Continental stretching: An explanation of the Post‐Mid‐Cretaceous subsidence of the central North Sea Basin

Abstract

The North Sea is a major continental basin filled with early Paleozoic to Recent sediments. Though graben formation started in the Triassic, the last major period of extension occurred between the Middle Jurassic and the mid‐Cretaceous. Following the faulting and graben formation associated with this extension, subsidence within the central North Sea was widespread and uniform and has created a saucershaped sedimentary basin. This was filled successively by chalks, sandstones, and finally, during most of the Tertiary, by shales and mudstones. We examined the subsidence of six wells down the middle and two on the flanks of the Central Graben. In the period of widespread steady subsidence the water‐loaded basement depth in the middle increased by 1100–1400 m. On the flanks the basement subsided 600–700 m. We suggest that most of this subsidence results from the thermal relaxation of the lithosphere which was thinned during a Middle Jurassic to mid‐Cretaceous stretching of the crust. Assuming a crustal stretching and associated lithospheric thinning of between 50 and 100% in the middle and decreasing on either side, we obtained a good match to the observed amplitude and rate of subsidence. The Middle Jurassic to mid‐Cretaceous subsidence which is found within the graben proper we relate to the fault‐controlled initial subsidence which occurred during the actual stretching. The measured heat flow is compatible with such a stretching model. Though there is no seismic refraction data across the Central Graben, this model is strongly supported by evidence of a thinner crust under the Viking Graben to the north and the Witchground/Buchan Graben complex to the east. Using the above observations as the basis for a geological interpretation, we examined the thermal maturity and hydrocarbon potential of certain sedimentary horizons in the northern section of the Central Graben. In analyzing the various wells we extended previous work on the compaction correction to handle overpressuring and mixed lithologies in backstripping studies. Further, we expanded these methods to include the variation of thermal conductivity, and calculations of the degree of thermal maturation of the deposited sediments, through time.