HPHT reservoir evolution: a case study from Jade and Judy fields, Central Graben, UK North Sea

3D basin modelling of a study area in Quadrant 30, UK North Sea was performed in order to elucidate the burial, thermal, pressure and hydrocarbon generation, migration and accumulation history in the Jurassic and Triassic high pressure–high temperature sequences. Calibration data, including reservoir temperatures, pressures, petroleum compositional data, vitrinite reflectance profiles and published fluid inclusion data were used to constrain model predictions. The comparison of different pressure generating processes indicated that only when gas generation is taken into account as a pressure generating mechanism, both the predicted present day as well as palaeo-pressure evolution matches the available calibration data. Compositional modelling of hydrocarbon generation, migration and accumulation also reproduced present and palaeo bulk fluid properties such as the reservoir fluid gas to oil ratios. The reconstruction of the filling histories of both reservoirs indicates that both were first charged around 100 Ma ago and contained initially a two-phase system in which gas dominated volumetrically. Upon burial reservoir fluid composition evolved to higher GORs and became undersaturated as a function of increasing pore pressure up to the present day situation. Our results indicate that gas compositions must be taken into account when calculating the volumetric effect of gas generation on overpressure.