Integrated study of the Judy Field (Block 30/7a) - an overpressured Central North Sea oil/gas field

The Triassic reservoirs of the Judy Field, an overpressured petroleum accum ulation in the Central North Sea, have been studied to determine their pres sure and petroleum filling history. The magnitude of overpressure in the Pr e-Cretaceous aquifer is similar across the field at about 24 MPa (3500 psi) , but with some higher pressure laterally in areas closest to the major dep ocentres. Pressure modelling shows that the magnitude of the overpressure c an be attributed almost entirely to disequilibrium compaction, and largely due to late rapid Tertiary burial, although modelling does require nanoDarc y (i.e. shale-like) permeability in the Cretaceous chalk section. The contr ibutions from other mechanisms, which may be relevant in this setting, i.e. gas generation and lateral transfer, appear to be small. The existence of both aqueous and petroleum phases in secondary fluid inclusions in microfra ctures in quartz and feldspar grains allows estimation of palaeopressure. T he data show fracture healing from 115 to 136 degreesC during a time when r eservoir palaeopressures were above hydrostatic. Reconstruction of the temp erature history of the reservoir places the timing of the fluid inclusion f ormation during fracture healing at between 3 and 1 My ago, coincident with a period of rapid burial. The palaeopressure estimation using fluid inclus ions validates the pressure modelling studies, which are derived independen tly from commercial basin modelling software. The fluid inclusions also rev ealed a low-GOR oil over the whole field, prior to late stage flooding by g as leading to the variable (low to high) GOR fluids plus gas condensates wi thin the field today. (C) 2000 Elsevier Science Ltd. All rights reserved.