Pressure Cell Mapping as a Tool for Evaluating Remaining Exploration Potential: Example from the Pre-Cretaceous Series in the Central Graben, North Sea, United Kingdom

Point, Olivier¹, Thierry Dugoujard², Neville Taylor², Thierry Rives³, Michel Sacleux⁴ (1) Total E&P Norge AS, Stavanger, Norway (2) Total E&P UK PLC, Aberdeen, United Kingdom (3) TOTAL SA, Pau, France (4) Total SA, Paris La Defense, France

In the Central Graben area of the North Sea, overpressures can reach such levels that the ultimate top seal integrity is overcome. Top seal breaching results in partial leakage, known in some fields of the area, but can also lead to complete emptying for other struc­tures.

The purchase of the 3D PGS seismic Megasurvey allowed TOTAL E&P UK to undertake a regional synthesis of the Central Graben province, which aimed at identifying the remain­ing exploration potential and assessing the value of undeveloped discoveries. In the process of assessing risks of identified leads and prospects, particular attention was given to pres­sure analysis. Mesozoic reservoirs are frequently overpressured with a pressure transition zone commencing in the Chalk. Fracture gradients were determined at a regional North Sea scale and used as tool to evaluate the risk of top seal breaching. A comprehensive pressure database was also used to build a pressure cell map. Pressure cells are bounded by major structural features (basement faults, salt ridges,…) which result from the rift history of the Central Graben province.

Practical examples of use of this method include evaluation of remaining potential updip of a dry Fulmar well and assessment of top seal capacity for a variety of Mesozoic prospects. In addition to top seal retention capacity, lateral and vertical compartmentalisation risks were also investigated using pressure analysis. Pressure cell mapping proved to be a very useful tool for residual exploration in a highly mature area.