Diverse Toe-of-Slope Depositional Systems Revealed in Large 3-D Seismic Coverage

Qunling Liu, David Steele, and kurt Steenson
Shell International Exploration, Houston, TX

Large continuous 3d seismic coverage enables the extraction of a diverse toe of slope depositional systems in deep water Niger Delta. Because of high resolution, amplitude extractions from the interval immediately below the sea floor shows very detailed information about the depositional fairways of the last depositional episode. A variety of depositional systems: channels, fans, and overbank deposits are revealed. These different systems develop side by side and interlink with each other in different ways. The map display of amplitude extractions creates an excellent framework for understanding deepwater depositional systems and for discussing analogs for deep marine reservoir systems.

Large-scale channel systems dominated this lower to toe of slope environment. They are often more than 200 kilometers long and 1 to 5 kilometers wide. They meander cross the slope through subtle valleys or troughs on the sea floor. Some large channels develop branches as they go down dip.

The dip angle of the slope is the most important factor that dictates what type of depositional system will develop in a particular location. Several deep marine fans with well-defined networks of distributary channels develop in areas where the dip angles of the slope are lowered and in ponded basins. Channels subsequently cut the fans and continue further down dip in deeply erosional canyons with meandering thalwegs. These fan and channel geometries are the analogues of the channel and sheet reservoirs in deeper prospect levels. Over bank systems also develop when a channel goes through areas where the dip angles are lowered.