Abstract

The seismic expression of submarine fan deposits within the lowstand fan systems tract is documented from the Gulf of Mexico, North Sea, Arctic Beaufort Sea, and the Permian Basin of Texas. Lowstand fans appear to have the same general seismic characteristics from basin to basin. The key recognition criteria include: 1) identification of lateral pinchout geometry, 2) high-amplitude continuous reflections onlapping paleobathymetric highs, 3) internal bidirectionally downlapping reflections, 4) low-relief external mounding, and 5) association with Type 1 unconformities basinward of the shelf edge. Pitfalls in the seismic recognition of submarine fan identification include 1) lowstand-fan deposition typically does not occur within submarine canyons, 2) apparent seismic mounding is not always submarine-fan related (in addition to data noise such as multiples, diffractions, statics problems etc., seismic mounding may also be indicative of slump, contourite and deltaic deposits, as well as erosional remnants), and 3) prediction of reservoir facies occurrence in sand poor systems can be problematic.

The seismic expression of submarine-fan deposits within the lowstand-fan and lowstandwedge systems tracts is compared. Both the lowstand fan as well as the slope and basin depositional systems of the lowstand wedge are typically “fan-shaped” or lobate in plan view and mounded in cross section. The principle factor which results in differences between these two units is the sand-to-mud ratio of the sediment supply, which is higher within the lowstand fan than within the lowstand wedge. The increased mud content of the lowstand wedge enhances the likelihood of mass movement processes occurring. In many instances, mass movement deposits within the lowstand wedge break up its seismic continuity. Consequently, the seismic response of the lowstand wedge in the deep water setting is commonly a contorted/chaotic seismic facies, in contrast with the more continuous higher-amplitude reflections within the lowstand fan.