Plumbing the depths of the Central Graben: towards an integrated pressure, fluid and charge model for the Central North Sea HPHT play

Abstract

The integration of detailed seismic mapping, overpressure studies, basin modelling and new drilling has provided new insights into the ‘plumbing’ system of the deep, High-temperature, high-pressure (HPHT) play in the Central Graben. Although overpressure distribution in the pre-Cretaceous section is broadly related to depth, large-scale compartmentalization of the main Jurassic/Triassic reservoirs has resulted in the generation of a number of distinct pressure cells. These pressure cells are bounded by major structural faults (e.g. Forties/Montrose High) or by ‘interpod’ settings where the Jurassic/Triassic reservoir sequence thins over salt ridges. Evidence is presented that shows overpressure is a key control on hydrocarbon retention in this area, with several structures known to have blown top seals or to be leaky (i.e. underfilled) at present. The commercial Shearwater structure is shown to be an example of the latter, having evidence for a palaeo-oil column before the seal presumably breached and the trap recharged with a later gas-condensate fill. The length of hydrocarbon column in the structure at present day is controlled by the aquifer pressure and, in turn, by the shallowest point in the Shearwater pressure cell. The overpressure and trap filling history has had important implications for hydrocarbon migration, retention and the resultant composition of hydrocarbon accumulations in this part of the HPHT area. A conceptual model is presented here.