Dipmeter and Log Motifs in North Sea Submarine-Fan Sands

Abstract

Submarine-fan sands and conglomerates are common in the North Sea basin, ranging in age from Middle Jurassic to early Eocene. Many ancient submarine fans are important reservoirs, both there and in rocks of diverse ages in other parts of the world. Deep-sea facies may be identified from seismic data, if they can be seen to occur at the foot of progradational delta slopes or fault scarps, and if submarine channel and fan morphologies can be mapped. They may also be identified from the sedimentologic study of cores. Once identified, the geometry and orientation of reservoirs in deep-sea deposits can be predicted from dipmeter and other wireline logs. In the North Sea, three basic log motifs can be recognized. (1) Slope and upper fan grain-flow sands and mass-flow conglomerates on gamma-SP logs show uniform motifs with sharp upper and lower contacts; dips are random. (2) Middle fan turbidite-infilled channels on gamma-SP logs show increased shale upward in section, serrate motif; dip motif declines upward, dipping into channel axis. (3) Lower fan turbidite lobes on gamma ray-SP logs show increased sand upward in sections, serrate motif; dip motifs increase upward, dipping in direction of progradation.