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Keywords:

  • hydrodynamics;
  • lateral drainage;
  • overpressure;
  • seismic attributes;
  • Ula Field

Abstract

The Ula Trend lies on the eastern margin of the Central Trough in the Norwegian North Sea and contains the Ula, Gyda and Tambar oilfields. In this Trend, the development of late Jurassic shallow-marine Ula reservoir sands is complex, because of the deposition of these sands in salt-dissolution features, separated by inverted Triassic pods. In this paper, attribute mapping (derived from newly re-processed 3D seismic surveys) has enabled a predictive model to be established for the presence of the Jurassic sands, corroborated by well data, in salt collapse and sand fairway grabens. This model suggests that the Ula sand is laterally continuous over large areas of the Trend. Pressure data from the Chalk in Ula Field wells, and from Jurassic reservoirs on the Cod Terrace, suggest that these continuous sands of the Ula Trend have significantly less pressure than would be expected for their depth of burial. Pressure escape of this nature is often associated with hydrodynamic aquifers, in which overpressure gradients are demonstrable. In this paper, we have used both virgin and postproduction water pressure data from the vicinity of the Ula Field (as well as a new sedimentological model for the reservoir) to test the applicability of a hydrodynamic aquifer versus other models to explain differing oil–water contacts. We establish that the hydrodynamic and structural spill-points are coincident on the SE of the Ula Field, and that, therefore, hydrocarbons have potentially spilt out of the Ula structure SE towards a series of salt-flank traps, generating prospectivity up-dip. The salt features themselves are considered to act as pressure-release valves or vertical ‘leak-points’, as Ula sands on their flanks are sufficiently shallow, because of salt movement that overpressures exceed the fracture pressure of the rock, therefore allowing the Ula sands to de-pressurise over geological time, setting up this hydrodynamic flow.