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 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 active 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 experience I have encountered many situations which defy explanation using the conventional normal-extensional approach. I think most workers in the field will acknowledge the existence of structures where wrench, rotation and oblique-slip mechanisms have all influenced the deformational regime. Add gravity collapse, slides and detachments and you can introduce 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 of the sedimentary section. That is to say, you can see compressional folds, shears, inversions, high-angle reverse faults and fault-plane décollements.

So should the potential for compressional structures not be more readily examined by the UK geoscience community? Obviously if we operated in an established thrust province like the Zagros mountains or North American Rockies belief and acceptance would not be a problem. But here in the UKCS, a passive continental margin mindset obtains. Graben tectonics are dominated by extensional processes so it can't happen. Well, why not. Who knows what happens when irresistable forces meet immoveable objects. In circumstances such as when, for example, a locally compressive system acts on a highly rotated fault block, mobilised by gravity, unweighted by overburden décollement, and activated by basement shearing?

Illustration 1 (GoM) shows the overthrust Cantarell/Sihil structure and is reproduced in amended form from The Leading Edge Vol.20 No.7, with acknowledgement to the authors (Aquino, Ruiz, Flores, Garcia). This supergiant field is located in the Bay of Campeche and was for many decades the most prolific field in Mexico. After 22 years of production a further one billion BOE of reserves were added by the discovery in 1998 of the underlying Sihil footwall block. The extension was revealed only after observations of a persistent material imbalance in the field prompted renewal of an equally persistent phase of seismic scrutiny.

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). The lessons of an alternative interpretative model are apparent. Not least in the way that field development strategies are planned and executed. From facilities design to production engineering, all depend on the recognition that reservoir maps are only as reliable as the seismic data from whence they derive. 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 systems must accommodate uncomfortable interpretations if they are to reflect reality.

Illustration 2 (Cote d'Ivoire) shows how structural rotation, sliding, slumping and localised thrusting of (in this case) the Cretaceous overburden can occur in response to wholly extensional (listric) faulting. Example is from the offshore Ivory Coast.