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Abstract

From deposition of source rocks, via maturation and migration, to alteration of oil in the reservoir, tectonics exert a fundamental control in the North Sea area. The southerly development of an elongate arm of the Boreal Sea during the Late Jurassic resulted from an early rifting attempt of the northern North Atlantic. This deep-water trough favored the development of bottom-water anoxia, while rapid sédimentation also enhanced the chances of preservation of algal debris falling from an upper oxygenated photic zone.

Maturation of the Upper Jurassic-basal Cretaceous source rock was then controlled by subsidence and heat flow in what is generally described as a techtonically quiescent period of thermal subsidence. If source rock deposition occurred as a result of crustal thinning and graben formation, the consequent thermal subsidence predicts falling sédimentation rates andheat flow. The facts, however, dispute this model, with increasing rather than decreasing (decompacted) sédimentation rates. With this continued subsidence oil generation has occurred from the Cretaceous to the Holocene.

Whereas long-distance oil migration is controlled essentially by tectonic tilt, local sedimentological changes at fault scarps interbedding source rock and coarser elastics can enhance the expulsion of hydrocarbons from the source rock unit.

Bacterial alteration in the reservoir is related to heat flow as a result of a 60-80 °C cutoff for active bacterial growth, and more speculatively by the presence of late-stage transtensional (open) faults allowing oil to reach near-surface reservoirs and ground water with bacteria to be introduced into the traps.

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