Abstract Decades of oil and gas exploration across the North Sea have led to a detailed understanding of its Cenozoic–Mesozoic structure. However, the deeper basin architecture of Paleozoic petroleum systems has been less well defined by seismic data. This regional structural overview of the Devono-Carboniferous petroleum systems incorporates interpretations from more than 85 000 line-kilometres of 2D seismic data and 50 3D seismic volumes, plus a gravity, density and magnetic study, from the Central Silverpit Basin to the East Orkney Basin. A complex picture of previously unmapped or poorly known basins emerges on an inherited basement fabric, with numerous granite-cored blocks. These basins are controlled by Devono-Carboniferous normal, strike-slip and reverse faults. The main basins across Quadrants 29–44 trend NW–SE, influenced by the Tornquist trend inherited from the Caledonian basement. North of Quadrants 27 and 28, and the presumed Iapetus suture, the major depocentres are NE–SW (e.g. the Forth Approaches and Inner Moray Firth basins) to east–west (e.g. the Caithness Graben), and WNW–ESE trending (e.g. the East Orkney Basin), reflecting the basement structural inheritance. From seismic interpretation, there are indications of an older north–south fault trend in the Inner Moray Firth that is difficult to image, since it has been dissected by subsequent Permo-Carboniferous and Mesozoic faulting and rifting.

Abstract Prestack depth migration is widely used as the best tool for imaging and velocity model QC and refinement, but is mostly applied isotropically, even when there is evidence of anisotropy. We outline a methodology of model-building and imaging for use in anisotropic contexts and apply it to a line of data from offshore West Africa dominated by massive shales. The results show that the anisotropy in the shale increases with depth, along with the velocity. The prestack migrated data are sufficiently sensitive to the anisotropy that a thin layer of sand showing little or no anisotropy can be detected, and they must be included in the model to get flat common image gathers everywhere. This suggests that the use of anisotropy as a lithology-discriminating attribute may be feasible on a depth scale of only a few wavelengths. In any case anisotropy determination is important for precise application of other lithoseismic methods such as AVO.

Abstract Observations of polarisations and slownesses at downhole receivers in walkaway or multioffset VSP experiments can in principle be inverted to give the anisotropic seismic velocity of the earth in the vicinity of the receiver array. In practice, such inversions have been only rarely reported, possibly due to a general lack of confidence in polarisation measurements. We apply such an inversion to the direct P-wave arrivals from a 3D VSP dataset acquired at the Oseberg field in the Norwegian North Sea. The large size of this dataset ensures that the result is more stable than those from comparable inversions applied to standard 2D datasets. Comparison with the result from the more established, but more restricted, method of inversion of surface and receiver slownesses showed a large discrepancy. Further investigation revealed that this is due to acquisition-related coherent noise on the estimated surface slownesses; there is no corresponding effect on the polarisation inversion because each shot is treated independently. We therefore prefer the result from the polarisation inversion in this case.