Tuning, AVO, and flatspot effects in North Sea Block F3
Abstract
© 2014 SEG The F3 block in the North Sea exhibits many large-amplitude reflections at shallow horizons, typically categorized as “brightspots” (Schroot and Schuttenhelm, 2003), mainly because of their bright appearance. In most cases, these bright reflections show a significant “flatness” contrasting with local structural trends. While flatspots can be easy to identify in thick reservoirs, we have often observed apparent flatspot tuning effects at reservoir edges and in thin reservoir beds, while only poorly understanding them. We conclude that many of the shallow large-amplitude reflections in block F3 are dominated by flatspots, and we investigate the thin-bed tuning effects that such flatspots cause as they interact with the reflections from the reservoir upper boundary. There are two effects to be considered: (1) the “wedge-model” tuning effects of the flatspot and overlying brightspots, dimspots, or polarity-reversals; and (2) the stacking effects that result from possible inclusion of post-critical flatspot reflections in these shallow sands. We modeled the effects of these two phenomena for the particular stratigraphic sequence in block F3. Our results suggest that stacking of post-critical flatspot reflections can cause similar large-amplitude but flat reflections, in some cases even causing a 'dimspot' interface to appear as a 'brightspot'. It is important to note here that muting based only on the basis of NMO stretch may cause some spurious features in the stacked section; care should be taken while applying muting in such cases.