Geopressured Jurassic reservoirs in the Viking Graben: modelling and geological significance

Abstract

Formation pressure data from Jurassic and Triassic reservoirs of 192 wells (68 in the Norwegian sector, 124 in the UK sector) in the Viking Graben have been analysed. Several different pressure regimes are distinguished. An ‘open system’ consists of hydropressured reservoirs adjacent to the East Shetland Platform in the west and the Horda Platform in the east. A ‘restricted system’ consists of moderately geopressured reservoirs (overpressure typically lower than 1500 psi) located at greater distance from the platforms, and is interpreted to be in partial pressure communication with the open system through lateral or vertical pressure drainage. A ‘closed system’ consists of geopressured reservoirs practically isolated from the open system. In this system, formation pressure is assumed to be representative of the various overpressuring mechanisms active locally. Pressure data analysis suggests that pore pressure is mainly controlled by rapid loading, aquathermal pressuring, oil generation and possibly by shale dewatering above the depth for the onset of gas generation (formation overpressure typically in the range 1500–2500 psi). Gas generation is interpreted to constitute a major overpressuring mechanism below that depth (formation overpressure higher than 2500–3000 psi). In the former case, a pore pressure gradient approximately 50% higher than the hydrostatic gradient is estimated. In the latter case, a much more drastic overpressure increase with depth is suggested. This model also applies to the Norwegian Central Graben where only a limited number of wells tested Jurassic reservoirs. Despite unavoidable simplification and approximation, this model is believed to provide a better understanding of the areal distribution of geopressured Jurassic reservoirs in the Viking and Central Grabens. It may be used to impose constraints on hydrocarbon migration models and for prediction of porosity preservation in relation to reservoir overpressuring.