--> Halokinetic Controls on the Evolution of Shallow Marine Facies Architecture: Upper Jurassic Fulmar Formation, UKCS

AAPG ACE 2018

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Halokinetic Controls on the Evolution of Shallow Marine Facies Architecture: Upper Jurassic Fulmar Formation, UKCS

Abstract

Documented outcrop examples of halokinetically-influenced siliciclastic shallow marine environments are sparse. As such, the effect of salt dissolution and sedimentation rate on the temporal and spatial distribution of facies remains poorly understood. Conventional wisdom dictates that the preserved sedimentology within salt-withdrawal minibasins is a consequence of subsidence and sedimentation rates; if sedimentation rates are high relative to subsidence, scouring of the sediment may occur, if sediment rates are low relative to subsidence rates the basin may be infilled with deep-water fine grained sediment.

In the absence of outcrop examples, subsurface data provides the key to generating predictive depositional models which may aid the understanding of facies distribution and resultant reservoir quality, both within and between salt withdrawal-mini-basins.

The Upper Jurassic Fulmar Formation of the Central North Sea provides an ideal opportunity to study the effects of halokinesis on shallow marine sediments. The shallow marine succession was deposited into salt-wall collapse basins formed by the dissolution of mobile Zechstein salts. The geographical distribution and facies of the formation were controlled by the complex interplay of basin-scale tectonics, combined with active salt migration and dissolution. Numerous closely spaced well penetrations and continuous 3D seismic data enable a detailed interpretation of the subsurface.

This multidisciplinary study incorporates 3D seismic reflection, ichnofabric and sedimentological analysis, wireline and biostratigraphical data from the Central North Sea. The study has three aims: i) to determine the architecture of the shallow marine salt-withdrawal mini basin-fill ii) to determine the spatial and temporal distribution of facies within and between salt-withdrawal basins iii) where possible, relate the variation in facies to active salt withdrawal and sedimentation rates.

Initial results indicate that the gross depositional environment of the sediments remain unaffected by the development of salt-withdrawal minibasins, minibasin subsidence may act solely to preserve thickened packets of sediment.

This work will clarify the relative influence of sediment supply, halokinesis and subsidence upon facies distribution, producing predictive depositional models of facies distribution and connectivity that are applicable to the Fulmar Formation and similar salt-influenced, shallow-marine sediment hydrocarbon plays.