Middle Jurassic Play, with about 40% of the resources, results from pre-rift uplift in the south providing clastic input for a delta system in the north, the thick sandstone reservoirs of which were preserved during the subsequent rifting. In late Jurassic times graben formation by rift collapse was accompanied by erosion of marginal uplifts, resulting in thick sand sequences, which interfinger with graben shales containing the source rocks. This ideal relationship provides the Upper Jurassic Play which contains 30% of the resources. The Lower Cretaceous Play occurs in submarine fan sandstones and is of minor importance. Chalk deposition in a tectonically unstable environment, with subsequent rapid burial beneath Tertiary muds and clays, is responsible for the Chalk Play. Although geographically limited, it is prolific and accounts for nearly 10% of the hydrocarbon resources. Geologically youngest is the Paleogene Play. Uplift of the Orkney-Shetland Platform in early Tertiary times was a consequence of sea-floor spreading in the North Atlantic. Great quantities of sand derived from this uplift spilled eastwards into the northern North Sea, blanketing the western flank of the Tertiary basin and extending axially far south along the Central Graben. These sands have captured vertically migrating hydrocarbons and have allowed extensive lateral migration. Although important volumes of oil and gas, about 20% of the total resource, are trapped within these sands, large volumes have probably been lost to the basin via their outcrop in the west.
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