Restoring the seismic image with a geological rule base

Stuart Bland, Paul Griffiths and Dan Hodge of Midland Valley Exploration, Glasgow, Scotland discuss a new conceptual model for understanding the development of structures. This paper presents a technique that optimises the use of seismic data through manipulation of the image using a geological rule base. The approach can be readily used in routine interpretation and saves time by quickly focusing effort on fruitful interpretational models and by increasing confidence in picking in poor data areas and in complex structure. Seismic imaging is a primary source of information used in the exploration of hydrocarbons. Analogies have been drawn between the uses of seismic in exploration and production (E&P) and that of medical imaging in healthcare. There are similarities in the core functions of the seismic interpreter and the radiologist: both rely on high resolution images as 2D sections or 3D models to reveal what can’t be observed directly. In both disciplines the key aims are to note salient features in order to produce an accurate diagnosis of the situation and to advise others of their conclusions. Neither driller nor surgeon will appreciate surprises and will expect to have been appraised of critical factors and potential risks. The surgeon is interested in the location and most efficient route. Likewise the drilling engineer in the hydrocarbon accumulation needs to define the target. However, in both scenarios, whatever leading-edge technology is applied, the outcome is dependent on the interpretation of the data that, until drilling or surgery, remains an estimation of reality. At a fundamental level in hydrocarbon exploration, the interpretation of data is the product of a continual stream of decisions - ‘What does the horizon look like?’, ‘Can it be correlated across faults?’, ‘Where do the faults terminate?’, ‘Are the faults linked?’, ‘Is the horizon folded?’ One technique available to help the geophysicist is to flatten the seismic on key marker horizons. This is the digital version of the interpreter taking a folded paper section and overlaying one part on another to check character and correlation. Taking this technique a stage further we can use it to mimic simple deformations where flat-layered rocks become folded or faulted. Since horizons are both spatial and temporal objects – they are defined by geometry and age - horizon flattening can reveal significant features present at a particular time. Unfortunately this process has a number of drawbacks that require the interpreter to overlook distortions in the image, artefacts of the flattening process. These artefacts can arise where the horizon is interpolated across a fault or more generally because the flattening does not replicate the deformation observed in the section. These artefacts can significantly mislead the interpreter if not recognised. Where the medical doctor can refer to records to gain an insight into the patient’s medical history, the geologist can restore the section to understand its evolution. By using structural restoration to sequentially remove the effects of sediment compaction, isostatic adjustment faulting and fault-related folding that have altered the present-day section since deposition, we have a geologically valid way of looking at the history of the development of our structure while referencing the seismic image of the present day. Structural validation aids the decision process between alternative interpretations by testing the results within the framework of our understanding of geological history and evolution. Inclusion of the seismic enables validation of the geohistory within the context of the data. Three case studies are presented to illustrate the techniques involved in restoring the seismic image and the bene- fits from adopting this approach. Each case study has a distinctive setting, characteristic, key issues and associated risks. The first example is set within an extensional fault system of the Gullfaks, northern North Sea and depicts an untested interpretation. The second, an inverted series of half Grabens in the southern North Sea, typifies the problem of degrading seismic quality at depth. The final case study is taken from a Foreland Thrust basin in the Alberta Foothills, Canada. Each example demonstrates an enhanced level of detail and reduced risk of error in the final interpretation from apparently simple structures.