The Palaeocene/Eocene to Mio-Pliocene sediments of the Central and Northern North Sea contain deepwater sediments with significant hydrocarbon reserves within submarine fan sands encased in mudrocks. Post depositional processes have significantly changed the original small and large scale geometry and reservoir characteristics of these deepwater sediments.
Examples of large scale remobilisation from 3D seismic, well and core data from the above intervals vary from cross cutting 10's of metres thick and 100's of metres in extent intrusions of sands, kilometer scale seabed pockmarks, sand mounds with kilometre wavelengths deposited between a mud mounded sea floor and syn to post depositionally controlled polygonally faulted deepwater sands.
Outcrop analogues of large scale remobilisation are difficult to identify as the scale of these sandstone intrusions are often larger than the outcrop available. Within Upper Miocene Santa Cruz Mudstone, Santa Cruz, California exist one of only two outcrop kilometre scale sandstone intrusion complexes in the world. The intrusion of these sands is postulated to have been related to expulsion of basinal fluids including hydrocarbons and related overpressure build ups, as proposed for several of the intrusive examples above in the Tertiary of the North Sea.
The geoscientist should be aware of the small to very large scale nature of sediment remobilisation which can significantly change primary depositional geometries and physical properties of deepwater sediments. These above examples of remobilisation highlight the great variation in reservoir character, which may significantly affect the petroleum exploration and development of such reservoirs. Ignoring such features will lead to incorrect reservoir modelling and subsequent exploration and development "surprises".