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 accumulation in the Central North Sea, have been studied to determine their pressure and petroleum filling history. The magnitude of overpressure in the Pre-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 depocentres. Pressure modelling shows that the magnitude of the overpressure can be attributed almost entirely to disequilibrium compaction, and largely due to late rapid Tertiary burial, although modelling does require nanoDarcy (i.e. shale-like) permeability in the Cretaceous chalk section. The contributions 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 microfractures in quartz and feldspar grains allows estimation of palaeopressure. The data show fracture healing from 115 to 136°C during a time when reservoir palaeopressures were above hydrostatic. Reconstruction of the temperature history of the reservoir places the timing of the fluid inclusion formation during fracture healing at between 3 and 1 My ago, coincident with a period of rapid burial. The palaeopressure estimation using fluid inclusions validates the pressure modelling studies, which are derived independently from commercial basin modelling software. The fluid inclusions also revealed a low-GOR oil over the whole field, prior to late stage flooding by gas leading to the variable (low to high) GOR fluids plus gas condensates within the field today. © 2000 Elsevier Science Ltd. All rights reserved.