The influence of halokinesis on shallow-marine sediments in salt basins: The Fulmar Formation, Central North Sea, UK

Abstract

Salt collapse structures host significant hydrocarbon reserves and have enormous economic importance, however despite this there is comparatively little research conducted on their evolution, structure and facies distribution. This thesis provides and in-depth multidisciplinary study on the influence of halokinesis upon the sedimentology of the Upper Jurassic Fulmar Formation of the Central North Sea. The thesis will: 1) examine the temporal and spatial variability of facies within and between salt collapse structures; 2) relate any observed facies variations to subsidence (salt withdrawal) and sedimentation rates; 3) and aim to produce predictive models of likely reservoir distribution, quality and connectivity that are applicable to the salt basins.
The Central North Sea area was selected for this study due to its extensive history of hydrocarbon exploration and production. This history has given rise to numerous, closely spaced wells and fields, providing the rare opportunity to conduct a detailed sedimentological study on the impacts of halokinesis on shallow marine sandstones.
Results from the study provide a new tectonostratigraphic model for the evolution of the West Central Shelf and the Central Graben, highlighting the importance of subaerial erosion on the West Central Shelf in creating salt collapse structures. This salt influenced topography was determined to be the dominant control on the preservation of the Fulmar Formation, with evidence from the Mallard Field showing the importance of these palaeovallys in preserving the Fulmar Formation.
A regional sedimentological analysis indicates that facies belts occur on a scale greater than singular collapse structures, suggesting the initial deposition of the Fulmar Formation would have been much more widespread.
The study reveals a clear and observable relationship between facies deposition, preservation and salt dissolution. In instances where salt dissolution rates are equal to sedimentation rates regional facies trends continue unabated. In situations where dissolution rates exceed the sedimentation rate deep water fine-grained sediments dominate, conversely where dissolution rates are outpaced by sedimentation rates erosion of the sediment occurs. The study hypothesises a ‘Goldilocks’ zone in which sedimentation and subsidence rates are balanced in such a way that preferred facies are deposited and preserved from later erosion.
The results presented here add to the growing body of literature surrounding salt collapse structures globally, and in particular can be used to mitigate risk during hydrocarbon exploration.