Overpressure and Lateral Drainage in the Palaeogene Strata of the Central North Sea

Abstract

Knowledge of subsurface pressures within the Palaeogene strata of the Central North Sea is an important aspect of the petroleum geology of the basin, enabling a better understanding of migration pathways, the potential for hydrodynamic trapping with tilted fluid contacts, and better assurance that drilling is carried out both safely and efficiently.
In this thesis, Palaeogene pressure data collected from over 300 offshore wells in the UK, Norwegian and Danish sectors of the North Sea are analysed. Regional overpressure distributions within individual Palaeocene and Eocene sandstone members are mapped across the Central North Sea, and lateral fluid flow pathways are interpreted within each individual sandstone member. Drainage within the older Maureen, Mey and Forties sandstone members is found to be predominantly south-east to north-west, with additional westwards drainage observed within the Mey and Forties sands, where smaller lateral fans fed into the basin from the Western Platform. Overpressure distributions within the Cromarty and Tay sandstone members indicate drainage westwards, where fluids probably escape through shelfal sandstones of the Mousa and Dornoch formations.
Locations of vertical drainage pathways between sandstone members were interpreted using local anomalies in the overpressure distributions and shared hydrocarbon accumulations, with vertical fluid flow found to be facilitated most commonly via areas of direct contact between sands, salt diapir structures, remobilised sands, and non-sealing graben-bounding faults. Areas with the greatest potential of hydrodynamic trapping were identified, although more accurate assessment of the prospectivity of such areas is required on a local scale.
Attempts at establishing the source of fluids entering into the Palaeogene system were made using 1D and 3D basin modelling. The results indicate a larger influx of fluid derived from the underlying pre-Cretaceous strata than from the overlying Tertiary mudrocks. These results, however, provide little quantification, and there is considerable scope for further work on 3D basin modelling.