Are seismic interpreters having a hard time? You’ve seen the symptoms - stressed, pressured, aggressive, erratic, irreconcilable - and that’s only the geological structures that they are more and more called upon to evaluate. This is not a plea for geophysicists, it’s a plea for structures, which are the cause of much interpreter chagrin.

Am I the only seismic interpreter operating in the UK North Sea region who believes he sees thrusted structures? In the received wisdom that informs the North Sea world, 'thrust' is anathema. Nonetheless in my everyday working experience I have encountered many situations which defy explanation using conventional normal-extensional strategies. I think most workers in the field will acknowledge the existence of structures where wrench, rotation and oblique-slip components of extension have all influenced the deformational regime. Add gravity collapse, slides and block emplacement mechanisms and you induce complexity of a high order. Despite prevailing opinion to the contrary, I believe there are at least two major fields - one in the Witch Ground Graben, one in the Viking Graben - where seismic data clearly demonstrate evidence of localised thrusting. That is to say, you can see compressional folds, shears, inversions, high-angle reverse faults and fault-plane décollements.

Why has the potential for compressional structures not been more readily examined by the UK geoscience community? Obviously if we operated in an established thrust province like the Rocky Mountains, belief and acceptance would not be a problem. But here in the UKCS, a rifted continental margin regime obtains. Graben tectonics are dominated by extensional processes So it can’t happen here. Well, why can’t it? Who knows what happens when an irresistable force meets an unmoveable object? What's wrong with locally compressive systems acting in conjunction with highly rotated fault blocks, enabled by gravity, induced by overburden décollement, or mobilised by basement shearing?

The illustration is reproduced in amended form from “The Leading Edge” Vol.20 No.7, with acknowledgements to the Authors. The field is located in the Gulf of Mexico. It represents a proposed mid-Tertiary structural analogue for two candidate fields in the North Sea. In the North Sea model, the timing of the main displacement of the overhanging structure is Late Jurassic/Early Cretaceous (Ryazanian to Valanginian age). Accepting a new structural paradigm has great implications. Not least how field development strategies are conceived. Oil field production teams must be aware that reservoir maps are only as reliable as the seismic data from whence they are derived. If generations of seismic interpreters pick only planar or listric normal faults and if visualisation and modelling systems identify only normal faults, then that’s how the reservoir will be characterised. By contrast, future 3D model building applications must accommodate uncomfortable interpretations if they are to reflect reality.