The edges of the wedges; a systematic approach to trap definition and risking for stratigraphic, combination and sub-unconformity traps

In recent years, stratigraphic and combination traps such as Buzzard (UK North Sea) and Jubilee (Ghana) have attracted much industry attention. Such trap types are generally considered higher risk than structural traps, and understanding them represents a challenge for explorers, as numerous less successful (often amplitude-driven) attempts have demonstrated. Owing to their perceived high risk, stratigraphic traps are often drilled late in a basin's exploration history; however, we assert that consideration of stratigraphic traps should be part of any frontier exploration programme because they occur in all basin types and depositional settings, and allow new plays to be opened up. Additionally, stratigraphic, combination and sub-unconformity traps offer the chance to rejuvenate exploration in mature basins, as recent discoveries like the Edvard Grieg Field (Norwegian North Sea) have shown. Focusing on clastic systems, and using a combination of seismic examples and models, we present two aspects of stratigraphic trap exploration: (1) the regional and local factors that favour the development of stratigraphic trap edges; and (2) a systematic method for defining and risking the trap edges, avoiding the common problem of over-risking. These two methods, used together and applied consistently, allow explorers to focus on the right area of a basin and to risk stratigraphic traps appropriately, for a fair comparison with structural traps.