Overpressures in the Central North Sea: implications for trap integrity and drilling safety

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Abstract
The Central North Sea Graben is, at pre-Cretaceous levels, a relatively under-explored province of the United Kingdom Continental Shelf. The hydrocarbon potential was recognized in the early 1970s, but both technology and economics prohibited any serious exploration efforts at that time. Exploration for deep targets increased significantly during the second half of the 1980s. This resulted in the discovery and appraisal of several fields, such as the moderately overpressured Marnock and Skua fields and the highly overpressured Erskine, Puffin, Franklin and Heron fields. From a regional viewpoint, the overpressures observed in the pre-Cretaceous reservoirs in the Central North Sea Graben follow a well-defined trend, increasing with depth. Similarly, the minimum effective stresses derived from leak-off pressures measured in the overlying seals follow a comparable trend related to the present-day depth of burial. This indicates that the present distribution of overpressures is controlled by retention and that the understanding of overpressure generation is of second-order importance. A more detailed analysis combining the areal distribution of overpressures with the structural framework suggests that the Central North Sea Graben can be subdivided into several pressure cells. The boundaries between the cells are controlled by faults which, in many instances, show signs of repeated reactivation. This reactivation has led in some cases to failure of otherwise well-defined structural traps, whereas in other cases it may have enhanced the trapping integrity. Examples of intermediate cases can also be recognized, which clearly show that the pressure cells form part of a dynamic system, controlled primarily by the structural evolution of the Central North Sea. A sound understanding of the structural framework and its evolution is therefore of paramount importance in predicting trap integrity and overpressure regimes prior to drilling. The drilling of deep high-pressure/high-temperature wells and the acquisition of modern 3D seismic data permit a continuous improvement in the understanding of overpressures and their distribution.